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ifS A F E T Y S E R I E S

No. 15

Radioactive Waste Disposal into the Ground

I N T E R N A T I O N A L A T O M I C E N E R G Y A G E N C Y

V I E N N A , 1 9 65

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RADIOACTIVE WASTE DISPOSAL INTO THE GROUND


The follow ing States are M em bers o f the International A tom ic Energy Agency:

AFGHANISTAN FEDERAL REPUBLIC OF NIGERIAALBANIA GERMANY NORWAYALGERIA GABON PAKISTANARGENTINA GHANA PARAGUAYAUSTRALIA GREECE PERUAUSTRIA GUATEMALA PHILIPPINESBELGIUM HAITI POLANDBOLIVIA HOLY SEE PORTUGALBRAZIL HONDURAS ROMANIABULGARIA HUNGARY SAUDI ARABIABURMA ICELAND SENEGALBYELORUSSIAN SOVIET INDIA SOUTH AFRICA

SOCIALIST REPUBLIC INDONESIA SPAINCAMBODIA IRAN SUDANCAMEROON IRAQ SWEDENCANADA ISRAEL SWITZERLANDCEYLON ITALY SYRIACHILE IVORY COAST THAILANDCHINA JAPAN TUNISIACOLOMBIA KENYA TURKEYCONGO, DEMOCRATIC REPUBLIC OF KOREA UKRAINIAN SOVIET SOCIALIST

REPUBLIC OF KUWAIT REPUBLICCOSTA RICA LEBANON UNION OF SOVIET SOCIALISTCUBA LIBERIA REPUBLICSCYPRUS LIBYA UNITED ARAB REPUBLICCZECHOSLOVAK SOCIALIST LUXEMBOURG UNITED KINGDOM OF GREAT

REPUBLIC MADAGASCAR BRITAIN AND NORTHERNDENMARK MALI IRELANDDOMINICAN REPUBLIC MEXICO UNITED STATES OF AMERICAECUADOR MONACO URUGUAYEL SALVADOR MOROCCO VENEZUELAETHIOPIA NETHERLANDS VIET-N AMFINLAND NEW ZEALAND .YUGOSLAVIAFRANCE NICARAGUA

The A gency 's Statute was approved on 23 O ctober 1956 by the C onference on the Statute o f the IAEA held at United Nations Headquarters, New York; it entered into force on 29 July 1957. The Headquarters o f the A gency are situated in Vienna. Its principal ob je c t iv e is "to a cce lera te and enlarge the contribution o f a tom ic energy to p e a ce , health and prosperity throughout the w orld ".

© IAEA, 1965

Permission to reproduce or translate the inform ation contained in this publication m ay be obtained by writing to the International A tom ic Energy A gency , K3rntner Ring 11, Vienna I, Austria.

Printed by the IAEA in Austria August 1965


SAFETY SERIES No. 15

RADIOACTIVE WASTE DISPOSAL

INTO THE GROUND

INTERNATIONAL ATOMIC ENERGY AGENCY VIENNA, 1965


International Atomic Energy Agency.Radioactive waste disposal into the ground.

Vienna, the Agency, 1965.I l l p. (IAEA, Safety series, no. 15)

621.039.7 614.876

THIS REPORT IS ALSO PUBLISHED IN FRENCH, RUSSIAN AND SPANISH

RADIOACTIVE WASTE DISPOSAL INTO THE GROUND, IAEA, VIENNA, 1965

STI/PUB/103


FOREWORD

Encouragement in the use of safe methods in radioactive waste management is a primary task of the International Atomic Energy Agency. In its Safety Series of publications it has already issued Panel reports on Radioactive Waste Disposal into the Sea (1961) and on the Disposal of Radioactive Wastes into Fresh Water (1963). These are now joined by a third complementary report, on Radioactive Waste Disposal into the Ground.

It has been prepared by the Agency1 s Secretariat on the basis of the discussions by an ad-hoc panel of experts convened by the Agency under the chairmanship of Mr. Mahmoud (United Arab Republic). Representatives of the Food and Agriculture Organization of the United Nations, the World Health Organization and the European Nuclear Energy Agency participated in the work of the Panel. The members represented a variety of disciplines and experience pertinent to this broad and often complex subject.

Most of the available information on disposal into the ground has come from establishments that have routinely practised ground disposal on a large scale. The present work has provided an analytical study of that information with emphasis on low and intermediate level wastes rather than the specialized problems of storing high-level wastes. It is hoped that it will be of direct interest to those who anticipate disposing of radioactive wastes into the ground, whether on a large or small scale.


CONTENTS

I. INTRODUCTION ....................................................................... 3

II. SITE CHARACTERISTICS AFFECTING GROUNDDISPOSAL AND ITS INVESTIGATION ......................... 6(1) Climate ............................................................................... 6(2) Hydrology ............ .............................................................. 6(3) Geology and sub-surface investigation ........................ 8

III. CHEMICAL REACTIONS OF WASTES IN THEGROUND AND THEIR PHYSICAL BEHAVIOUR.............. 12(1) Chemical reactions with minerals ............................... 12(2) Physical behaviour in the ground ................................ 15

IV. MODES OF RELEASE............................................................. 21(1) Liquid wastes .................................................................... 21(2) Solid disposals .................................................................. 30

V. EVALUATION OF SITES AND METHODSOF GROUND DISPOSAL ....................................................... 35(1) Potential exposures......................................................... 35(2) Site evaluation ................................................................. 37(3) Choice of shallow or deep disposal ............................. 39(4) The sm all-scale disposal of solid wastes ................ 41

VI. STANDARDS AND CONTROL TECHNIQUES ..................... 45(1) Standards ....................................................................... 45(2) Control ........................................................................... 45(3) Monitoring.......................................................................... 47(4) Accidental Release .......................................................... 52

VII. CONCLUSIONS........................................................................... 53(1) D isposal.............................................................................. 53(2) L iquid.................................................................................. 54

LIST OF PARTICIPANTS ............. ............................................................. 1


(3) Solid ................................. ................................................. 55(4) Storage............................................................................... 55(5) Accidental Releases ....................................................... 56

APPENDIX I Pre-treatment for ground disposal ............... 57

APPENDIX II Physics and chemistry of the movementof radioactive wastes in the ground .................. 64

APPENDIX III Ground disposal operations .............................. 76

APPENDIX IV Methods of site investigation .......................... 87

GLOSSARY OF TERMS ...................................................................100

REFERENCES .................................................................................... 104

BIBLIOGRAPHY 1 1 0


L I S T OF

Chairman

K. A. Mahmoud

Panel m em bers

A. Barbreau assisted by C. Gailledreau

L. Ber£k

G. Di Lorenzo

E. Glueckauf

W. J. Kaufman

P.J. Parsons

J. Rotnicki

K. T. Thomas

Consultant

P. Dejonghe

Representatives

B. H. Dieterich

T. Siggerud

P A R T I C I P A N T S

United Arab Republic

France

Czechoslovak Socialist Republic

Italy

United Kingdom

United States of America

Canada

Poland

India

Belgium

World Health Organization

International Council of Scientific Unions


G. Wortley Food and Agriculture Organization

E. Wallauschek Organization for Economic Cooperation and Development/ European Nuclear Energy Agency

Scientific secretary

J. F. Honstead International Atomic EnergyAgency

Note:In the months immediately after the Panel, a number of inter

national meetings were held on allied subjects. For instance, a colloquium on "The Retention and Migration of Radioactive Ions in Soils" was held at Saclay sponsored by The Commissariat & l 1 Energie Ato- mique and the French section of the Health Physics Society; there were also IAEA symposia on the "Treatment and Storage of High- level Radioactive Wastes" (Vienna 1962) and on "Radioisotopes in Hydrology" (Tokyo 1963). Both of these broad programmes included fringe subjects overlapping ground disposal that could usefully complement the results of the Panel meeting.

This report is thus a presentation of the results of the Panel meeting augmented and modified as necessary by details from other international meetings and recent publications considered appropriate for inclusion in the text. It was drafted initially by J. F. Honstead and expanded to its final form by P. J. Parsons from the Division of Health, Safety and Waste Disposal. Advice from C. A. Mawson and D. W. Pearce is also gratefully acknowledged.

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I. INTRODUCTION

Radioactive waste from nuclear establishments must be treated, contained or disposed in such a way that it will endanger neither the surrounding population nor the natural environment. Of the various methods used, disposal into the ground has sometimes proved to be an expedient and simple method. Where ground disposal has become an established practice, the sites have so far been limited to those remote from population centres; but in other respects, such as in climate and soil conditions, their characteristics vary widely. Experience gained at these sites has illustrated the variety of problems in radioactive waste migration and the resulting pollution and environmental radiation levels that may reasonably be anticipated at other sites, whether remote from population centres or otherwise.

Radionuclides can enter the soil either directly by the introduction of liquid wastes, or indirectly by water infiltrating through the soil and leaching contaminants from the surface of solid waste buried with insufficient protection. The release may not be deliberate but may result, for example, from an accidental rupture in a buried pipe-line causing an escape of radioactive solution.

However the release occurs, the soil and its attendant pore water become contaminated, and this leads to the special case of radioactive pollution of the ground water. The pollution may disperse and become diluted or may remain close to the point of introduction, according to the chemical composition of both the soil constituents and the water. The contaminants may retain their toxicity for long periods, depending on their radioactive decay rates; so even though they may stay underground for many years, there remains the possibility that the longer-lived nuclides will survive in a large enough quantity and move far enough below ground to pollute a potable source of water.

The pollution of ground water is a fairly common problem caused normally by domestic sewage, detergents or industrial wastes being released with little or no control [1]. Radioactive wastes, by contrast, are discharged under controlled conditions, except in the case of accidents; but there is always apprehension that the pollution could pass inadvertently into the human food chain. Anxiety may be further increased when low-level ingestion can continue for some time without noticeable effect and when the pollution can be recognized only by special instruments.

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The soil is a medium in which many pollutants are reduced in potency, either by oxidation, by chemical and physical sorption, or by dilution or delay. Some chemical wastes [2, 3, 4], e. g. nitrates and chlorides, attenuate only by dilution but radioactive wastes normally attenuate through sorption, dispersion, dilution and with the passage of time. Thus any environment has a potential capacity to receive limited quantities of radioactive waste material without creating an unacceptable exposure potential.

This report has attempted to analyse the factors that control this capacity and to assess the technical problems associated with ground disposal. It has not attempted to compare the merits of ground disposal with those of other methods of radioactive waste management.

It may be expected that the discussion is of greatest pertinence to sites that have a significant potential for retaining wastes released into the ground. For other sites, the nature of the geological structure, the hydrological conditions or the proximity of population centres may reduce the amount of safe permissible release to uneconomical amounts. In this situation ground disposal may appear futile and unnecessarily hazardous; but it is often expedient nevertheless to bury solid waste or to sink containers of radioactive liquid underground, in order to take advantage of earth shielding. This procedure is more appropriately termed ground storage rather than ground disposal, since the objects may be unearthed and retrieved for processing at any time.

In contrast, ground disposal implies a more or less irreversible practice in which solids are intended for permanent burial and liquids or leached radionuclides merge with the naturally occurring ground water. These radioactive solutions are carried along by the ground water mass at a velocity that may equal that of the water but which is frequently far slower as a result of chemical interaction between the radioactive ions and the earth materials.

Eventually the contaminated water will emerge from below ground by seepage at a spring or stream or by penetrating the region of an aquifer tapped by a well. It is at that potential area of emergence or point of usage that the Health Physicist has to determine the level of permissible contamination based on any possible hazard to population, livestock or biota. The corresponding permissible amount of emerging contaminants at such a point and the probable attenuation afforded to contaminants in their passage through the soil from the disposal area to this point, provide bases for determining the type and permissible magnitude of ground disposal operations.

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Numerous disciplines are required in the evaluation of a site for ground disposal. Those principally involved are geology, soil chemistry, hydrology and engineering, but numerous other scientific topics may have to be specially examined. This report has considered the relevant sectors of each discipline without becoming too detailed in any one subject. It discusses the subsurface investigations undertaken to disclose the nature of rock and soil, and associated studies of all the contained water-bearing formations. Conventional exploratory tools may be applicable or geophysical methods may be used to supplement the lithological data. The chemistry of the soil is of major importance since the sorptive capacity or affinity for certain radionuclides will determine the useful attenuation by the soil while migrant contaminants are present in the ground water. The movement of water underground is discussed together with its effect on the migration of radioactive ions through both the aerated and saturated zones. Liquid wastes may be discharged into shallow formations and so may modify the natural ground-water flow pattern or they may be injected at depth through boreholes penetrating porous formations. Experience of most types of liquid disposal operations has been described in addition to experimental disposals into deep aquifers and rock strata and the proposed use of salt formations.

In evaluating sites for waste disposal the preferred environmental criteria seldom occur together, so the balance of conflicting requirements has to be examined. The merits and disadvantages of shallow and deep disposal are considered in relation to the accuracy in predicting the future behaviour of the waste, and the ability to monitor or control any subsequent movements.

For the disposal of solid radioactive waste the potential hazard is normally much less than for liquid disposals. Various solid disposal operations have been examined as well as those experimental procedures aimed at solidifying liquids and sludges, thus making them less leachable and therefore acceptable for burial in the less exacting conditions of a solid disposal area.

Most of the report is necessarily based on the experience gained at establishments where ground disposal has been practised on a large scale. For this reason a section is devoted to assessing the minimal requirements necessary before small quantities of solid waste may be buried in the ground; for these small operations it is felt that the pre-operational survey could be curtailed to make it more commensurate with the low inherent risks involved.

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II. SITE CHARACTERISTICS AFFECTING GROUND DISPOSAL AND ITS INVESTIGATION

The natural features determining the suitability of a site for ground disposal include the climate, the type of soil and geological structure, the hydrology, particularly in relation to underground water sources, and the proximity to population centres.

(1) Climate

Where the climate is consistently damp there is obviously greater likelihood that radioactive materials will be leached. As the moisture, from rainfall or other precipitation, infiltrates through the soil at waste disposal sites it may come into contact with buried solid wastes or elute radionuclides already sorbed by the mineral components in the soil. A rough guide to the measure of infiltration likely to cause this condition is the climatic feature known as the "Net Annual Precipitation Surplus". This is defined as the average annual gross precipitation minus the total annual potential for evaporation. Unfortunately, it has to be applied with discretion since much depends on the frequency and intensity of the precipitation with its corresponding "run-off". It is possible, for example, for a re gion with a negative net precipitation surplus to have surface water infiltrating the sub-soil if the rainfall occurs during infrequent violent storms.

Although wind and water are both natural transport media, it is the naturally occurring water below ground surface that plays overwhelmingly the greatest part in spreading contamination from buried radioactive waste. If the material becomes exposed to wind or surface water, contamination may spread relatively fast, and any delay anticipated from burial will be lost immediately.

(2) Hydrology

Because of the special significance of the ground water, much of the exploratory and research work has been devoted to the detailed examination of sub-soils or rocks and the precise determination of ground water flow through these formations. P or the special re quirements of waste disposal, a typical examination would encompass the environs of a proposed waste disposal ground with particular emphasis on the underground flow path of water at the site. The

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general information normally obtained in a ground water investigation would certainly be useful [4, 5], The depth and thickness of the aquifer would be ascertained, the hardness, solids content, pH and chemical composition of the water would be measured and the yield determined after a period of sustained pumping. Such data would only indicate broadly the average transmissibility of the waterbearing formation with associated estimates of the mean permeability. While these details are pertinent to waste disposal problems, interest is centred more on the behaviour of specific water bodies within aquifers together with these subterranean flow patterns. Aquifers must be identified and their boundaries determined; pressure measurements are required to map the piezometric contours that may indicate, for example, unsuspected horizontal subdivisions within an aquifer. Ideally, spot measurements of local ground water flow should be taken. This information must then be examined in the light of conditions prevailing at the time of measurement. Owing to seasonal changes in climate the sub-surface behaviour of ground water should be expected to vary accordingly, particularly in humid regions.

It is most unlikely that there will be prior information on local ground water details before the investigation. However, there may be records of rainfall or even records of flow in nearby rivers to indicate historically the anticipated variation in annual precipitation and run-off. Records of rainfall and ground water levels should be maintained at a site as essential hydrological data which, as necessary, may be related to the prior long-term records. In this way it may be possible to predict probable annual variations in ground water conditions and, if flow nets have been described, to estimate their seasonal variation. In particular, it should be possible to distinguish longer cyclical changes from extreme abnormal conditions.

One of the main objectives in the ground water investigation would be the determination of the flow path for water beneath the proposed site and, in particular, of the points at which it is likely to appear at the surface [2, 6]. This "area of emergence" may be at a spring, or where there is seepage into a stream, or near a well that penetrates and taps a connecting aquifer. Frequently this subsurface path for the water may be fairly obvious from topographical features, but it may be difficult to define if the water table lies at great depth, making observation and measurements proportionately more complicated [7, 8, 9],

7


Probable variations in the ground water path should be examined; for instance, if a gravity aquifer is shallow the flow path would be influenced significantly by the lower boundary profile and therefore more susceptible to variation with the recharge rate. Flow paths in deep aquifers, conversely, would be less influenced by variations in recharge rate.

The time of passage for ground water to traverse the possible routes will have to be estimated and this is probably one of the most difficult yet important facets of the investigation. Without an estimate of the travel time it will be impossible to assess the probable retention of migrating radioactive ions.

(3) Geology and sub-surface investigation

It is axiomatic that no investigation of aquifers can be complete without a thorough understanding of the associated geological features [10,11]. To examine precisely the successive; horizons of strata, samples of geological materials must be collected and examined. The objects of such investigations, for the special requirements of waste disposal, are directed towards the permeability of the various formations [12] and the isolation of their mineralogical components for sorption and ion-exchange measurements. For consolidated rock, the conventional method of rotary drilling and the extraction of core samples is sufficient for these purposes. For the permeable sedimentary rocks, physical examination of undisturbed samples and permeability checks may be carried out; whereas for the intrinsically impermeable rocks, water flow is restricted to fissures whose presence may be revealed only by in-situ measurement.

The investigation of unconsolidated and granular deposits may be undertaken by conventional methods of drilling but sampling re quirements may be different in order to secure specimens suitable for non-standard examination. In soils that are partially dry,a borehole may be sunk by means of a churn drill or auger, and conventional sampling carried out with tools appropriate for the type of soil. Those that have a large range of particle sizes, e. g. gravels, are difficult to sample except in a disturbed state by auger or bailer, and reconstituted samples of such soil may be the best that can be obtained for examination. Where the soils are sorted into smaller, more uniform grain size, e.g. sands, it is sometimes better to use washboring methods and certainly preferable to sample in the un


disturbed state re g a rd le ss o f w hether the s o il is dry o r saturated. A visual exam ination o f undisturbed sam ples would indicate the h o m ogeneity o f the so il o r whether it was obviously com posed of su ccess ive lam inations o f contrasting w e ll-so rted so il. An exam ple of the lam inated type o f structure, a coh esion less fine sand containing le ss than 5% silt, is shown in F ig. 1. Such observations are n e ce s sary when the p erm eab ility o f the m edium is one o f the p rin cip a l details sought from the investigation .

F ig -1

Cross-section through an undisturbed sample o f laminated sand.

Lam inated m a teria ls have d ifferen t p erm ea b ilit ie s a ccord in g to the d irection o f w ater flow and are term ed a n iso trop ic . When sam ples o f undisturbed granular so ils are tested fo r p erm eab ility in the laboratory, water is passed along the axis of each cylindrica l

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sam ple that was prev iously oriented v ertica lly in the ground. The induced flow th erefore correspon ds to v ertica l flow in the fie ld and passes a cross the layers o f so il that are norm ally oriented c lose to the horizontal plane. It is usual therefore for laboratory perm eabilities to be low er than the horizontal field perm eability parallel to such layers . Visual examination will also indicate whether the fine fr a c tion of a so il sam ple (silts , clays), which includes any clay -m in era l component, is distributed throughout the m atrix or whether it is con centrated in pockets or layers. This is important since the sorption capacity is often associated d irectly with the quantity and availability o f clay m ineral. Clays in th em selves are not sub jected to the in tensive exam ination they re ce iv e in n orm al so il investigations d irected, for instance, to the determination of shear strength and com p ress ib ility . The in terest in this m ateria l is centred on its in trin s ic im perm eability and whether clay strata are sufficiently "tight" to form an im penetrable boundary to an aquifer. With consolidated m ateria ls anisotropy can be as prevalent as in unconsolidated so ils but it may no longer be assumed that the higher perm eabilities lie close to the h orizonta l plane. Such form ation s m ay have been subjected to earth m ovem ents and the resu ltin g faulting o r fo ld ing m ay have left the strata inclined at any angle.

In deep form ations where econom y dictates that drilling should be fast and where detailed sam pling is m ore d ifficu lt, the requ ired in form ation is obtained by appropriate alternative m ethods. G eophysical logging [13] may be applied fo r exploratory studies, p articu larly where severa l aquifers may be stacked above each other as in sedim entary form ations or basalt. T ypical w ells [14] bored with percu ssion equipment in dry m aterials may be as much as 40 cm in d iam eter, which on entry into w a ter-sa tu ra ted m a ter ia l would be cased at a reduced d iam eter of, say, 15 cm . G eophysical logging in such holes may include hole-d iam eter, gam m a-ray, e lectr ic -flow - m eter, tem perature or w ater-resistiv ity logging. In drilling through lava the wall of the hole may be sm ooth and only slightly larger than the bit, whereas in blocky basalt it would be rough and la rger and in unconsolidated m ateria l the walls would cave in to produce a large over-cu t. In such m ateria ls the hole -d iam eter method and gam m a- ray logging can usefully supplement the lithologic log from drill cuttings. W a te r -re s is t iv ity , tem perature and flo w -m e te r logg ing appear m ore useful in the recogn ition o f aquifers, indicating v a r ia tion in the concentration of d issolved salts, the differential tem perature between water from different o r ig in s , and v ertica l flow within

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a w ell. W here only few boreh oles are sunk the resu lts may be augm ented by s e is m ic su rvey m ethods to in terpolate the approxim ate details between borings. There are other methods whereby television equipm ent m ay be em ployed to exam ine the free -s ta n d in g w alls o f a b oreh ole fo r details o f stratigraphy and orientation .

To m eet the demand fo r detailed so il and ground water surveys, existing tools have been used or adapted and specia lized tools [15, 16, 17] have been invented to m eet new requ irem ents. N orm ally, a tool developed fo r use in one type of so il is unsuitable in other types, p erhaps as a resu lt o f changes in consistency , water content, sampling depth, hardness, etc; and because of these limitations the new equipment has frequently been used only fo r specia lized conditions. New w ater-sam plin g techniques w ere developed w here rad ioactive con tam ination in the ground w ater had m igrated fro m d isp osa l s ite s . They w ere developed to delineate p rec ise ly the boundaries o f m igration and proved them selves equal to these exacting demands. W ater- sampling devices w ere constructed fo r driving into the so il to extract water from a narrow d iscre te stratum [18] and an extension o f this princip le produced a device capable o f extracting water sam ples s i multaneously from severa l horizons with no intercontam ination [16]. Instrum ents have been developed fo r low erin g inside dry tubes in serted in the so il; these m easu re radiation fie lds and con firm the presence of a particular g a m m a -emitting nuclide by plotting its energy spectrum .

F or investigating la rg e -s ca le ground water transport as w ell as detailed stream lin es and flow patterns the u se o f ground w ater tra cers has expanded. M aterials suitable as tra cers have to be s o lutions that are com pletely m isc ib le with w ater, should have no adsorption on the so il, should be detectable in very low concentrations and should not rea ct ch em ica lly with the ground w ater. The use of ground w ater t ra ce rs such as flu oresce in dye and e le c tro ly tes has long been estab lished but the p re c is e concentration m easurem ents attainable with rad ioactive tra ce rs coupled with im proved in jection and sam pling tech n iqu es, have in trodu ced a standard o f a ccu ra cy hitherto unequalled. Am ong these the use o f tritium as tritiated w ater d e se rv e s sp ec ia l m ention s in ce its beh aviour in the ground water in speed and d isp ersion is v irtually identical to the naturally residen t w ater. It has the sligh t d isadvantage that equipm ent r e quired to detect and m easu re tritium concen tration s is frequently m ore elaborate than that needed fo r other radioactive tra cers . However, if these other tra c e r s are intended fo r use, p r io r fie ld e x

11


perim ents com paring their rates o f m ovem ent relative to that of tritium may assist in choosing the m ost suitable tra ce r com patible with a particular so il and ground water. To avoid disrupting the ground w ater stream lin es by in jection , frozen s o u rce s have been in troduced [19]'that thaw slow ly to re le a se a plum e o f ra d ioa ctive tra ce r . Chem isorption techniques have been investigated to su persede water sampling, which itse lf inevitably causes some interference to the natural flow . W here w ater and so il sam pling have been n e- .cessary , tools have been developed to sam ple these sim ultaneously from several p re -se lected horizons in sm all quantities to cause m inimal in terference with ground water flow. M ore details of these p ro cedures are given in Appendix IV.

M ost of these refinem ents have been adopted for use in relatively shallow unconsolidated saturated form ations. In deeper form ations the tasks are m ore difficult. A ll equipment has to be heavier, m ore com plicated and expensive and at present the standard o f m easu rements is le ss p rec ise . Pumping tests have proved useful when con fined within the boundary of a particular stratum. By packing a cased w ell above and below the stratum , p erfora tin g the casin g betw een, and then pumping from this section, som e hydraulic ch aracteristics may be m easured. Flow measurem ents within a single well are being investigated electronically with the object of measuring the horizontal flow through the well and the vertical flow within it.

III. CHEMICAL REACTIONS OF WASTES IN THE GROUND AND THEIR PHYSICAL BEHAVIOUR

(1) Chemical reactions with minerals

When solutions o f rad ioactive w astes are re le a se d into the ground the d isso lved m ateria l w ill often re a ct ch em ica lly with the c la y -m in era l o r organ ic constituents o f the su b -s o ils . The resu lt o f such reaction is to reta rd the m ovem ent o f the rad ioactive m a te r ia l re la tive to that o f the solvent. Since the tim e req u ired fo r movement of the m aterial to a point where it is accessib le to the public determ ines the fra ction that w ill decay en route, the ch em ica l reaction s are im portant. In m ost ca s e s , the solutions re lea sed to

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the ground are com plex and contain many chem ical constituents, both stable and rad ioactive. The m edia through which the solutions flow are also com plex m ixtures of various m inerals and organic m aterials. Under these circum stances it is im practical to consider the chem ical behaviour o f the solution in term s o f individual ch em ica l reactions and a m ore e m p ir ica l v iew is n e ce ssa ry [20], It is often p oss ib le to lim it the consideration to a few im portant rad io isotopes, e. g. strontium -90, ca e s iu m -137, ruthenium -106 or coba lt-60 .

Studies [10, 21, 22] have shown that som e m in era ls d isp lay a m arked sorption p re feren ce or "se le c t iv ity " fo r certa in ions. The effect appears to be related to the ion ic dim ensions and som e stru ctural dim ensions in the crysta l la ttice o f the m ineral. Pronounced se lective sorption o f caesium has been dem onstrated fo r m ost m ineral constituents of the so il and the effect of this selectiv ity may be o f m ore s ign ifican ce than the m easu red total exchange capacity o f the m inera l. F o r exam ple, i l lit ic c lays have a h igher affinity but low er capacity than m ontm orillon itic clays. Thus i f the concentration o f caesium ion is low the affinity fa cto r ov err id es the capacity fa ctor and the illite w ill sorb the g rea ter amount o f caesiu m . But if the concentration o f caesium ion is in crea sed , the exchange c a pacity becom es predominant and the m ontm orillonite w ill sorb m ore caesium than the illite .

N early all sorption reactions with natural m inerals are lim ited to cation ic sp e c ie s , so that those elem ents o f rad ioactive waste in cationic form are subject to retarded m ovem ent through the so il. In con trast, elem ents in anionic fo rm exp erien ce no such delay and m ove re la tiv e ly unhindered through the s o il at a speed re la ted to, but rather le s s than, that o f the natural ground w ater. T here are, as m ight be expected , excep tion s to these gen era liza tion s. When certain, cations are brought into contact with chelating agents, they are com plexed and henceforth m ove rap id ly , exp erien cin g no e x change reactions with the so il. An exam ple o f this [23] is the com plex form ed by E. D. T. A. with co b a lt -60. Again, anionic behaviour is frequently changed if organ ic m a teria ls are p resen t in the so il; such m ateria ls m ay have som e anion exchange capacity and a c cordingly retard m igration. Fortunately m ost of the hazardous radionuclides present in waste are n orm ally in cation ic fo rm but among the notable excep tion s is ru th en iu m -106, which m ay be p resen t in cationic, anionic or neutral form accord ing to its pre-treatm ent and its h istory after d isposa l. With such unpredictable behaviour it is difficult to know whether this radionuclide is likely to m igrate or not.

1 3


There are various types of sorption m echanism s (elaborated in Appendix II) but p re lim in a ry la b ora tory exam ination fo r sorp tion should determ ine the suitability o f a subsoil fo r retaining radio - cations, irresp ectiv e o f the m echanism s. The cation-exchange ca pacity may be m easured by estab lished standard techniques in so il chem istry indicating the num ber of equivalents o f exchangeable ions contained in unit weight of so il (m eq/100 g) [24]. Typical values for sands [25J lie between 0. 5 and 20 m eq/100 g, the variation depending largely on the clay m ineral component that frequently represents less than 1% o f the total so il. A frequently used dynamic method involves passin g a ra d ioa ctive solution at a u n iform rate through a colum n o f so il . Sam ples o f the effluent solutions are taken reg u la r ly and their radionuclide concentration com pared with that o f the influent solution. The "break-through", represented by the arrival of rad io activ ity at the outlet, w ill be fo llow ed by a subsequent in crea se in concentration until both influent and effluent solutions are the sam e. If the effluent volum e is ex p ressed in te rm s o f "co lum n v o lu m es" the resu lting breakthrough curve (C /C 0 versu s vol. effluent) shown in F ig . 2 may be n orm alized re la tive to the s ize o f the so il colum n. When these results are extrapolated to predict behaviour in the field , it should be borne in m ind that fie ld conditions are m ore variab le , the s o il m ore h eterogeneous and the liquid m ovem ent su b jected to m echanism s of d isp ers ion [24, 26],

The equ ilibrium d istribu tion co e ff ic ie n t K j is the ra tio o f the sp ecific activity of the so il to that of the contacting solution. Its use is norm ally restricted to conditions where the concentration of radioactive ions in solution is v ery low com pared with the host ions that saturate the solid phase. The coe ffic ien t varies when the contam inant concentration r ises above such dilutions, and is also dependent upon the prevailing pH value. The m ost notable use of the Kd value is to estim ate the re la tive flow rates betw een the ground w ater and the s p e c if ic rad iocation (see Appendix II). It is evident that such resu lts w ill indicate whether the so il is suitable at a proposed site and w ill assist in predicting the reactions of se lected radionuclides when introduced into the su b -su rfa ce environm ent.

At sites w here the saturated zone lie s c lo se to the su rface the actual m igration ra tes o f nuclides m ay be m easu red and checked against labora tory p red iction s [27]; but i f the d isp osa l form ation s are deep, there is little likelihood of being able to conduct such p re c ise m easurem ent. So great re lia n ce is p laced on the resu lts o f ch em ica l in teraction tests in the la b ora tory .

1 4


1. TEMPERATURE2. SOIL EXCHANGE CAPACITY3. SPECIES OF FOREIGN ION4. CONCENTRATION OF FOREIGN ION5. FLOW RATE

CONTROLLED VARIABLES:

■

i - *

/’

3cmLEN6T

V

/ 1 1f TEMP. = I8°C /l20c mrLuw KAit-4tni/cmyn r i 1000 ppm Mg PRESENT /EXCHAN6E CAPACITY /

r s u i t

hA10 20 30 4 0 50 60 70

VOLUME-LITRES

Fig. 2

T ypica l results o f soil colum n experiments.

(2) Physical behaviour in the ground

Studies of the m ovem ent o f liquid through porous m edia have many p ra ctica l applications; fo r shallow depths these include the drainage of swamps and irrigation pro jects , while at greater depths exam ples are found in petroleum exploration and ground w ater developm ent. The d isposal of industrial effluents may be included in. either category but in few of these applications is the demand for p recise prediction of subsequent behaviour so exacting as in radioactive waste disposal.

A diagram m atic representation of a sim ple hydrological system is shown in F ig. 3. The so il -w a te r -a ir system is typ ica l of an unconfined aquifer with forces of piezom etric gradient, gravity and surface tension in dynamic equilibrium.

When rain fa lls on to the surface depicted in the diagram there are three routes that it may take. F irst, it may run off the surface, collect into rivulets and enter the stream - this represents the most

1 5


rapid m ode o f rem ova l. Second, it m ay pass into the so il and in filtrate downwards through the partia lly dry so il until it eventually reaches the saturated zone. Third, it may evaporate d irectly or be absorbed by root system s o f vegetation after it has entered the p artia lly dry so il.

It is the second o f these routes that is of predominant interest, s in ce it is a sou rce o f rep len ishm ent o r "r e ch a rg e " to the gravity aquifer and a means fo r leaching and transporting radionuclides from waste buried in the partially dry so il. The m oisture content in this so il, known as the aerated zone, in crea ses with depth, and the degree of saturation is a continuum with no rea l discontinuity between unsaturated and saturated zones. N evertheless the term "water table" is adopted by com m on usage, represen tin g the upper p ro file o f the saturated zone. It is in fact the locu s o f water leve ls in w ells that just penetrate the saturated zone.

The water table in Fig. 3 is continuous with the r iver level, and movement within the aquifer is directed towards the river,continuously replenishing it by seepage. W ells sunk through the upper deposits penetrating this aquifer have their standing water leve l coincident with the water table at that point. The rem ainder o f the diagram is related to a deeper aquifer, isolated from the upper gravity aquifer by an im perm eable form ation.

N orm ally there is no v ertica l flow interconnecting the two and if a w ell is driven into the low er aquifer and carefu lly sealed against seepage from the upper, the standing w ater lev e l w ill be d ifferent from the leve l o f the free w ater table. In the diagram it is higher, indicating the natural artesian effect when the confined aquifer outcrops at a higher altitude than the overly ing stratum .

If radioactive w astes w ere introduced into the system depicted in Fig. 3 they would lie in the aerated zone. Contamination may o r i ginate either from infiltration leaching the buried solids or from e ffluent d ischarged d ire ct ly into the so il . N orm ally the liquid waste would be an aqueous solution but if it contained an organ ic solvent, im m isc ib le with w ater, a com plex th ree -flu id flow system o f a ir, w ater and solvent would develop that would need sp ecia l analysis .

In the n orm al ca se , an aqueous solution would blend with any natural water present in the aerated zone and infiltrate through the so il. The path would be predom inantly vertica l until a le ss p erm eable stratum o r lens was encountered causing the liquid to spread la tera lly within the stratum through cap illa ry action. If the th ick ness and perm eability o f this layer was sufficient to induce satura-

1 6


Fig. 3

Diagram m atic illustration o f ground water details.

tion, water would accum ulate above it b e fore the downward m ov e ment was resum ed either by slow infiltration through the layer or by oversp ill at the edge if it was a lens. Under these conditions the flow would be com posed of numerous discontinuous accumulations of m oisture. They would next enter the cap illary zone where they could be influenced by pronounced horizontal flow before they finally entered the fully saturated zone. At this stage the transition from p re dominantly v ertica l flow to predom inantly horizontal flow would be com plete.

When the m igration is incorporated fully into the saturated zone, the residual vertica l component of flow depends upon the rem aining differential in sp ec ific gravity between the ground water and the d iluted contaminated solution. If the solution is denser than the water it tends to sink through the saturated so il . In the subsequent flow , contaminated solution displaces norm al ground water and the boundary betw een the two b ecom es p ro g re ss iv e ly d iffuse. In the ca se o f a single d isposal, theoretica l pred ictions agree reasonably w ell with experim ental results [27], showing that the solution is carried downgradient with the ground w ater in a c ig a r -lik e plum e. Its s ize in cre a se s with p ro g re ss io n ow ing to continuous diffusion both along and a cross its path, and its trailing edge may be ill-defin ed if there is significant sorptive delay.

With slow continuous discharge the shape o f the resultant plume is norm ally divergent but depends on the configuration o f the p iezo m etric contours which in turn re fle c t the lo ca l boundary conditions.

1 7


The plume contains relatively few radionuclides since many will have decayed and others may be highly sorbed by the soil and remain close to the point o f d isposa l. The length of the plume depends upon the rate o f ground w ater flow and the retentive capacity o f the so il fo r the sp ecific m igrant cations. The principal m echanism s of d isp ersion are caused by liqu id d isp lacem en t and lo c a l in h om ogen eities [14, 26, 28] in the so il, but by com p arison , the e ffe ct o f m o lecu la r diffusion is insignificant.

The flow -path of contaminants in a gravity aquifer may thus be sum m arized as an in itia l gravitation through the aerated zone f o l lowed by a horizontal m igration within the saturated zone. It is the fir s t stage of this p ro ce ss that is difficu lt to p red ict. Im m ediately after discharge the waste solution com es into d irect contact with the so il. A certain amount of water w ill be already resident in the so il but the quantity depends on the p articu lar percola tion o r rech a rge rate at that tim e. D ifferen ces in sp e c ific gravity and tem perature affect the percolation rate; the pH of the solution and its concentra:- tion in the so il have a profound e ffe c t on the reten tive capacity o f the so il. In addition to these points, the so il may w ell be inhom o- geneous and significant lateral dispersion may ensue if, for instance, the percolation is confronted by a less perm eable silt stratum, which need only be a few m illim e tre s th ick . Any accu rate p red iction o f flow through this zone req u ires deta ils o f all the re levan t fa c to rs m entioned, m any o f w hich are m ost d ifficu lt to c o lle c t .

In deep deposits the above p rob lem s m ay be s im ila r but m ore com plex if the saturated zone lie s at great depth; but much depends on the heterogeneity o f the form ation s. As depth in cre a se s it b e com es correspondingly difficult to extract p recise information. Sites fo r boreh oles have to be chosen with care and, because o f the time and expense of deep borin gs , som e of the geophysica l logging m e thods p rev iou sly outlined have to be adopted to supplem ent p e tro - graphic data and hydraulic tests . F low system s in such ca ses are obviously com plex, and resea rch is continuing on their analysis by com puter techniques but the su cce ss o f this depends u ltim ately on the quality o f the fie ld data that can be co llected . Many o f the p roblem s are associa ted with v e rtica l rather than h orizonta l flow . There are, fo r instance, exam ples where adjacent w ells penetrating an apparently hom ogeneous aqu ifer show ed v e r t ica l w ater flow in each casing - but while one was upward, the other was downward [14 ].

Some o f the la rg es t d isp osa ls ever m ade into the ground have been ca rr ied out where the w ater table lie s nearly 100 m below the

1 8


su rface, but even with the se lf-la b e llin g o f the m igrant waste, p re c ise flow paths have yet to be plotted. Waste solutions with high concentrations o f d isso lv ed so lid s and o f high s p e c if ic gravity , d is charged as on e-sh ot d isposa ls into the ground at the su rface , have seldom contam inated the ground water to m ore than a few thousand p. p .m . [ 14 J (see F ig. 4). This is because the w astes reach the s a turated zone through a m yriad o f w idely sca ttered d is c re te sm a ll stream s rath er than as a so lid fron t o f diluted solu tion .

Fig. 4

Diffusion o f radionuclides through the aerated zone.

In m u lti-aqu ifer conditions w here there are se v e ra l w aterbearing strata loca ted above each other, p oss ib ly in clined at d ifferin g attitudes and th ick n esses , it b e com es in crea sin g ly com plex to delineate their areal boundaries and determ ine where there is any

in terconnection . An iso la ted accum ulation o f ground w ater may gather above a gravity aquifer to fo rm a perched water table. Such a condition is often only tem porary but w hether it w ill d isp e rse or continue depends upon the d iffe ren ce betw een the lo c a l in filtra tion rate from recharge and the transm issib ility of the partially saturated soils that encircle and underlie the zone.

19


Flow within confined aqu ifers o ccu rs through the en tire depth o f the porous stratum and there is no partia lly saturated zone co m parable with the unconfined aquifer. Interconnection betw een con fined aquifers may be investigated by chem ical or tem perature com parisons, natural tritium concentrations or pumping tests, but care must always be taken to ensure that the standpipes or m easuring probes are adequately sea led and do not th em selves contribute to c r o s s - f lo w connnections. E stim ates o f ground w ater flow in deep confined aquifers are usually based on cla ss ica l pumping tests where num erous conditions are assum ed. F or instance in pumping a w ell, the system is subjected to a different o rd er of magnitude in gradient and flow rates that m ay not be ex trapolative to n orm al cond itions. The introduction of sodium flu oresce in into deep aquifers has shown that longitudinal d ispersion m ay be very pronounced, particu larly with high flow rates (100 m /d a y ), and that the fastest stream line of water may be three tim es the com puted average value [14],

Deep geologica l form ations, below the horizons of aquifers, are reg ion s o f in terest fo r d isp osa ls ow ing to th e ir iso la tion fro m the b iosp h ere . Outstanding, fo r its apparent su itab ility as a deep r e pository for waste, is the m assive deposit o f rock salt f?-9, 30]. E xperien ce in salt m ining has dem onstrated the stru ctu ra l su itability o f this m ateria l where la rge caverns have been form ed at depths o f 300 m without perceptib le deform ation of the supporting salt p illa rs . Of particu lar s ign ificance is the fact that such form ations are chara cte r is tica lly fre e from ground w ater and norm al m igration p rob lem s would th ere fore not be met.

Most rock form ations, however dry and seem ingly im perm eable, contain som e entrained water. In a form ation of m assive crystalline rock , below 300 m etres of unconsolidated sedim ents, the behaviour and res id en ce tim e o f the entrained w ater was studied to exam ine whether it was p oss ib le to store h ig h -lev e l liquid w astes in vaults engineered in the rock by mining [31]. Perm eability tests on packed sections of w ells produced w ater-level changes in observation wells; and it was subsequently deduced that the structure of the ro ck was traversed by a wide network of hair*-like cracks interconnecting the entire entrained water into one hydraulic system . By assuming rea sonable values fo r the porosity , which varies typically between 0. 1% and 6% in such m etam orphic rock , and com bining the m easured hydrau lic gradient and p erm eab ility , it was p red icted that the w ater m oved at 50 cm /y e a r through sound ro ck and 220 c m /y e a r through fractured rock . H ow ever, an independent check by estim ating r e

2 0


charge into the form ation w here it outcropped at the su rfa ce in d icated the flow to be only 0. 6 cm /y e a r . In this instance it was p o s sible to estim ate the age of the entrained water from sm all quantities of helium detected in solution. Since the helium was probably form ed by the a lph a -decay o f uranium and thorium , presen t as tra ce m in era ls , it was concluded from the p rop ortion s presen t that the water had been resident in the form ation fo r 5X105 years. The p re lim inary evaluation showed that if liquid wastes were introduced into specia lly prepared caverns in this rock , any potential hazard would be extrem ely low .

IV. MODES OF RELEASE

(1) Liquid wastes

Liquid w astes m ay be in troduced into the ground e ither at the surface or by in jection into deep p re -se lected form ations. Introduction at the surface is s im p ler, and is done by a variety o f m ethods, o f which the lea st soph istica ted is to d isch arge liquid d ire c t ly into a rtific ia l ponds as at H anford. This is a good exam ple o f su rfa ce disposal into deep deposits where the ground water lies at great depth (60 m) and the intervening zone of partially saturated soil is the prin cipal ion-exchange column for extracting radionuclides.

At Oak Ridge ponds have been form ed in open excavations in the underlying shale, the perco la tion being lim ited to the in terfissu ra l zones and cracks within the shale. To reduce contamination to w ildlife , w ire netting has been installed over the ponds at the Savannah River Plant, and at Chalk R iver the same object is achieved by filling the pits with pebb les . H ere the rate o f d isch arge is con tro lled to ensure that the free water surface remains covered by stones. These are exam ples o f su rfa ce d isp osa l into shallow dep osits w here the bases o f the pits lie only a few m etres above the w ater table. Many o f the rad ionuclides rem ain in the unsaturated zone and the others are sorbed within the saturated zone, where they m igrate h orizon tally with the ground w ater but at a fraction o f its speed.

Y et a further m easure o f su rfa ce protection is a fforded i f the aggrega te -filled pits o r r e s e r v o ir s are th em selves buried beneath the so il. This procedure is carried out both at Hanford and Oak Ridge

2 1


where the waste is piped to " c r ib s " , which may be covered trenches or buried wooden boxes with open bases.

At the National R esearch Testing Station both shallow and deep disposals have been practised fo r severa l years [8], In the shallow disposals low -leve l effluents have been discharged into a ponded ex cavation above deep dry alluvial sed im ents, w hereas the deep d is posals have been introduced through a borehole extending 50 m below the water table, in basalt, at a depth of 200 m. At Grants, New M exico, thousands o f litre s o f lo w -le v e l w astes originating from a uranium m ill are d ischarged daily into a sandstone stratum 315-450 m below the su rface . An im perm eable clay b a rr ie r , 100 m thick, ov er lie s the sandstone, preventing any translocation of wastes into the potable w ater-bearin g strata above. M ore details o f such operational p r o cedures are given in Appendix III.

The s ize o f suitable fa c ilit ie s fo r introducing waste w ater into the ground depends on the volume of waste, its chem ical com position, and the infiltration rate of the proposed form ation. The infiltration rate m ay be difficu lt to a ssess because the dynam ics o f in filtration change with tim e. During initial flow into nominal dry so il there is a high capillary pressure that provides high potential energy gradients with usually sh ort flow g e o m e tr ie s . This resu lts in a high in itial rate o f in filtration . As tim e p a sses the flow geom etr ie s , i. e. wetted volum e, b ecom e m uch la rg e r and cause low er energy gradients and a dim inishing rate of infiltration . Coupled with this, a reduction in tra n sm iss ib ility m ay be caused by the deposition o f suspended fin es, the settlem ent o f co llo id s o r the growth o f algae and bacteria l s lim es. In som e cases e fforts m ay be made to m aintain the hydraulic ch aracteristics by p r ior filtration or chlorination. A s an exam ple [8] o f d im inishing in filtration the pond at the National R eactor Testing Station had an original filtration rate of 560 litres /m 2 day through the bed of the pit. During the first 5 years ' operation , in which it accepted roughly 3 X 108 li t r e s /y e a r , the in filtra tion gradually d im inished, establish ing a mean value o f 400 litres /m 2 day for this period. But in the following 4-year period, in which the acceptance in creased to 7 X 108 lit r e s /y e a r , the rate of infiltration dim inished further to 250 l i t r e s /m 2 day.

Other experience in shallow liquid d isposal [32J has shown that pits excavated in suitable sands can accept, fo r long p eriod s , low - leve l active w aters provided that they are fre e from acids o r com - plexing agents, and have passed through appropriate settlement tanks b e fo re d isch arge . R em aining co llo id s are deposited on the bed o f

2 2


the pit and with tim e form an additional layer of ion-exchange medium that extracts a large proportion of the radiocations from subsequent infiltration. It is thus a partially compensating com plication that will eventually reduce the infiltration rate to unacceptably low levels , but during this p r o ce s s new deposits tend to retain many o f the ra d io cations on the base o f the pit.

The usual backwashing and cleansing of a sand filte r bed [5] is not norm ally poss ib le in the base o f an infiltration pit. Plugging o f the so il may also occu r through peptization, in which there is chem ical interaction between the water and so il. Peptization has in som e cases been alleviated by adding calcium salts to the water, but only at the expense of low ering the retention capacity o f the so il fo r strontium ions.

T here have been se v e ra l ca se s w here con cen trated solu tion s have been d e libera te ly o r acciden ta lly in troduced into the s o il and these have serv ed as an in d icator o f the p oss ib le resu lts that m ay be anticipated elsew here. At Chalk R iver three separate single liquid d isposa ls o f w aste solutions high in salt content, one o f which was strongly acid, w ere poured without treatment d irectly into holes dug a few m etres above ground water in shallow sands.

The m igrations after 10 years w ere accurately mapped (an example is shown in F ig. 5) and their future m ovement was predicted

Fig. 5

Cross-section showing migration and longitudinal dispersion between a disposal pit and an "area o f em ergence" in a swamp.

fo r the next century [27, 33]. H ere, although the underground flow - paths w ere fa ir ly short (600 m and 1000 m ), the sorp tion capacity of the sands coupled with the longitudinal dispersion ensures that the ultim ate re lea se o f s tron tiu m -90 into su rfa ce w aters w ill be below

2 3


current M PCW values. If, however, such disposals had been carried out in the sands at M ol or Brookhaven the retention o f stron ium -90 would have been much low er and the hazard unacceptably high.

If the waste is in jected into deep confined aquifers the resu lts are s im ila r except that the w astes are concentrated in the annulus o f so il surrounding the borehole, the screened portion of which p re sents a sm all c ro s s -s e c t io n . The infiltration speeds are th erefore corresp on d in g ly h igher and any tendency to plug is ch a ra cte r ized im m ediately by higher in jection p ressu res . It is obvious th erefore that the useful working life of such liquid ground d isposa l fa c ilit ies depends, in general, d irectly on the quality o f the discharged fluid.

Experiments have been carried out to inject waste solutions into a confined aquifer in such a way that the solution, instead of spreading out into a d iffuse p lum e, is con centrated in a lim ited volu m e. The method is a m odification of an o il-w e ll procedure in the U. S. A. w hereby waste is in jected into an exhausted o il stratum . H ere the rad ioactive solution was experim en ta lly in troduced into a cen tra l w ell and ground water was pumped sim ultaneously from four re lie f w ells disposed sym m etrica lly around it. The arrangem ent o f w ells and pumps rela tive to the strata in jected is shown in F ig. 6a. The object was to rem ove m ost of the ground water within the boundary of the peripheral w ells and replace it with the solution in a manner likely to ensure good con trol and confinem ent of the waste. In this way a continuous flow was established producing an accum ulation of sorbed, radionuclides on the so il until the zone was "exhausted"; at this stage breakthrough would be im m inent, with contam inants appearing in a r e lie f w ell (F ig . 7). A flow net [34 J, shown in F ig. 6b, ind icates the pattern o f underground flow in an id ea lized situation w here th ere is u n iform p erm eab ility and no natural ground w ater m ovem ent. Sim ilar fie ld experim ents in the C zechoslovak Socialist Republic are being extended to in ject sludges of various v isco s it ie s under the impetus o f high lo ca l p iezom etr ic gradients [35].

E xperim ental deep perm anent d isposa ls have been ca rr ied out at Oak Ridge [36] using the hydrofracturing technique, where a large hydrostatic p ressu re is applied down a borehole into a se lected h orizon. The pressure causes the stratum to fracture along its bedding plane, raising the entire overburden by a few m illim etres m easured at the surface. A cem en t-clay grout, liquefied with the aqueous radioactive waste, m ay then be in jected under the maintained p r e s sure and forced into the crev ice , where it remains to set and harden. In this way the waste m ay b e im m ob ilized in a wide lam in ar sheet

2 4


RELIEF WELL W E L L- m

Fig. 6a

Layout o f equipment for in jecting waste into a central well and simultaneously pumping water from four re lie f wells.

thin enough to d issipate any se lf-h ea t generated, and the procedure may be repeated severa l tim es in one boring by stacking the laminae at p re -se lected horizons above each other. Several waste injections have been su ccessfu lly com pleted, but the theory o f hydrofracturing has long been a controversia l subject in the petroleum industry, which is fraught with num erous problem s of rock m echanics. M ost of the lo ca l p rob lem s have now been ov ercom e at Oak R idge, w here it is be lieved that the m ethod m ay have potential fo r wide application in the d isposa l o f m oderately h igh -leve l w astes.

A lm ost all ro ck strata fra ctu re under p re ssu re s betw een 0. 23 and 0. 41 k g /cm 2 p er m etre o f depth (1. 0 - 1. 8 lb / in 2 p er ft depth), but if large volum es of liquids are to be disposed of, fracturing alone w ill not create an adequate re se rv o ir [37], Only natural porosity in a ro ck form ation prov id es the n ecessa ry r e s e r v o ir capacity . F or

2 5


Fig. 6b

Flow net for inverted 5-spot system with 2. 5°Jo o f total flow conducted between any two adjacent solid streamlines.

low-pressure operation the greatest single hazard to successful disposal is the accidental reduction of permeability in the annulus of the formation surrounding the bore. It may be caused by suspended solids in the disposal liquid or by chemical incompatibility between the injected waste and the rock or the naturally resident fluids. Suspended solids do not usually enter a rock formation but cling to the face of the injection area,creating an impermeable blanket. This may be dispersed by acidizing or hydraulic fracturing, but fractur-

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INJECTIONWELL

RELIEFWELL

FEET

„__________________HUNDREDS OE J iE E I___________________________ _ 1

Fig. 7

Radial m igration o f radionuclides from in jection w ell towards re lie f wells.D ifferential sorptive characteristics causes separation between nuclides.

ing works only if the permeability has been so reduced that it will sustain the necessary pressures.

Provided that the waste solution and the formation are compatible, other desirable criteria are that the reservoir be preferably made of sandstone, water-saturated, and of sufficient areal extent that the injected fluids may flow at low velocities from the bore to displace and compress the formation fluid. It is essential that the over- lying strata permit the well-casing to be cemented completely throughout its length and that they have low vertical permeability so that there is no migration of waste solution upwards in the surrounding annulus nor intercommunication with fluids in superimposed formations.

The examination of salt formations has been promoted because of their abundance and broad distribution [38], because they are inherently dry and because rock salt has unusually attractive properties. It has a compressive strength similar to that of concrete blit unlike concrete and most other common rocks it flows plastically when subjected to high stress concentrations and in so doing relieves

2 7


the stress. Owing to this plastic behaviour it is essentially impermeable since any cracks that may develop tend to be self-healing. Another attractive feature of rock salt is that its thermal conductivity is higher than that of most rocks, so it is better suited to dissipate heat generated by radioactive waste. Because of its chemical and physical homogeneity it is dissimilar from all other soils although the bedded deposits frequently contain interbeds of shale with a higher permeability; it is nevertheless considered essentially as a storage medium.

In investigating its suitability for high-level liquid storage, experiments were conducted in which solutions, chemically similar to the waste, were introduced into 16 m3 pits in the floor of a salt cavern (Fig. 8a) [39]. Electrical heating was installed to simulate that of the actual waste, causing the solution temperature to increase over a period of one month until equilibrium was achieved (50°C) (Fig. 8b). From these results the in-situ thermal conductivity and diffusibility were measured and found to agree, within 10-20%, with laboratory results on single crystals.

A complication with intensely radioactive aqueous solutions arises from their instability under high radiation. Aqueous solutions are radiolytically decomposed, producing a hazardous mixture of hydrogen and oxygen, and pressures of several atmospheres may develop. It is probably not feasible to expect salt formations to contain such pressures for long periods. Laboratory irradiation experiments have shown that radiolytic stability may be assured for doses below 109 rad if the waste solution is introduced into granules of crushed salt and allowed to permeate the interstices without forming a free surface above. For doses approaching 109 rad a liquid phase forms over the salt accompanied by a build-up of pressure. The formation of vapour and gases may also cause a hazard to storage. A serious problem is caused by the interaction between the nitrates of the hot radioactive solution and the chloride in the salt, which together form a highly corrosive condition in the vapour space above. Acid waste produces NOC1, C02 and oxides of nitrogen at temperatures above 50°C but this effect may be avoided if the acidity is re duced to below 4M and the temperature maintained below 60°C.

In liquid storage, isolation is best assured by introducing the solution into specially prepared chambers in the salt, but at present numerous difficulties remain. In addition to the corrosion and off- gas production, the self-heating problem is assuming greater im portance because it has increased by at least an order of magnitude

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Fig. 8a

Experimental disposals, below pyramid shaped covers, in Carey Salt Mine, Hutchison, Kansas, USA.

Average temperature rise in waste.

since the Oak Ridge experiments, as a result of higher fuel burn-up. It is believed that the combination of self-heating and the density difference between the solution and the salt could result in an upward

2 9


migration of a "waste bubble". In view of the economics of transporting waste, the operation of the mine, and the extensive research on the disposal of solidified high-level wastes, it seems likely that solid rather than liquid wastes will feature in any development of salt deposits.

Experiments have been conducted with high-level solid wastes by placing them in suitably sized holes in the floor of a salt cavern and backfilling with crushed salt for shielding. The problems are similar to those for the liquid wastes except that the temperature rise may be much higher and there is no production of off-gases. The limitation is set by the maximum temperature that the salt can withstand, which varies from formation to formation. Contained in the salt there may be numerous small inclusions of water that explode violently when the temperature reaches a certain value (typically 250-350°C). Steam is released and the shattered debris forms a medium of lower thermal conductivity than the homogeneous parent material. The compressive strength of salt is unimpaired by doses of radiation below 108 but above that value there is some reduction. For a 2-year cooled calcined waste this dose may accumulate within a radius of 30 cm but, since the floor of the cavity does not support the load of the overburden and the support pillars are beyond the local radiation fields, the structural stability of the cavern is reasonably assured.

(2) Solid disposals

In the routine monitoring of solid radioactive waste, materials are usually segregated according to their activity levels: those that require shielding, those that are contaminated with alpha-emitting nuclides, and those that require no special shielding. It is in this last category that large volumes of contaminated trash and garbage are being produced at an ever-increasing number of sites around the world; its disposal is a problem for most nuclear establishments [40, 41, 42]. The varied nature of this material, which may range from paper to glassware, rubber gloves, wooden boxes or metal pipe, may require additional handling if, for instance, physical segregation is needed to permit baling or incineration. For this reason shallow land burial is frequently adopted because of its simplicity; it may sometimes be wasteful in consuming large areas of land but it does allow the piecemeal direct disposal of miscellaneous wastes. The simplest disposal procedure is similar to that used in municipal

3 0


garbage dumps where the waste is tipped over the edge^of a short slope to accumulate and assume its natural angle of repose. With continued tipping the sloping face of garbage advances and the exposed crest is covered with soil and consolidated to form an access for vehicles bringing more waste. Although the simplicity and cheapness of this method is attractive, the disadvantages are fairly obvious. There is the danger of containers or bagged waste rupturing when they are tipped; this would spread contamination down the face of the slope, possibly overspilling beyond the base. The resulting exposure to wind and rain is particularly undesirable if there is the possibility of contaminated surface water collecting at the base of the slope and thus spreading activity in the resulting run-off.

Shallow ground disposal is more safely carried out by tipping the waste into trenches excavated in soil known to be free from waterlogging and nominally dry under typical conditions (Fig. 9). The advantage of this over the "tip and fill method" is that contaminated rain-water is automatically directed underground and the spread of surface contamination is minimal. The packaging of this m iscellaneous material is generally only sufficient to prevent the spread of particulate contamination while it is being transported from the laboratory or workshop to the disposal ground. Paper or plastic bags and sheeting are usually used so that after tipping and consolidation the waste is only nominally protected.

One of the recurring problems is to assess satisfactorily the magnitude of this type of disposal without emptying the contents of each bag and subjecting articles to laboratory scrutiny. Owing to the nature of the materials and the variety of radioisotopes with which they are contaminated, it is impossible, at present, to measure accurately the number of curies and the specific radionuclides present in a disposal. In this respect, precise estimation of the magnitude of solid disposals is much more difficult than for liquid disposals. A common method of assessing the activity in a packaged disposal is to measure the radiation field with a portable counter held at a standard distance from the bag or in contact with it. Each establishment has its own interpretation of these results, based on the typical spectrum of contaminants present in their waste. In this way their radiation readings are converted empirically to "nominal curies" of activity in the disposals.

When low-level waste is buried without containment, the principle is accepted implicitly that some contamination will be leached from the solids and carried away by the percolation from rainfall.

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Fig. 9

Trench excavated in sand for typical lo w -lev e l solid waste.

The disposal is thus a planned continuous release of radionuclides into the environment at a rate dependent upon the infiltration and the leachability of the solid material. If the climate is wet and consistent leaching may reasonably be anticipated, quite obviously the sorption by the soils determines whether the migration will be held within acceptable limits or whether the site will be unsuitable. The acceptability of the potential exposure therefore depends on the ion- exchange characteristics of the soil for the "biologically hazardous radiocations", the rate of deposition of contaminated waste, and the resultant rate of migration. In particular, one would expect the migration of strontium-90 to be in the order of metres per year rather than tens of metres per year. At Savannah River it has been demonstrated [43] that the leaching potential may be reduced by covering the local area, containing the buried waste, with an impermeable membrane or "umbrella". In a full-scale experiment (Fig. 10) the

3 2


Test area 15 m X 15 m surrounded by 8 m deep trench and sprayed with bentonite "um brella" on top and sides.

ground surface of a selected area was sprayed with clay to form the membrane. This was covered with more soil to arrest drying, shrinkage and cracking, and infiltration measurements beneath the area were compared with those from the surroundings. Results showed a pronounced increase in run-off and reduction in infiltration that would reduce further the likelihood of any migration.

Solid wastes of higher activity must be isolated from natural water and contained in structures [44] that may be expected to retain their integrity for many decades. If they are made from concrete, which has so far proved to be one of the more suitable and economical materials, then adequate precautions should be taken to ensure that they will withstand erosion or chemical decomposition from natural waters. In addition, they must be made watertight and strong enough to resist stresses caused by settlement of the foundation. These


types of underground containers are also suitable for storing the small quantities of radioactive liquids, such as solvents, that defy normal liquid treatment processes and must be stored indefinitely in sealed bottles. When containment is practised, it is important that water is excluded from the container while it is being filled so that no erosion may start from the inside. To ensure that there are no voids in the waste, concrete or sand may be poured in before the container is capped and.sealed. Thus settlement of the waste itself is forestalled, residual water is incorporated or absorbed, and differential stresses are avoided in the resistance against outside earth pressures.

Between the extremes of complete containment and calculated slow release from low-level solid wastes, there are intermediate procedures that are often suitable. For instance, solid equipment may be subjected to a decontamination procedure to remove most of the radioactivity, and the residual surface contaminants may then be sealed with paint or lacquer. Preliminary pre-treatment of this type reduces leaching from objects buried without containment by denying access of ground water to the contaminated surfaces. The principle may be extended to sludges where individual particles are coated with an insoluble film such as asphalt [15, 45]. Here the process, which is described in greater detail in Appendix I, extends the insolubilization beyond sludges and ash residues to concentrates from evaporation that are corrosive and have high salt contents. Particular emphasis is laid on the ability of the coating material to withstand degradation when subjected to prolonged irradiation from the concentrates. The final disposal of this type of material may be directly to the soil, but with asphalt it is convenient to allow the mixture of waste and asphalt to solidify in drums at the mixing site and later transport the drum and contents for disposal.

Pre-treatment methods for concentrated waste solutions of high solids content are more appropriately dealt with under the separate subject of solidifying high-level wastes. These methods include calcination and conversion into ceramics [46] and glasses, but at present none of these solidified products is routinely disposed of into the ground, although experimental burials have indicated that the low leachability expected from laboratory measurements is reproducible in the soil [47]. These products appear to release radiocontaminants under conditions of constant saturation, at acceptably low rates, corresponding to a surface erosion of roughly 10"8 g/cm2 day.

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V. EVALUATION OF SITES, AND METHODS OF GROUND DISPOSAL

(1) Potential exposures

The use of the ground for disposal operations is based on two fundamental properties. First, the minerals from which the subsoils are composed generally have a marked sorption capacity for most uranium fission products as well as for many other radioisotopes. Secondly, the mass of soil covering a disposal provides good radiation shielding and permanent physical protection to people above ground.

Ground disposal of radioactive waste is a method by which the the waste may be disposed in such a way as to release radioactivity slowly into the environment. If the rate of release from a waste is expected to be unacceptably high, the waste is put into a container. This is merely an additional safeguard to postpone the eventual re lease of contaminants until the container eventually fails through corrosion or erosion, when it is anticipated that the radionuclides in the stored material will have decayed to sufficiently low levels to permit their slow release into the soil. In such a case the action may be considered to have been permanent disposal; but if the activity remains high at the end of the estimated useful life of the container, the action is but temporary storage.

Where large volumes of slightly radioactive effluents are produced, they may possibly be discharged directly into a river; the turbulent diffusion and dilution would be relied on to reduce the contaminant concentrations to acceptable levels downstream. It may, however, be possible to discharge the effluent into permeable soil nearby, where the water will infiltrate underground before emerging into the river. Such practices have a special advantage if occasionally high transient releases should occur, perhaps through errors in processing or control. When the hazard level is inherently low or of limited significance, the soil in the line of underground flow will serve to disperse and delay such transients. The seepage concentrations issuing from soil to river would, in consequence, be more uniform, even with those radionuclides that experience little delay due to exchange.

In the normal case of ground disposal by shallow land burial the disposal site is remote from the lake, reservoir or river draining the catchment and the intervening path may be tortuous, lying partly

3 5


below ground and partly above. In the underground section a long enforced residence with restricted flow rates and sorption reactions provides overwhelmingly the delay needed before release. Once the contaminants emerge in surface water they are subjected to large dilution and comparatively rapid transport, but reconcentration of specific radionuclides may be expected in the aquatic biota and on minerals in suspension and on bed deposits [48], At the end of this flow-path the residual concentration of contaminants has to be below the maximum permissible concentrations in water and foodstuffs for consumption by the general public.

The importance of the type of sub-soil, the proximity to streams, depth to water-table, permeability, etc. have already been emphasized, but simple procedures on the site can do much to restrict an initial spread of contamination by wind, rain or surface water. In particular, elementary precautions to stop surface water flowing into a trench or container, or temporary roofing to prevent rain or snow from coming into contact with the waste, can greatly reduce any hazard. It is generally true to say that the possibility of spreading contamination is greatest when the disposal facility is being filled; if it is to be capped or sealed there is normally an immediate reduction in the potential hazard.

An important requirement in the choice of a site for ground disposals [49] and in particular for shallow land burial is that the area be reserved exclusively thereafter for waste disposal. It must be adequately fenced, must have no alternative use, and its presence may well inhibit the development of surrounding areas. The finality of the choice is so absolute that complete confidence is essential that tlie area would be required for no other conceivable purpose and would be solely a ''restricted area’1. It is conceivable that mineral r e sources themselves could become contaminated by radioactivity but with shallow disposal operations the concentration levels would be unlikely to inhibit mineral recovery if it was commercially economical. This situation is more likely to arise where liquid wastes are injected into deep formations and it indicates that deep disposal sites should be chosen only after a careful investigation of neighbouring strata for potentially exploitable minerals.

The evaluation of a proposed site must include a prediction of the probable stability of the existing ground water equilibrium. One may assume that there exists in the soil an equilibrium [50] in the reaction between the radioisotopes in the liquid and in the solid phase. The equilibrium is influenced by the mineralogical, chemical and

3 6


physical environment, and equilibrium conditions may be attained more or less quickly. Accordingly, one must take into account that any later changes in the characteristics of the contaminated aquifer, caused for instance by the introduction of a new waste of different composition, the building of a dam, the use of a large new production well or the tapping of a spring, can introduce a new condition that may increase or decrease rate of movement and change rate and direction of underground flow. The evaluation should also include the probability of natural occurrences, -such as earthquakes or changes in erosion level with time, and how these may affect the behaviour of the radioisotopes in the disposal area. Establishments intending to ma:e planned disposals should consider these questions in consultation with earth scientists when examining a prospective disposal area.

(2) Site evaluation

To summarize the significant natural phenomena affecting ground disposal, Table I has been assembled. It embodies the factors of climate, hydrology, geology and geography as well as the condition of the waste, and indicates which characteristics are, in general, favourable and which are unfavourable. Some of these factors (e. g. pH of the waste) may appear in either column according to the characteristics of the environment. However, the list is intended to represent the more common cases.

Obviously the factors listed must be considered individually since certain combinations of so-called favourable features are inconsistent with naturally occurring situations. They are therefore an indication only of the qualities generally found to be favourable, but many exceptions are possible. For instance, the slow ground water movement noted as a favourable hydrological feature may be due either to a low permeability or a low hydraulic gradient. If l i quid disposal were anticipated, conditions of low permeability would be quite unsuitable, since the effluent would be unable to percolate fast enough into the soil. If the low hydraulic gradient were present in a highly transmissive medium - a normal condition - then the boundary conditions of the aquifer would control the extent of any disturbance to the equilibrium through the introduction of large volumes of effluent. If the aquifer were shallow or of limited areal extent, rapid sub-surface flow could be anticipated.

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TABLE I

ASSESSMENT OF GROUND DISPOSAL FACTORS

FactorsFavourable

characteristicsUnfavourablecharacteristics

Physical state o f waste Solids Liquids; sludges.

C hem ical com position o f waste A lkaline; neutral; low salt.

A c id ; high salt.

R adiochem ical com position o f waste

Presence o f short-lived nuclides.

Presence o f (a) long -lived nuclides, (b) anionic species.

Geographical situation Low precip itation ; rem ote from open water sources; rem ote from population centres.

High precip itation ; close to rivers, lakes; close to population centres.

Geology (geochemistry) Uniform unconsolidated m aterials; high sorptive s o il ; low soluble ca lcium content.

Fissured rocks; inert materials; high content o f ca lcium or other soluble salts.

Hydrology Deep water ta b le ; slow -m oving ground water; slow ion m igration ; long path to point o f discharge.

Shallow water ta b le ; fast ground water; fast ion m igration; short underground flow path.

Another example may be where the-"highly sorptive materials", classified as "favourable geological" features, are present in the form of clay. This material has the other favourable characteristic of low flow rate, in fact so low that it is effectively impermeable. In such a case the massive ion exchange potential of the deposits is not used since they remain largely inaccessible to the bulk of radioactive liquid. Where solid radioactive waste is deposited in open trenches excavated in clay, water slowly accumulates, saturates the

3 8


waste and, unless protected from the ingress of surface water, eventually overflows, spreading contamination above ground.

The preferred conditions lie somewhere between the extremes, i. e. where the soil is coarse enough and of low enough clay content to permit easy percolation of water, yet of such texture that an adequate admixture of clay mineral is available for exchange with the radiocations.

(3) Choice of shallow or deep disposal

The choice between shallow or deep disposal cannot be generalized and each proposal has to be considered on its merits. It is perhaps generally acknowledged that the disposal of radioactive waste into deep formations is, in principle, preferable to shallow disposal, particularly for liquids. If the solutions can be injected into formations well below any potential aquifers, their permanent isolation is reasonably assured, and as a result it is probable that wastes of far higher activity levels than those judged prudent for shallow disposal might be introduced. But the procedure and the investment involved must be examined against the comparative potential hazards resulting from simpler procedures.

The shallow disposal of liquids into sites where there is a great depth of dry soil is another method which may ultimately introduce waste into deep formations, but unlike the example given in the previous paragraph, the solutions are disposed above rather than below the aquifer. Such terrain, where the water table is at great depth, often occurs in arid regions where recharge is negligible. The uncertainties about the vertical migration rates and the attendant difficulties in monitoring and measurement make difficult the accurate prediction of migration rates and the probable potential hazard to neighbouring water supplies.

Although deep disposals have much to commend them, shallow ground disposal may have attractions even though the local water supplies and the environment generally seem susceptible to immediate hazard. The choice of a site and the depth of disposal may well be governed by the ability to collect adequate geological and hydrological data. If the sub-soil is known or shown to be regular and homogeneous a few exploratory borings and sampling may be adequate. But usually such a picture is an over-simplification of heterogeneous deposits whose physical characteristics vary much more than their chemical characteristics. In these circumstances the investigator must be

3 9


prepared to estimate the importance of local heterogeneity and to establish how representative are the results from any selected boring.

The measurement of most of the necessary fundamental data can be carried, out for shallow formations where migration will be limited to depths probably less than 25 m. Under these conditions, migrations can be predicted and,if necessary, experiments conducted with incremental releases of radioactive solutions to check the predictions quantitatively. The means of following the track of migration lie in the numerous shallow sampling techniques simplified, if necessary, by prior introduction of non-radioactive tracers into the groundwater.

The contrasting philosophies of shallow and deep disposals must be clearly understood. Shallow disposal methods envisage a slow continuous release into the environment that can be monitored re gularly. The delay caused by migration underground provides adequate warning if hazardous quantities of radio-contaminants have been erroneously introduced into the soil. The condition can probably be corrected by pumping the contaminated water to the surface and treating it by other methods [51 J. But the normal condition envisaged is that the rate of seepage and the corresponding concentrations of pollutants do not cause the maximum permissible concentrations to be exceeded in the immediate body of water that they enter. This condition would be expected to last throughout several decades, during which the decayed contaminants would be eventually released to surface waters.

Deep disposal methods, by contrast, do not envisage any re lease when they are introduced into a permanent repository below known potable ground water bodies. The installation of bores suitable for injection would be accompanied by an extensive examination of all formations intersected, particularly for their permeability. However, the need for exact quantitative data decreases with depth and increasing water salinity, especially if the hydraulic head on the deeper aquifer is lower than in the shallow potable aquifer. Even when large volumes of low-level waste are introduced into a porous formation, migration from this reservoir to higher strata is thus unlikely although the degree of confidence depends on the activity level of the disposed solution. If there is a horizontal migration there is at present no simple way of tracking it, but the resultant hazard is probably insignificant. The method probably lends itself most economically to the adaptation of formations previously tapped for extracting other naturally occurring fluids.

4 0


Where the stratum to be used for deep disposal lies above an aquifer, some migration is likely, but measurement and control over the migration is not comparable with the control possible for shallow disposals.

(4) The small-scale disposal of solid wastes

All the descriptive details have so far been related to large- scale disposal operations, from which most of the current experience has been derived. Yet many cases may arise where a small producer of radioactive waste would question whether it is necessary to embark on a large pre-operational survey or whether a curtailed investigation might be sufficient and more commensurate with the minimal hazard involved. With solid waste the latter premise may be valid, and the points that have to be considered are:(a) The choice of site;(b) The environmental condition most likely to produce migration;(c) The limitation on the type of waste and how it should be contained.

(a) The choice of site

If a site is to be evaluated completely there is no alternative to a thorough examination of the geology, ground water hydrology and soil chemistry. But since experience at many sites has shown that the contamination on solid waste buried in the aerated zone tends to remain stationary unless leached by water, it may be adequate to bury the waste in unsaturated soil, all necessary precautions being taken to avoid rainfall infiltrating the waste. The question remains: howmay such a site be evaluated?

A system has been developed [52] to assist sanitary engineers who are frequently faced with similar problems, such as choosing a site for a septic tank system with assurance that it will not pollute nearby wells or a water supply. The geological and hydrological complexities are evident, but with limited data at hand preliminary evaluation must be made in planning and sometimes the same data are used even for construction.

The Point-Count System provides a standard for comparing the merits of a number of sites by evaluating each with reference to five environmental factors, namely:

Depth to water tableSorption

4 1


Permeability Water table gradient Distance to point of use.These are composed in a point-rating chart, shown in Fig. 11,

in which the conditions at a site are checked against each of the factors, and points are awarded in each category. The larger the total

WATER TABLE

r----- --------- 1----- 1--------1——i------------i— ---------- r5 20 30 AO 60 100 200 300

DISTANCE BELOW BASE OF DISPOSAL UNIT-m

LIMESTONE | COARSE CLEAN SMALL AMOUNTS EQUAL AMOUNTS

FRACTURED ROCK SAND OF CLAY IN SAND OF CLAY AND SAND

I I IFINE SAND FRACTURED ROCK COARSE SAND

GRADIENT

3 2 1 0|------------- 1------------- 1------------------- ,------------------------------,----------------- 10 2 5 10 30 60

PERCENTAGE

DISTANCE

0 1 2 3 4 5 10 13

H --------1--------- h---------1---H— i ---------- 1----- '— i---- *— i----------- t—1----- 13 10 30 50 75 100 200 500 m 1km 2 16km

Fig. 11

Rating chart for radioactive waste disposal sites.On all scales the point values are indicated by the upper scale.

The chart is not applicable i f more than 50% o f the distance is through lim estone or fractured rock.

number of points accumulated by a site, the better it is for disposal purposes.

The point-count system does not purport to be anything more than a rough approximation and has been developed empirically from widespread experience in cases of well and water supply pollution. The chart shown for radioactive waste is slightly modified from that applied for other pollutants and should be used preferably where the soil is granular. It should be noted that the points allotted for certain factors are based on highly simplified concepts. For instance, the

4 2


chart for sorption takes into account none of the selectivity of certain clay minerals for specific radionuclides; all it does is to recognize that soils containing clay are better than those with no clay. Similarly the points allotted for permeability are greatest (only 3) for a sandy-clay soil, decreasing inversely with permeability and decreasing also towards pure clay where there is practically no flow to promote sorption. Little significance is therefore attached to the magnitude of permeability in sands, which may be hundreds of times greater than for clays. Note that those factors in the chart that do most to enhance the value of a site are: the distance down to the water table, the sorption of the soil, and the distance to the point of use. When this system is used it should be realized that in such an imprecise evaluation it is futile to obtain an exact value for only some ' of these factors if the precision is to be lost in the crude estimates of the remaining factors.

As an example of its use, consider the hypothetical case of a site where the base of a disposal trench is anticipated to be 3 m above the highest estimated ground water level (i point). The soil is a fine silty sand (S = 2, P = 2, say) and the slope on the water table is roughly 5% (1) towards a perennial stream 150 m away (3). Total points = 8j. This result by itself is not meaningful, but when a series of prospective sites are compared those potentially the more suitable may be selected for later judgement in the light of more detailed investigation.

(b) Probable conditions likely to produce migration

If the soil has a large clay content and is of low permeability the greatest hazard will probably develop from surface water entering the trench and overflowing on the surface. In such material it is frequently possible to excavate suitable trenches with almost vertical sidewalls without risk of cave-in; the soil surrounding the disposals is thus relatively undisturbed. With such clearly defined boundaries to a disposal trench it is simple to carry out suitable precautions such as thoroughly compacting the filling, raising the filling profile relative to the surroundings to divert surface run-off, and capping the trench. If, on the other hand, the soil is granular a hazard may develop from migration and the base of the trench must always be above the saturated zone even at times of flood. Low-lying areas should preferably be avoided and the trenches should be sited where there is gentle relief to the land profile. In this way run-off is en-

4 3


couraged, although the trenches should still be capped with a flexible impervious material such as clay or asphalt capable of assuming the modified surface profile after the waste has settled. It is not usual to go to the added expense of providing a rigid concrete roofing for low-level disposal trenches.

The packaging is more important for the handling of waste than for its disposal, where minimal covering by waxed paper or plastic has sufficed at numerous disposal sites, and has resulted in negligible migration hazards. However, the more substantial packings such as drums and wooden boxes, sometimes used, have normally reduced the voids volume by permitting the waste to be placed selectively and so the best use is made of the space available.

(c) The type of waste

The other feature that should be examined is the radionuclide content in the waste and the length of its half-life. Here it is of interest to note the proposals in the United Kingdom, where ground disposal is practised only on a limited scale. It is suggested [53] that small quantities of waste should be permitted to be discharged into selected municipal refuse dumps. These dumps would be chosen where there was minimum risk to the contamination of water supplies. The amounts proposed are, however, very small, being less than 100 /uCi per package for isotopes of half-life greater than one year and 1 mCi per package if the half-life is less than one year. In carrying out disposals the general limitations may be modified or expanded to suit the kind of waste envisaged. An example of a typical modification is that the alpha-activity of any package should not exceed 1 mCi and that the main activity on the surface of unshielded material should not exceed 0. 1 nCi per cm2 of alpha activity or0. 75 R/h gamma radiation.

Finally, it should be appreciated that waste normally destined for land burial includes not only low-level packages of juCi or mCi contamination, but large volumes of waste that are possibly radioactive but generally not. Because of the expense of monitoring and segregation it is disposed as low-level waste for convenience and the result is that the mean level of contamination is very much lower than the limits given may suggest.

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VI. STANDARDS AND CONTROL TECHNIQUES

(1) Standards

Any standards proposed for ground disposal are based mainly on the primary return route to man by migration through sub-surface waters. The standards should be such that adequate health protection is assured both to operating staff and to neighbouring communities and that all avenues of reconcentration in biological systems are recognized. The standards applied to effluent seepage concentrations should, in addition to meeting human requirements, be acceptable for the subsequent re-use of the water by industry. Commercial undertakings may take water from downstream points in the hydrologic basin; thus the residual contaminants should be within acceptable concentrations at these water intakes. Although the limits are not normally exacting in purity requirements, certain industries, for instance the photographic industry, are particularly sensitive to changes in the background concentrations of radionuclides in their water supplies.

Most problems in ground disposal are basically of a local nature. Ideally it would be useful to set contamination standards for the soil, taking into account the sorption phenomena relative to time and concentration. But because of the complex structure and inhomogeneity of most soils it is more realistic to work from the contamination levels of the groundwater that are permissible at the point of emergence. Such levels can be derived from the Agency Basic Safety Standards [54] or the ICRP recommendations [55], taking into account such matters as the use of the water, population densities, mineral resources, reconcentration in biological systems, and permissible body burden.

General guidance in evaluation techniques for similar problems is given in detail in the International Atomic Energy Agency1 s Reports on "Disposal of Radioactive Wastes into Fresh Water" [56] and "Radioactive Waste Disposal into the Sea" [57].

(2) Control

The total volume of disposals to be permitted at a given site needs to be estimated. If the area is to be used solely for low-level solid waste, given reasonable soil and ground water conditions, the limitation will probably be set by the size of the area as for any normal refuse dump. For liquid disposals this may also be true if se

4 5


veral disposal basins are used and insignificant migration develops before they are ultimately abandoned because of diminished infiltration. But it is more likely that the limit will be set by the maximum attenuation afforded by the soil before seepage into surface waters reaches the maximum permissible concentrations. The complexities and uncertainty involved in predicting the detailed behaviour of waste from laboratory experimentation encourage a more empirical approach to determine the working limits. Field experiments may be made with tracers in the ground water and when flow rates and directions are estimated further measurements may be made by re leasing incremental amounts of typical waste and studying intensively their subsequent behaviour. Such investigations would indicate which radionuclides were the more mobile and from their biological importance the limits would be established. This empirical approach would be extended to predict the flow and dispersion experienced by these pollutants and the limits of the disposals would be set accordingly. Such predictions may be possible only with liquid disposals where their magnitude is known with some accuracy.

It is usual for an ensuing operational practice to be ultraconservative to begin with, but as experience grows and favourable results from environmental surveillance establish confidence in the safety of such disposals, the limitations may be relaxed accordingly. All long-term predictions must naturally be tempered by the reservation that they are valid only if the ground water pattern or regime remains unchanged. Such predictions cover short periods in geological time so it may be that the greatest possibility of change will come from man-made developments that may alter the projected paths of radioactive migration.

The control and surveillance of ground disposal operations re quires two forms of regulation: (i) Control before discharge;(ii) Surveillance and monitoring after disposal. Before the waste is released to the soil it must be monitored or analysed to assure conformity with the accepted disposal standards. This is of outstanding importance in all liquid disposals. The examination may be carried out by a continuous monitoring device automatically scanning the flow, or it may involve sampling and analysis of batches of waste before discharge. Special techniques are sometimes re quired to collect accurately representative samples from a batch, particularly if the waste contains two or more phases, e. g. suspended solids, precipitates, or globules of immiscible organic solvent. It is quite common for most of the radioactivity to be concentrated in the dispersed phase; this requires great care in control sampling.

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An important feature in control of ground disposal operations is an efficient system to segregate wastes. The segregation of solutions containing acids, detergents or complexing agents fromlarge- volume low-level effluent can contribute significantly to the efficient ground disposal of liquids by ensuring that the sorption potential of the soil is fully mobilized. In solid disposals also, efficient segregation can contribute not only to the safety but also to the economy of the operation. Permanent storage bunkers are expensive and it is essential that the various types of waste are directed to their appropriate facility. These may vary from heat-dissipating structures or concrete containers, to open trenches, and quite obviously space is at a premium in the more elaborate facility.

Efficient supervision before and during disposal is far more effective in controlling the spread of contamination than any later control, which is concerned almost entirely with environmental monitoring

(3) Moni toring

All ground disposal operations, whether liquid or solid, have to be carried out under careful surveillance, and the precautions taken before disposal - for example the pre-treatment of liquids or solids, packaging, segregation - have to be followed by appropriate precautionary measures after disposal. If underground migration is anticipated the monitoring of such movement is best achieved by the sampling and examination of ground water. Trace quantities of radioactivity, except of tritium, may be concentrated if necessary by evaporation before assay or identification. The assay of soil samples may be more troublesome; gamma-spectrometry may be used for suitable nuclides but for pure beta-emitters the contaminants must be leached for analysis. However, if they are previously known and only the count is required, soil samples may be counted directly in an end-window counter provided that the sample is no greater than a single layer of grains spread over the counting tray. In this way it is possible to counteract much of the attenuation due to self-absorption by the soil, and reproducible results may be obtained for comparison against a standard previously analysed.

For the monitoring to be most effective the sampling tubes or wells must be positioned to intercept and overlap the anticipated path of migration, and unless this area is indicated by radioactivity in the foliage of vegetation or in nearby surface waters, the position of this will have to be determined from an examination of the water

4 7


table profile and probably the underlying soil. Numerous points may have to be examined and it is often convenient to use a sampler (Fig. 12) that is easy to drive and withdraw, precise in the horizon from which it samples, and re-usable from site to site. In Fig. 13a and b such a sampler is shown suitable for attachment to drill rods for driving directly in the soil. Once the path has been found, monitoring may be conducted either from continued water sampling or from direct radiation measurement by sinking dry aluminium tubes in the soil into which suitable counters may be lowered. Such arrangements cannot be recommended if beta-emitters only (e. g. strontium-90) are present but for gamma-emitting nuclides the contaminated horizons may be identified with precision. This method is particularly satisfactory for monitoring the aerated zone, where normal water sampling methods are generally inapplicable. Soil sampling is of course possible in this zone but it is a non-repetitive procedure; each act of sampling disturbs the soil and the method is therefore little used when a continuous monitoring record is needed.

Where liquid disposal is practised, routine ground water monitoring, tracer tests, and occasional soil sampling would be normal to keep a check on the progress of migration. It frequently occurs that tritium is present in liquid disposals, either from the activation of deuterium or as a product from the bombardment of lithium, which is a corrosion inhibitor sometimes used in reactor cooling systems. Tritium can therefore be expected from the'se sources in the effluent from certain reactor cooling loops or in waste streams from fuel processing. Whereas the monitoring of migration is normally concentrated on the biologically hazardous nuclides, tritium, owing to its low hazard potential, has sometimes been overlooked. Yet the occurrence of tritium in waste effluents provides an ideal tracer for monitoring purposes and an early indicator of the path to be followed by the slower moving radiocations [58, 59], There is thus a good case for monitoring for tritium at any site where liquid effluents are generated and routinely discharged into the soil. Often the tritium concentrations in the samples are high enough for them to be assayed directly in a liquid scintillation counter. When this is not so, the tritium concentration has to be increased by electrolytic methods if the same type of counter is to be used.

In solid waste disposals the problems are simplified since the water infiltrating from recharge is the only carrier of leached contaminants and sampling devices may be installed beneath any struc-

4 8


Water sampling in granular soil.Samples extracted by suction through plastic tube into evacuated flask.


‘ 7

Fig. 13a

View, of dismantled water sampler.

ture to lead what little moisture there is to one collecting basin.From here it may be withdrawn, if it is within the range of suction lift, by pumping through plastic or metal tubing extending to the surface. The advantage of such devices is the ease with which the source of contamination may be identified with a particular installation. Where low-level waste is buried in trenches, reliance again is placed on ground water sampling. Samples may be withdrawn from small- diameter sampling wells sited close to the trenches in undisturbed soil or from a sampling point positioned in a sump pre-form ed in the base of a trench.

A broader surveillance of the environment by monitoring water, vegetation and natural livestock is an essential check on the efficacy of the principal monitoring devices and establishes a record of background radioactivity from natural effects and weapons fall-out. The routine collection and monitoring of water samples from any nearby streams during base flow periods is a particularly good check on the purity of the ground water that continuously replenishes the stream. Seasonal records of radioactivity in the foliage of local vegetation are a useful indicator of radiobiological uptake and concentration from subterranean sources. But where the ground water is

5 0


Fig. 13b

Cross-section through water sampler.

deep and beyond the reach of the root system, no activity would be found in the foliage. Finally, a check on the dissemination of con

5 1


taminants by rodents and other wild-life, may be carried out by sampling the species regularly and determining contamination levels in flesh, bone and selected organs.

To ensure the protection of the general public and the continued safety of a disposal area it must be permanently fenced and appropriate notices must be displayed. Entry should be restricted to those people operationally engaged in the area and standard monitoring procedures of clothing and equipment should be undertaken at the exit to stop any spread of contamination outside the fence. Also accurate records, registered both at a central office as well as at the site office, must be maintained of the whole operation. In view of the long life of the hazard, the magnitude of each disposal must be noted and its position charted accurately relative to a prominant and permanent landmark above ground. Such procedures are needed not only to safeguard the present public but also to ensure the protection of future generations who through ignorance could inadvertently re- excavate the buried waste.

(4) Accidental release

An additional situation arises from the large array of possible spills, leaks or equipment failures which may cause accidental introduction of radioactive material into the ground. Normally, such an accident would be confined to liquid wastes, although it is conceivable that airborne contaminants, settling and subsequently leached by rainfall, could constitute a low -level introduction into the so il.

It is common practice to retain high-level waste solutions in tanks placed underground to take advantage of the natural shielding offered by the covering of soil. If such a tank develops a leak, or if a leak develops in an underground pipe-line carrying a radioactive solution, the resulting release of radioactive material creates problems similar to those arising from normal liquid disposal operations except that the specific activity of the solution is probably much higher.

Steps that may be taken to prepare against accidental release are relatively few although where they occur at a permanent installation the transport hazards are avoided. Here the local soil and ground water conditions are known and the subsequent behaviour more predictable.

With underground storage tanks it is desirable to provide measurement devices to detect leaks. Internally this may be in the form

5 2


of a sensitive level gauge and externally it may be by radiation detection instruments either operated in adjacent boreholes or installed in lateral channels constructed beneath the tanks [60], In particularly hazardous situations, a double-tank arrangement is used, by which any leakage from the inner tank is collected in the outer vessel and monitored. A tank design which provides for the rapid transfer of one tank1 s contents into another may prevent a large release into the soil.

After the specialized details of containment have been decided, probably the most useful preliminary studies, to prepare against accidental leaks, are those that will indicate beforehand the probable behaviour of the radioactive liquid after it has escaped into the soil. Such details may normally be taken into account where an installation is planned, say, near a river. However, in other situations where the effects of accidental releases are less obvious, these subterranean investigations must not be overlooked.

To summarize the various procedures, one may say that the introduction of radioactive materials into the soil is generally likely to cause less pollution if the waste is in the solid than the liquid stata But owing to the nature of solid waste it is generally not possible to predict the actual quantity of soluble radionuclides that will be leached from the waste. Liquid disposals, by contrast, may be precisely evaluated except where there are accidental escapes, and their introduction will be carried out only after adequate prediction of the behaviour of the disposals has been evaluated.

VII. CONCLUSIONS

(1) Disposal

The merit of ground disposal as a means for disposal of radioactive waste must be judged according to the safety afforded by the natural environment of a site. Geological and hydrological studies alone cannot provide absolute assurance about the nature of radionuclide movement from disposals; but in most situations, if the laboratory studies on interaction between the soil and typical radioactive solutions are backed by adequate field investigation at the site,

5 3


reasonable assurance may be secured on the magnitude of any potential hazard.

The main concerns of planned large-scale ground disposals are the possible long-term effects. The half-lives of certain radionuclides normally present in waste are long when viewed against the processes that cause migration through the ground. The half-lives may be similar in time to the duration of processes that change hydrological patterns, and thus pollution could conceivably reappear in a populated environment at some distant time if geological disturbances altered the pattern of river or ground water flow.

(2) Liquid

In densely populated areas the discharge of medium or highly active liquid wastes into the ground cannot be recommended. However, low-level liquid radioactive wastes are discharged into the ground at several establishments where conditions are favourable, and waste products are retained substantially in the sub-soil within their boundaries. Where large volumes of slightly radioactive effluents are to be discharged it may often be preferable to introduce them into the ground even when it is clearly evident that they will ultimately reach a surface water body. The soil offers additional delay and therefore time for further decay before discharge to surface waters. The various processes in the ground even out high transient releases to a more uniform seepage concentration.

The capacity of the ground to accept liquid wastes safely may be realized fully if the liquids are pre-conditioned to be fully compatible with the natural minerals in the soil. If this is achieved the soil will afford the maximum attenuation to contaminants and their eventual release into surface waters will be diminished. Pre-conditioning may be by filtration, by pH adjustment or by simple segregation.

A more positive form of treatment, in which less emphasis is laid upon the sorptive properties of the soil, is to remove specific long-lived nuclides from the wastes before disposal. This greatly reduces the time needed for the radioactivity to diminish sufficiently before the seepage may enter surface drainage waters. One pre- treatment method under development uses beds of highly selective sorbantsfor strontium-90, caesium-137, radium-226 and plutonium-239.

Another treatment method is to convert the solutions into solids of low leachability such as glass or ceramics or alternatively to concentrate the solutions and enclose the concentrates or sludges in an

5 4


insoluble material such as bitumen. The resultant solids may then be stored or disposed of in the less exacting environment required for solids.

(3) Solid

Solid radioactive wastes buried in the ground normally present a much lower potential hazard than liquid radioactive wastes, so the restrictions on disposal procedures may be less exacting. The prior inspection and monitoring of material destined for disposal ensures that the more highly radioactive waste is segregated and disposed into containers appropriate in size and shape for the level of radioactivity. The large bulk of low-level or potentially contaminated material discharged into the aerated zone in the soil enters an environment where nuclides are anticipated to remain stationary unless leached by water. In general, the minimal packaging afforded to low- level solid waste makes it the most susceptible to leaching, but the ensuing contamination is likely to be low. For waste of higher activity it is generally arranged that the higher the potential hazard of the waste, the more protective is its container. Thus the container for a potentially hazardous disposal will have a built-in monitoring device to detect the first signs of leaching and provide warning of the need for repair or replacement. It is unlikely that hazardous quantities could be inadvertently released but if the situation did arise the offending material could be removed and handled separately.

The economics of a ground disposal scheme will certainly take into account the initial capital investment commensurate with the predicted activity levels in the water supplies of population centres and their development, but the cost of maintaining the site and of monitoring and surveillance could well be ultimately more expensive.

When the choice of disposal site depends on the type of soil and the hydrodynamics of the ground water system, great responsibility is thrown on the correct interpretation of sub-surface investigations. Owing to the present shortcomings in measurement or sampling techniques in certain types of ground the choice of site may well be l i mited to those from which the maximum information can currently be extracted and so provide the basis for the most accurate prediction.

(4) Storage

Where a particular plant site is not suitable for ground disposal, or where the radioactivities are so high that direct ground disposal

5 5


is. unacceptable, some form of long-term storage must be used. It is probably inevitable that economic and potential exposure considerations will dictate that the earth be used as a storage and shielding medium for such materials. Storage facilities are intended to isolate and shield radioactive materials for a time that is long enough for their activity to decay to a level at which they may be treated or disposed. The decay interval includes:

(i) Short-term initial storage in thoroughly controlled containment, until there is adequate decay of short-lived radionuclides,

(ii) Long-term storage in isolated vaults or burial grounds until the container fails.

(iii) The period of slow transport of waste components through the ground after failure until final release by seepage into surface waters.

The necessary length of the decay interval is established by the relative hazard of the radioisotopes concerned, their concentration, and their half-lives. With some materials the transport interval alone might be sufficient to accomplish the necessary decay, the long underground residence being achieved by slow ground water movement, or long flow-path accompanied by satisfactory mineral sorption to retard migration.

The most important examples of storage are the tanks for storing high-level liquid waste solutions. Except in the most isolated of sites, shallow'tank storage cannot rely on the soil as an adequate safeguard if a leak should develop. Research is continuing on the solidification of such material where the soil can serve as a primary or secondary site for permanent storage.

(5) Accidental releases

Since some accidental release of radioactive substances is inevitable in any operation, it is particularly necessary to evaluate carefully the consequences of such an event. By examining the pertinent characteristics of the site, it is possible to make an advance selection of those parts of it where accidental releases would result in minimum exposure of populations. It is similarly possible to recognize the most sensitive parts of the waste handling system. These might include such units as underground storage tanks or pipelines, settling basins, transfer points, and pump pits. In most situations reasonable estimates of design limits can be provided through appropriate sub-surface studies; these will indicate the maximum ac

5 6


cidental release that could be tolerated without causing unacceptable exposures from environmental pollution.

The chemical and physical forms of the stored radioactive wastes have a great influence on the consequences of an accidental release. The form of wastes stored as liquids should be so adjusted to assure maximum sorption by sub-soil components in the event of the tank leaking. It is possible to provide chemical or physical barriers (e. g. beds of sorptive minerals, clay, asphalt, or grout barriers of low permeability) to reduce the rate of movement of spillage or leakage.

A P P E N D I X I

PRE-TREATMENT FOR GROUND DISPOSAL

This Appendix deals with methods for converting liquids, sludges or concentrates into solidified forms more capable of resisting solubilization through contact with ground water.

Much research has been carried out on methods of insolubilization and most of this has been directed towards developing methods for the high-level wastes at present held in liquid form. In general the methods are not viewed as a pre-treatment for ultimate disposal directly into the soil, but rather it is planned to store the wastes in vaults or engineering caverns below ground. One exception is the method developed at Mol, Belgium, for insolubilization by enclosure in asphalt; here the process is in operation but the final disposal to the soil remains at present in the experimental stage. Details of the method follow.

I. INCORPORATION OF INTERMEDIATE AND LOW-LEVEL CONCENTRATES IN A LOW-MELTING INERT MEDIUM [45]

Intermediate and low-level concentrates may be mixed intimately with asphalt in order to envelop completely each particle of sludge and so protect it from leaching by ground water. The method has been developed at Mol and a 100 litres/h processing plant is in operation. The apparatus, shown in Fig. 14, is composed basically of

5 7


S T A C K

Insolubilization of treatment concentrates by dispersion in asphalt.

a batching plant to measure the input of radioactive waste, a pre- heater and a mixer.

The mixer is fitted with a set of blades remotely adjustable in number and pitch to suit the viscosity of the mix and so obtain opti

5 8


mum speed of rotation. It is powered by a 10-h. p. electric motor and heated by an external jacket and immersion elements totalling 90 kW.

The asphalt is preheated before entering the mixer, where its temperature is raised further to 200-230°C. Meanwhile the waste is proportioned with a drum filter or in the case of liquids by a dosing pump and, with the mixer agitating the asphalt vigorously, the waste is slowly introduced. The objective is to obtain a high degree of dispersion until the mixture is homogeneous enough for discharge into 200-litre drums, where it cools and solidifies.

Experience has shown that the water content in the waste is immaterial since it boils away smoothly without spattering, 'but at present the maximum concentration of dry radioactive material in the mix is limited to 45%. Experiments with various types of asphalt indicate that the preferred type should have good mechanical and chemical stability, a low volatile oil content and a high "ring and ball" softening point (70°C).

Experience has been gained with various types of Waste, such as radioactive ash, evaporator concentrates in NaNOg and Na2S04 solutions, and sludges from radioactive chemical-treatment processes with a 55% water content. The entrained wastes have all been subjected to leaching tests by distilled water, ground water and sea water and satisfactory reproducible erosion rates of the order of 1 0 " 6 g/cm 2 day were achieved. Irradiation of the asphalt to 108 rad produced negligible change in the leaching rates.

An electrostatic filter is installed in the plant to remove volatilized oils and the amount of residual radioactive contaminants in the off-gases after condensation is roughly 0. 003% of that immobilized in the asphalt. Another attractive feature of this process is that the final volume of the waste is less than that of the original sludge owing to the evaporation of moisture and the suppression of voids.

During a 12-month period, all sludges resulting from chemical treatment of liquid waste at Mol were mixed with asphalt, and an estimate of the cost was made, taking into account maintenance and amortization within 5 years. The cost was estimated to be $31 per 200-litre drum of mixture processed for storage, corresponding to $0. 067 per m3 of processed effluent from which the sludge was formed.

5 9


II. INCORPORATION OF HIGH-LEVEL LIQUIDS IN GLASS

There are no sites where high-level liquids are operationally converted into solid glass, but there are two experimental procedures that are worthy of note in their association of this method with ground disposal.

(1) Glass manufacture at Chalk River, Ontario, Canada

In this process an aged fission-product solution of concentration 20C i/litre, containing 8N nitric acid, was converted into glass hemispherical blocks of 14 cm diam. The glass was made from nepheline syenite mixed with 15% lime, pelletized by tumbling in water and introduced into crucibles in alternate layers with the radioactive solution. The nitric acid reacted with the syenite pellets to form a gel of silicic acid, which was dried at 900°C. This was then fused at 1350°C and the volatilized gases were passed through firebrick absorbers containing iron oxide to remove ruthenium and caesium.

Twenty-five of these blocks (1000 Ci) were then buried in sandy soil and leaching was deliberately encouraged by ensuring that they lay beneath the water table. They were placed in a vertical grid oriented normal to the direction of ground water flow, which was found to be 23 cm/day. After 2. 5 years, only 4 mCi was released to the surrounding soil and ground water, representing a leaching rate of 2 X 1 0 ' 8 g/cm 2 day or about one-tenth of the value anticipated from previous short-term leaching experiments with distilled water. The front of strontium-90 migration was less than 5 m from the blocks and the experiment was felt to be a successful demonstration of safe disposal into the ground [47].

(2) Glass manufacture at Harwell, England

A conceptual waste treatment is envisaged [61] that will convert high-level liquid wastes from fuel processing into glass cylinders 15 cm diam. X 1. 5 m long. The specific activity of the glass would be 600 times as large as that in the Chalk River experiment, with each cylinder incorporating an estimated 9 X 1 0 5 c i after one year's decay.

A pilot plant using inactive materials was developed until the reliability and reproducibility, necessary for the final process, were

60


achieved. From the experience attained engineering components were developed suitable for the commissioning of a full-scale plant. In the process the waste solution and a silica/borax slurry are premixed and introduced at an appropriate flow-rate into a stainless- steel cylindrical casing placed in an electric furnace (Fig. 15). The

FISSION PRODUCT WASTE

SILICA/BORAX SLURRY

STAINLESS STEEL

CYLINDER

6* ID » 5'LONG

FURNACE WITH — . 6 INDEPENDENTLY \CONTROLLED __ i

HEATING

ELEMENTS

-© ©-

T 5PRIMARY

; Tfilter/absorber

Fig. 15

Sim plified flow -sheet o f process for incorporation o f fission product wastes in glass.

heating applied to various levels of the casing is adjustable to ensure that evaporation, ' denitration, sintering and melting occur simultaneously over a narrow zone to form a bubble-free glass.

The resulting blocks of glass would be self-heating and it is intended that they should be stored permanently underground or in appropriate structures where convective air-currents would c ir culate naturally and so provide the necessary cooling. It is envisaged that the soil be used only as a shielding medium but leaching experiments indicate that if ground water came into contact with the exposed end of a cylinder, the estimated release would be 10 mCi/week. If, however, the entire container corroded, leaching from the exposed surface would be increased to an estimated 500 mCi/week.

6 1


During the next 20 years it is estimated that 4 X104 tons of fuel will be processed in Great Britain. If all the wastes were vitrified and stored in their steel cylinders it is believed that they could be suitably contained in an area of 900 m2.

III. PRE-TREATMENT PROCESSES BEFORE DISPOSAL THAT CONCENTRATE RADIOISOTOPES FROM LOW- OR INTERMEDIATE-LEVEL WASTES INTO SLURRIES, SLUDGES, OR CONCENTRATED SOLUTIONS

The treatment processes described in this part of the Appendix are mainly applied to low-level radioactive waste. They may be designed to remove specific radioactive constituents from the bulk of the liquid and to prepare the solution for re-use or disposal to the ground. Only the more common treatment processes, most applicable for pre-conditioning for ground disposal, will be reviewed. A more complete discussion can be found in the numerous references [61, 62, 63, 64].

(1) Flocculation

Standard flocculation treatment of waste water, using a wide range of flocculating agents, yields overall decontamination factors of about 10 [63]. Where one particular radioisotope is of critical importance and the flocculation treatment can be designed to optimize removal of that element, much better results can be obtained. The equipment required for flocculation treatment is of simple design and relatively inexpensive to install and operate. Wet sludge volumes of the order of 0. 1% to 2% of the original waste must normally be stored or otherwise disposed of.

Conventional lime-soda water treatment including coagulation, settling and sand filtration provides a total beta decontamination factor in the range of 3-10 for wastes containing mixed fission products [62], Small improvements can be realized by including activated silica, alum, ferric chloride, F u ller 's earth or bentonite clay in the treatment process. The use of alum and iron as coagulating agents is effective for removing high-valence cations (e. g. rare earths), giving decontamination factors of 10-100. The addition of a small amount of copper sulphate, silver nitrate or activated carbon results in the concurrent removal of halogens (e. g. iodine-131) with

6 2


decontamination factors of 10-100. Better flocculation decontamination is generally obtained with phosphate floes, such as calcium phosphate. Decontamination factors of several hundred can be obtained for some wastes with calcium phosphate floe at high pH and by using excess phosphate [63], but the floe is bulky, with a high water content.

For specifically removing the long-lived fission products caesium-137 and strontium-90 a precipitation scavenging process has been demonstrated [65 J. Ferric ion precipitated as a mixture of hydroxide and phosphate in a high pH system [66] removed most of the strontium (99%) from high-salt solutions containing 20 ppm inert strontium. A precipitate of nickel ferrocyanide scavenged up to 99% of the caesium-137 from a high-salt solution at pH 10.

(2) Sorption processes

Most low-level waste solutions may be described as dilute solutions of salts of stable isotopes containing traces of radioactive isotopes. That is, the radioactive constituents are generally an insignificant fraction of the total salt content. As a result, sorption reactions tend to be more controlled and limited by the non-radioactive constituents than by the radioactive ones.

Sorption reactions applied to radioactive waste solutions can provide decontamination factors ranging up to 104, depending on the composition of the wastes, the nature of the sorption bed, the flow rate, etc., but for low-level waste solutions decontamination factors of 10 to 100 are most common [63], The sorption beds may be re used by regenerating them, in which case the concentrated regenerated solution must then be treated as low-volume intermediate-level waste However, if inexpensive sorbent materials are used they may be disposed of as solid waste without regeneration. Development has been directed largely towards minerals or inorganic materials for once- used beds and towards high-capacity organic resins for regenerative systems [67].

Promising materials include commercial grades of expanded vermiculite, clays, zeolites, phenolic-carboxylic resins, phenolic- sulphonic resins, and sulphonated polystyrene resins. Sorption processes are particularly useful for cationic radionuclides that do not readily form soluble complexes, e.g. caesium-137 and strontium-90.

Several minerals have been shown to have a selective sorption capacity for caesium, even in the presence of larger concentrations of other monovalent ions. For example, clinoptilolite, a naturally

6 3


occurring zeolite (one of a group of hydrated aluminosilicates with exchangeable sodium and calcium), has demonstrated an unusually high selectivity for caesium [68, 69]. In one experiment a laboratory bed of this mineral removed 99% of the radiocaesium from more than 5 X 1 0 4 bed volumes of water containing 24 ppm calcium and magnesium together with a trace of caesium -137. This capacity is more than 30 times that obtained with a non-selective com m ercial ion- exchange resin. Many other minerals, including heat-treated mont- morillonitic clay, have been shown to have a selectivity for caesium [70], while other sorbents may exhibit selectivity for other ions.

(3) Evaporation

Of the various treatment processes, evaporation provides the highest decontamination factors, can handle the widest variety in types of waste, and is the most reliable. However, it is also the most expensive method and would not normally be considered complementary with ground disposal for large-volume low-level effluents.

A P P E N D I X II

PHYSICS AND CHEMISTRY OF THE MOVEMENT OF RADIOACTIVE WASTES IN THE GROUND

I. CHEMISTRY OF THE MOVEMENT OF RADIOACTIVE WASTESIN THE GROUND

The chemical reactions of radioactive wastes with soils may involve a variety of mechanisms. Some of the common reactions may be largely included in the single term "adsorption", defined as "any reaction that results in the concentration of dissolved material on a surface". Thus the fraction of fine material present, particularly of clay minerals, often determines the reaction capacity of sub-soils because of the high surface area exposed by finely divided material. However, by no means all of the adsorption capacity is located on external mineral surfaces. With some minerals, notably certain

6 4


clays and zeolites, a large part of the adsorption capacity is associated with internal surfaces. To utilize this capacity, the ions being adsorbed must diffuse into the mineral grains. Since the mechanism of reactions involving the internal pores of mineral grains is not easily characterized as a "surface" reaction and is sometimes referred to as "absorption", the chemical reaction between minerals and radioactive ions is sometimes described by the inclusive term "sorption". Frequently sorption proceeds by an "ion exchange" mechanism in which an ion attached to the mineral surface is replaced with an ion from solution. Here the displaced ion must diffuse out of the crystal as part of the reaction process. These diffusion mechanisms frequently control the rate of mineral sorption reactions.

The three most important parameters that determine the sorption behaviour of a sub-soil towards a particular constituent of a solution are: (i) the exchange (sorption) capacity; (ii) the equilibrium constant; and (iii) the general reaction rate coefficient [50J. The exchange capacity is usually measured by a standard soil- chemistry technique which permits comparison of the relative equilibrium capacities of various materials. The numerical value obtained for the exchange capacity depends upon the technique used to determine it. The equilibrium constant reflects the distribution of the replaced and replacing ions between the solid surface and the solution under equilibrium conditions. If the concentrations of the competing ions Ca+2 and Sr+2 are C c a and C s r respectively, and their corresponding equilibrium capacities on the solid are qca and qsr , then the equilibrium constant K for the replacem ent of Ca+2 is:

K = ^ C -- • (1)(-Sr ^ C a

This simple equation represents the case of ions having the same valency. It is a modified mass-action equation and must be expanded in the usual way in the case of ions having different valencies. If theion of interest in solution, e. g. Sr+2, is present in very small concentration relative to the ion on the sub-soil, the values of Cca and qca are sometimes considered to be constant and are combined with the equilibrium constant to define a new constant Kd, called the distribution coefficient. This convenient simplification is often justified and results in a parameter that is readily measured:

_ sorbed radioisotope/gram soil . \d dissolved radioisotope/cm3 solution ’

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However, the case of the radioisotope tracer, in the presence of a single macro-ion, was believed to be unreal and for this reason studies were undertaken in several laboratories [71, 72] to anticipate the behaviour of radioelements in the presence of two or more m acro-ions. In most cases it is possible to demonstrate that, in fact, the influence of one macro-ion is so predominant that the others may be neglected.

An estimate of the retardation effect of sorption reactions by sub-soils may be made from measurements of Kd and the assumption that chemical equilibrium prevails in the ground water. Here the travel time of a radioisotope, TR, relative to the travel time of water, Tw, is:

i ^ ' l + K d f (3)s

where p is the weight of sub-soil per unit volume and s is the porosity of the sub-soil. In general, laboratory data indicate better sorption of caesium than of strontium on sub-soils, with rare-earth sorption usually being intermediate between them. This generalization is subject to many exceptions according to the solution composition and the nature of the materials comprising the sub-soil. For example, Mol reports a strong relationship between strontium sorption and the lignite or organic content of the sub-soil [73], which can reverse the relative order of strontium and caesium sorption capacity. If typical reported values for packed density p of 2. 0 and porosity s of 0. 2 are chosen it is possible to estimate the probable ranges of T r / T w for various isotopes from the reported values of distribution coefficients [33, 74, 75, 76, 77, 78, 79]. For caesium T r/ T w = 20-5 X103, for strontium T R/Tw = 50 - 105 and for rare earths T R/ T W = 103-104. Thus, the effect of sorption reactions would be to extend the travel time by one to four orders of magnitude.

In the above example it was assumed that chemical equilibrium conditions prevailed in the ground water during the movement of the radioactive material. The validity of this assumption depends upon the bulk flow-rate of the ground water together with the third im portant sorption parameter, the general reaction-rate coefficient. Failure to achieve complete equilibrium would be equivalent to a reduction of Kd in the sample calculation, and a similar reduction in the. retardation of the radioactive ions. Laboratory experiments have shown that some mineral reactions require very long periods

6 6


to achieve equilibrium. A period as long as 4 weeks is required to achieve maximum exchange of caesium with sodium on some forms of vermiculite [80]. However, most of the exchange reactions with sub-soils occur relatively rapidly, and usually only the last few per cent of the total exchange capacity requires extended equilibration time to be utilized. The good correlation often obtained between laboratory determinations of exchange capacity and that measured in field experiments tends to support the assumption that nearequilibrium conditions often prevail in the ground water [24],

II. CHEMICAL REACTIONS WITH SOIL MINERALS

The constituents of waste solutions can react with soil minerals in a variety of ways, mainly in accordance with the composition and properties of the two phases. The intention in this section is to outline in the first place some of the better-known facts on the structure and reactions of the clay minerals abundant in nature and largely responsible for the sorption properties of soils. In the second place, a few other mineral reactions will be described that play important roles in waste treatment and disposal.

(1) The structure and reactivity of the clay minerals

It is well recognized that the crystal structure of clays is principally determined, as are all silicate structures, by the spatial arrangement of the large oxygen (or hydroxyl) ions that surround the small silicon ions in tetrahedral co-ordination. Shared oxygen ions, at the corners and edges of tetrahedra, bind these fundamental units into sheets of considerable strength while cations may occupy appropriate positions between the sheets. In all alumino-silicates aluminium ions are found in octahedral co-ordination surrounded by six oxygen ions also bound into sheets; they occasionally occupy in addition silicon positions within tetrahedra. The numbers and arrangements of silica and alumina sheets determine the general clay type while the choice of cations between multiple sheets determines the mineral species. On these principles, the clays may be classified in one of two types, the kaolinite arrangement or the montmorillonite arrangement. Fig. 16a illustrates the arrangement of atoms in the silica and alumina sheets, and Fig. 16b is an edge view of such sheets in kaolinite and montmorillonite; the small size of the metal atoms

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SILICA SHEET ALUMINA SHEET

4 Si

6 OH

40-20H

60

4 Si

4AI

4 Si

6 0140+20H

40+ 20H

>60

I : I TYPE 2 :1 TYPE

Fig. 16

The crystal structure o f clays,(a) Silica and alumina sheets view ed'norm al to the sheets;(b) Kaolinite (1 :1 type) and m ontm orillonite (2 ; 1 type) sheets viewed from the edge.

relative to that of the oxygen ions is a significant consideration and is approximately to scale in the figure. The hydroxyl ion is approximately the same size as the oxygen ion in these structures.

(a) Clays of the 1: 1 type

In the clays of the 1: 1 type, for example in kaolinite, the crystal layer consists of one silica sheet and one alumina sheet as shown in Fig. 17. In that figure, the ionic components of the crystal are not shown in true relative size in order that their structural relationship may better be illustrated, but the significant crystal dimensions are given because of their importance. Spacing of the layers in the clays of the kaolinite type is generally close and there is little opportunity for water molecules or cations to enter the lattice between the layers. The ability of the kaolinite crystal to accept ions from a solution will therefore be due largely to sorption at exposed crystal faces or to replacement of H+ from the ionization of the OH' ion. The sorption is enhanced by a decreasing particle size of the clay (exposing a larger area of crystal faces) and the replacement behaviour by an increased pH of the medium; in either case, the uptake of cations is comparatively .low.

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- r - U V b t x , to4 Si 6 O

b(OH)4 AI4Ch-2(OHJ4 Si 6 0

C -A X IS

KAOLIN ITE $i4 O )0 H O N TM O R ILLO N ITE (O H ) AI SI 0 „ . *H ,0 4 4 8 20 2

O

o <O

' oC -A X IS

- B - A X IS -------------- --

I LUTE (DHj4 K7 (AI4- F.,. Mg4 Mg6) ( S i ^ - Alyj O20

Fig. 17

The crystal structure o f clays.The sheet edges show crystal dimensions.

(b) Clays of the 2 : 1 type

With clays of the 2: 1 type, the crystal lattice is composed of layers made up of an alumina sheet sandwiched between two silica sheets as shown for montmorillonite and illite in Fig. 11. Because of the origin of the many species of clay of this type, substitutions of other cations in the alumina layer are frequent and substitutions of Al+3 for Si+4 in the silica layer are also notable. In the latter case, illite, vermiculite and beidellite are a result. In either case, the valency changes due to the substitutions bring about changes in the

604-ySi --yAI 2(0H]+40 Al4 -F -Mg Mg 2(0H)+40 4 ySi *yAI

6 9


electrical charge of the layers and this, in turn, allows compensation to be made by the addition of mobile ions between the layers. It is thus apparent that the exchange capacity of clays of the 2: 1 type is in general considerably higher than that of the 1 : 1 type. The spacing between the layers in montmorillonite particularly can be modified in various ways. Treatment with KC1 collapses the lattice along the C-axis from 14 to 10 A and binds the layers together to form an illite-like mineral. This simultaneously reduces the exchange capacity although the selectivity for caesium-137 is enhanced at the 10 A spacing [81, 82]. Heating to 600-700°C releases water molecules from interlayer positions and from hydroxyl ions, collapsing the lattice and producing the same result. Treatment of vermiculite-biotite minerals with sodium ion increases the exchange capacity by enlarging the spacing along the C-axis.

(2) Some other minerals capable of sorbing radionuclides

In addition to the clays there are many other types of minerals that have been found to sorb cations [83]. The zeolites offer a long- known example. These minerals make up a large family of several types of hydrous alumino-silicate s. In addition to being closely re lated in composition, in origin and in mode of occurrence, the zeolite groups all possess an open three-dimensional network structure of linked polyhedra with considerable opportunity for lattice substitution. For these reasons they are all capable of reaction with solutions of various ions; some possess a remarkably large exchange capacity and some are ion-selective to a valuable degree. The particularly useful properties of the zeolite, clinoptilolite (mentioned in Appendix I), are enhanced by its selective exchange capacity for caesium and strontium even in the acid pH range. Mordenite is a related zeolite of similar properties, found to be highly strontium-selective by the Czechoslovakian investigators [84]. The relative ease of synthesis by hydrothermal reactions gives promise that zeolites of a desired stability and selectivity will eventually be able to be prepared.

Among synthetic materials, aluminium and cerium oxides are surface-active in the sorption of strontium; increase of pH and heat treatment increase the sorption capacity of aluminium oxide.

The successful use of beds of granules coated with ferric oxide for the removal of radioruthenium from gas streams has prompted investigation of the feasibility of similar reactions for removing ruthenium from aqueous solutions. The finding of a naturally occurring

7 0


or cheaply synthesized m ineral substance fo r this application would so lve one of the m ajor rem aining problem s in the d isposal o f ra d io active waste stream s. Some progress has been made by dem onstrating that ferrou s ion used with m ineral ca lcite is efficient in the r e m oval o f ruthenium sp ec ie s from certa in Hanford w aste solu tions. Although the m acro-chem istry of ruthenium has been studied in some breadth [81, 85, 86, 87,88], it is still not clear which m olecules or com plexes are responsible for its non-retention by the soils under trace conditions.

The C zechoslovakian w ork ers have shown that B aS04 cry sta ls can be prepared, purposely contaminated with co-precipitated CaS04,. so that anom alous cry s ta l-su r fa ce la y ers ex ist containing ca lcium ion. The m aterial reacts readily to accept cations such as radium, strontium or lead at the lattice points occupiedby the calcium ions [89].

(3) Metasomatic replacement and related precipitation reactions

M etasom atic rep lacem en t and re la ted p recip ita tion rea ction s illustrate another m echanism by which radioactive ions m ay be r e m oved from waste solutions by m ineral reactions. The m ost typical exam ple of the form er was d iscovered and cla rified at Hanford [90]. When basic phosphate solutions are brought into contact with m ineral ca lcite , the calcite is converted to the new m ineral species , hydroxy apatite:

Na3P 04 + NaOH + 5CaC03 - Ca^PO^gOH + 5Na2C 03

During the conversion o f the one solid to the other, lattice points becom e a ccess ib le and various ion substitutions are facilitated . Im portant among these are the substitutions o f various cations in the calcium ion positions, and strontium -90 and other bone-seeking is o topes are notable exam ples [90]. The reaction kinetics were studied at Saclay [91], where it was shown that the stron tium -90 concentration in a column of ca lcite decreases exponentially with column height during the reaction.

At Oak R idge it was noted [92] that a s im ila r rea ction o ccu rs with exchangeable calcium in the so il and that ca lc ite is not n e ce s sary as a reactant. Another exam ple o f this m echanism appears to be the use o f phosphate ion with verm icu lite , when a m agnesium phosphate precip ita te resu lts from the reaction with exchangeable magnesium from the m ica; stron tium -90 is w ell retained on the precipitate.

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III. PHYSICS OF LIQUID M OVEM ENT THROUGH A POROUS MEDIUM, AND HYDRODYNAMICS OF GROUND W ATER

The m ovem ent o f liquids through so ils o r rock s o f varying p o rosity and the hydrodynamics of ground water have direct application to the understanding o f the flow and the flow rate of radioactive w astes d isposed to the ground.

The rate o f m ovem ent of liquid radioactive w astes is governed by com plex relationsh ips among such variab les as the p orosity and vectora l perm eability of the form ations; the physical characteristics o f the solution, such as density, v iscos ity and surface tension; the geo logy o f the stru ctu res used fo r in troducing liquid w astes to the soil; and the paths of water movement in the soil from other sources such as rainfall. When significant amounts of liquid radioactive wastes are to be disposed into the ground, som e prelim inary, evaluation o f the rate o f m ovem ent and d irection w ill be o f value. Thus som etim es ground disposal may be restr icted to areas where the d irection and rate of m ovem ent of w astes can be predicted with great assurance.

One of the sim plified concepts used in describ ing the flow of l i quids through a porous medium is term ed D a rcy 's Law and may be written:

Q = - KAi (4)

where Q is the flow rate o f liquid through a section o f area A under the impetus of p iezom etric gradient i. K is the proportionality con stant, com m only ca lled the p erm eab ility o f the m edium . The r e lationship is valid under conditions of saturated lam inar flow. If the system is only partially saturated the proportionality term becom es a function o f the cap illary pressu re and is usually designated as the capillary conductivity o f the medium (K 1). As used here K' is con sidered a scalar quantity, being a function o f both capillary pressure and spatial location . It is th ere fo re associa ted with partia lly saturated flow in heterogeneous m edia.

The piezom etric head, or hydraulic potential, may be defined as:

w here p is hydraulic p re ssu re , Z is the position head o r potential energy due to the location in the gravitational fie ld , p ' is the liquid

7 2


density, and g is the gravitational sca le r . F or capillary (partia lly- saturated flow ) system s the te rm p /p 1 g is negative ( le s s than atm ospheric) and is term ed the capillary pressu re. To define experim entally the hydraulic p roperties of a su b -so il would requ ire m easurem ents o f ca p illa ry conductivity at variou s values o f ca p illa ry pressure, and determination of the relationship between the capillary pressu re and the m oisture content 9 of the m ateria l. F or partially saturated or m ultip le-phase flow of air and w ater, the p ressu re in the air phase is usually con sidered constant and at atm ospheric or zero gauge p ressu re . In such ca ses the cap illary p ressu re pc, d e fined as the p re ssu re in the a ir phase m inus the p re ssu re in the water phase, is then sim ply the term -p /p 1 g. The cap illa ry p r e s sure ,is related to both the capillary conductivity K' and the m oisture content 6. Such re la tion sh ips are determ ined experim en ta lly fo r the so ils of in terest. F o r a stated m oistu re content'tw o values o f cap illa ry p ressu re and o f cap illa ry conductivity m ight be obtained, one fo r the im bibing cy c le , and one fo r the draining cy c le . F or a flow system defined in term s of the usual x, y, z Cartesian space c o ordinates, and the tim e variable t, the relationship o f D a rcy 's Law m ay be com bined with the law o f m a ss con serva tion to d er iv e the fundam ental flow equation [93]:

fd 2(j) d2(j, d2(j)\ 9 K '.9 0 9K '_9 </> 9K 1 9 <j> 90\^9x2 + 9y2 9 z 2 y 9x 9x 9y 9y 9z 9z 9t

This equation is quite general, but suggests that fo r all but the most s ter ile m odels the ba sic equations w ill be non -linear and likely not subject to general analytical solution. Some num erical solution technique must be applied as a general approach to these problem s. Com puter m ethods are helpful in obtaining solutions to sp ec ific p rob lem s [ 94 ].

The solution o f equation (6) and the appropriate boundary co n ditions by n um erica l m eans prov id es the ground w ater potential <f>. From the ground water potential the interstitial ve locity components v x, vy, and vz in the three co -o rd in a te d irection s x, y , and z are d irect ly obtained, using the p oros ity f, that is:

— _ 9j£Vx ' f 9x

v = - ^ ^ (61)y f 9y { '— = 9£Vz " f 9z

7 3


Equations (6 1) p rov ide the ground w ater tran sport e ffe c ts r e quired fo r waste m ovem ent analysis.

The factors affecting the distribution of an in jected radioactive waste may be categorized in three groups:

(i) Sorptive influence o f natural ion exchangers;(ii) Hydrodynam ic d ispersion ;

(iii) Convective transport.The general d ifferentia l equation incorporating these three e le

ments has been reported as:

9c i l f 9 i U A 8 A , *£at f ’ p at + ax v ° xax J + ay v ° y 3y / + 9z \ z 3z

(7)

where t is tim e, c and q are the concentrations of a sp ec ific ra d io isotope in the liquid and solid phases resp ective ly , p is the median density, f is the porosity of the form ation , v x, vy and v z are the average interstitial v e loc ities , and Dx, Dy and Dz are the dispersion coe ffic ien ts fo r the th ree co -o rd in a te d ire ct ion s . The f ir s t term o f the right-hand side o f equation (7) depicts the sorptive influence of the solid phase, the second the diffusional transport, and the third group the convectional transport.

In the passage o f radioactive w astes through ground form ations it m ight be reasonable to assum e that w ater v e lo c it ie s w ill be su fficiently sm all to allow the establishm ent of an exchange equilibrium between the solid and liquid phase:

Kd=K ^- (8)Cg

where Kd is the distribution coefficient, K is the mass action constant, Q is the exchange capacity of the medium and cg is the concentration o f the gross interfering cation. Equation (8) perm its the estim ation o f the influence o f m inor changes in waste com position on the equ ilibrium distribution o f the radio-contam inant.

Using equation (8) and the tim e transform ation

7 4

t = ( l + ^ K d)T = K fT (9)


the sorption term o f equation (7) is elim inated and this equation is converted to a purely hydrodynam ic expression . K f is term ed "the tim e transform ation fa c to r " .

9c _ _9_ 9T " 9x ( y 9y

9c) )

(10)

The exact mathem atical solution of equation (10) is possible only fo r the sim ple case o f one-d im ensional flow through a column. F or com plex flow nets in natural form ations it is not p oss ib le to arrive at exact solutions, but by energy and dynam ic m ethods n u m erica l solutions of adequate a ccu racy m ay be poss ib le in som e situations. Another approach is the use o f a w ater t ra ce r to ascerta in e x p e r imentally the d ispersive and convective properties of a form ation and, through the use o f the tim e transform ation relationship, to then delineate the c(t) function o f the exchanging nuclide between the point o f in jection and som e distant point o f observation .

The above-m entioned tra ce r approach can be applied to tra ce cation breakthrough when the follow ing four conditions are satisfied:

(i) The form ation p oros ity f m ust rem ain constant within rather narrow lim its , s in ce the tran sform ation fa c to r is v e ry sen sitive to changes in p oros ity

(ii) The distribution coefficent K d at equilibrium must be known and must rem ain reasonably constant in both tim e and space

(iii) Equilibrium must prevail at all points in the form ation between cations in the solvent and those so rb ed on the adjacent so lid

(iv) Conditions o f "steady flow " m ust p reva il in the form ation fo r any constant rate of injection. Furtherm ore, any change in the injection rate should im m ediately be reflected in a proportionate change in solvent v e lo c it ie s at all points in the form ation . It would be equivalent to state that the flow net must rem ain unchanged.

It m ust, o f co u rse , be re cog n ized that none o f the above fou r conditions w ill be com plete ly m et in any actual in jection operation. However, in many situations where analysis of waste flow is required, the receiv ing form ations are sufficiently close to the ideal in m inera- lo g ic and lith o log ic h om ogen eity fo r the d isp e rs iv e th eory o f co n tam inant tran sp ort to p rov id e usefu l in terpretation .

7 5


A P P E N D I X III

GROUND DISPOSAL OPERATIONS

Probably the m ost extensive operating experience in the disposal of radioactive liquids to the ground [77] has been accumulated at the Hanford p ro ject in Washington State, U. S. A. The site has a sem i- arid clim ate (rainfall 18 cm /year) and the w ater-table lies at a depth o f roughly 60 m below deposits of gravels, sands and silts. The d is posal points lie roughly 16 km from the nearest em ergence of ground w ater in the banks o f a la rg e r iv e r (F ig . 18) and although the so ils

Plan o f radioactive m igration (> 8 x 10*8 fiCi/m l) showing associated water table contours and boundaries at outcrops.

have a low ion -exchange capacity, the long underground flow paths that waste waters must follow guarantee contact with a so il column o f great total capacity. Any radionuclides that rem ain unsorbed by the s o il w ill enter the r iv e r along an iso la ted stretch and b ecom e

76


thoroughly m ixed and diluted b e fo re the w ater reach es a populated area dow nstream .

The effects o f disposing of radioactive wastes in these soils have been studied fo r many years. The early disposals form ed bases for re se a rch in the sub ject and fie ld investigations o f the underground m igrations indicated that la rg e volum es o f certa in types o f w astes could be d isposed sa fely to the ground. When such d isposa ls w ere m ade and the e ffects determ ined by sam pling and analysis o f the water from deep w ells , a further degree o f confidence was obtained that ground disposal at the Hanford site could be practised with wide m argins o f safety.

The types o f fa cilities used fo r the disposal of wastes of various activity levels have been described in the main text. The structures are sim ple and the main design cr ite r ia are a percolation rate adequate fo r the disposal rate, and the provision o f sufficient so il cover as shielding fo r the appropriate activity level.

The volu m es and a ctiv ities o f in term ed iate le v e l w astes d is charged to cr ib s and trenches from 1944 to 1961 amount to 2.52 X 106 cumulative total cu ries [95] and it is interesting to note that ra d io active decay has so decreased the activity that only 6% of the original now rem ains. This includes 86% of the ca e s iu m -137 and 88% o f the stron tium -90 . It is s ign ificant that the annual d isch a rge rate and activ ity have been p ro g re s s iv e ly red u ced as a resu lt o f im p roved tech n ology p a rticu la r ly in fis s io n -p ro d u c t separation and through the in crea s in g re c ircu la tio n o f w ater.

Tritium , included in the fission -product waste [96], has migrated rapidly underground without sorptive delay and has shown broadly the future path to be traversed by s low er-m ov in g rad iocation s. It has spread alm ost to the r iv e r in a pattern o f flow in good a cco rd with predictions from known geology and hydrology. It has a lso brought to light, through p re fe rred rapid flow paths, som e in teresting fe a tures of early r iv er channels. These had been filled with m ore p ermeable sedim ents and later concealed beneath m ore recent deposits.

As an exam ple of sm aller liquid d isposal operations the exp erience at Chalk R iver under conditions in complete contrast to those at Hanford is of interest. The surface deposits are shallow, consisting o f im perm eable g la cia l t ill which is som etim es eroded down to the bedrock and elsew here covered by m ore recent accumulations of sand. The granite bedrock is irregu lar in p ro file , frequently outcropping, and ra re ly deeper than 20 m below the su rface . In a gen era lly fo rested area the liquid d isposal pits [32] are situated on a sm all pla-

7 7


teau (40 000 m 2) o f dune sand ra ised slightly above neighbouring swam ps. F rom 1953 to 1956 all the lo w -le v e l active waste effluent (2 X 1 0 5 litre s /d a y ) was d isch arged into a sm all natural dep ression in the sand (50 m diam. and 4 m deep). The water table lay 2 m b elow the pit and the aquifer was but 2 m deep. The underground flow path to a spring was 65 m away, and ru then ium -106 em erged from the spring after 4 months o f d isch arge . A fter 3 y ea rs the pit was abandoned owing to surface contamination in vegetation and w ild -life and to the imminent breakthrough of strontium -90. The effluent was then directed to two p eb b le -filled seepage pits excavated in sand nearby. The sm aller (30 m d ia m .) was reserved for chem ical wastes containing acids, detergents e t c . , and the la rger (66 m X 33 m) r e ceived only low -lev e l effluent (10-5 fjC i/m l). The la rge pit (F ig . 19)

Fig. 19

Cross-section through a shallow liquid disposal pit in sand showing m igration towards nearby "area o f em ergence".

accomm odated a daily inflow of 2X 105 to 2. 5X105 litres /day and from 1956 to 1962 accum ulated 1. 1X 104 Ci o f soluble beta-contam ination la rgely concentrated in the sand base o f the pit.

A fter 6 y ea rs o f continual use a m igration of f is s io n products (strontium -90, cobalt-60, caesium -137, ca esiu m -144) totalling 50 Ci had developed along the sub-surface flow path. Although tracer tests showed that the ground w ater m oved at 70 cm /d a y along this path, the stron tiu m -90 lagged behind at a m igration rate o f 1. 9 cm /d a y . At this rate a breakthrough of strontium -90, in weak concentrations, is anticipated in 1966 when the pit w ill have been in use for 10 years. H ow ever, the in creasing use o f w ater recircu la tion through ion -

7 8


exchangers is dim inishing the quantity o f effluent d ischarged and it is likely that the usefu l life o f the pit w ill be extended accord in g ly . If it w ere later abandoned the reduction in recharge would cause the water table to drop and m ost of the fission products would be stranded in the aerated zone. N eglig ib le further m igration would be an ticipated. As at Hanford the lo w -le v e l liquid wastes have contained tritium , which has experienced no hold-up by the so il. Over 800 Ci have spread beneath the neighbouring swamps but since the eventual re le a se into su rfa ce w aters w ill be slow no environm ental hazard is envisaged [59].

The solid d isposal facilities at Chalk R iver [44] are in fine sand, up to 4 m thick, that overlies either granite bedrock d irectly or thin interm ediate deposits o f g lacia l till. L ow -leve l wastes are dumped, packaged in paper or p lastic, into trenches roughly 4 m wide X 3 m deep. They are covered with sand and com pacted by bu lldozer. For convenience lo w -le v e l wastes are signified as those packages of juCi quantities estim ated by radiation readings taken 25 cm from the package. Experiments have shown that typical consignments of waste in the w aterproof paper liners o f garbage cans have a beta-radiation equivalent to 1 -y e a r -o ld m ixed fiss ion products. They have th ere fore defined their "nominal m illicu rie" of waste as that giving a reading of 20 m R /h at 25 cm using a particular beta-radiation m eter. Any single reading may be in e rro r by a factor of two but this is regarded as acceptable.

F or solid w astes of higher activity, including bottles o f im m iscib le flu ids, tren ch es are constructed o f re in fo rced con cre te typ ica lly com p osed o f a 2 0 -cm base slab , 3 0 -cm r o o f and sidew alls not less than 15 cm thick but appropriate for the external earth pressure. The trench is subdivided by bulkheads every 13 m so that a section m ay be drained, iso la ted and ro o fe d b e fo re being fille d (F ig . 20). N orm ally the sidew alls protrude roughly 30 cm above ground to stop su rface w ater flow ing in and after the capping slab has been cast a m etre of so il is piled over it. Other types of experim ental disposals w ere carried out with asphalt-lined trenches, which proved too weak to be sa tis fa ctory , and by using drum s of so lid ified active m orta r cast in an enveloping m atrix o f sound concrete. This was expensive but has proved satisfactory after 10 y ea rs ' surveillance.

Other facilities consist of spun-concrete pipes (Fig. 21) cast v e r tica lly into a concrete base slab set in the sand above the water table. These are used for highly radioactive m aterials, which may be transferred from the shielded transport container d irectly into the buried

7 9


Fig. 20

Concrete disposal trench with the contents boxed or wrapped and arranged to save space.

pipe without exposing the operators. The diam eter of the pipes varies between 25 cm and 125 cm ; they are assem bled in short section s , jointed with asphalt and coated externally with bitum en or asphalt.

8 0


Concrete pipes installed vertica lly in the ground.

F u el-rod storage holes are s im ila r except that the external 6 0 -cm pipe contains a con cen tr ic 15- o r 2 5 -cm -d ia m . s tee l p ipe. The annulus is filled with con crete and the pipe capped either with con crete or a rem ovable steel plug accord ing to whether the storage is permanent or tem porary. Testing has shown that when fuel elements, aged fo r 6 m onths, are p laced inside, heat d issipation to the s u r rounding dry sand is sa tis fa cto ry and the tem perature r is e in the con cre te p ipes is acceptably low .

Both liquid and solid disposals to the ground have been practised routinely at Oak R idge, T en n essee , fo r many y e a rs . R adioactive effluents w ere d ischarged fo r 10 years into three open pits each of 4X106 litre s capacity , .a ccom m odating a total da ily in flow o f2. 8X104 litres [2], The m ore hazardous radiocations have been r e tained in the Conasauga shale and investigations have shown that there is no apparent danger o f stron tiu m -90 seeping into nearby stream s.

However, ruthenium -106 experienced no such retention and appeared in a stream after 5-15 days from one pit and after 150-300

8 1


days from the best situated pit. In this case the concentration o f ruthenium -106 entering the main r iv e r with a dilution factor of3. 9X1015 established the lim iting concentrations, if the MPCW values w ere not to be exceeded [97] . Such lim itations, enabling an annual discharge of 2.4X104 Ci (ruthenium -106), were insufficient for these pits to handle if they w ere to retain a safety m argin fo r high tra n sient re lea ses . New pits were therefore built. They were constructed as trenches, filled with lim estone and covered with 2 m of so il, so avoiding the high radiation fields that surrounded the old pits. One trench was 140 m X l m X3 m deep, had a void volume of 4 .0 XI05 litres and an estim ated daily in filtra tion capacity o f 2. 0 X 1 0 4 l i t r e s . A c r o s s -s e c t io n o f this type o f tren ch is shown in F ig . 22.

Fig. 22

Cross-section through a liquid disposal trench.

L ow -level solid disposals w ere made also in trenches excavated in the weathered shale. The waste was com posed of all types of rubbish som etim es clad in m etal, wood, p lastic fib res or concrete and som etim es dumped unprotected. Often the waste was placed, covered, and com pacted by backhoe and bu lldozer causing containers to rupture, but only where alpha-contam inated waste was buried was the trench capped perm anently with con crete .

Ground water levels have played an important role in these operations [98] since in the low -ly in g areas the saturated zone was only

8 2


1 m below the su rface and the waste was continuously wet. In the higher areas the position was im proved with the water table at a depth of 3-5 m. Owing to the low perm eability of the shale and the sloping ground profile , any re lease of activity has been caused m ore by su rface water than by sub-surface flow paths into neighbouring stream s. R ainfall has p erco la ted the porou s tre n ch -fill in g and accum ulated until it overflow ed at the low er end, and only where the stream has been v ery c lo se to a trench (3 m) has there been any underground seepage.

In spite o f the apparent escape o f rad ioactiv ity from the so lid disposal area, it is significant that the contribution of contamination to the main drainage creek is seldom detectable above the background activity previously introduced upstream from a water-treatmentplant. Relatively high concentrations of Na+, NOg and SO ^ions found in contaminated ground water sam ples indicate that chem ical leaching has accom panied the re lease of radionuclides.

An im proved type o f d isposal trench was introduced roughly 3 m w ideX 4 . 5 m deep (F ig . 23a and b). H ere the depth o f the trench , though variable, was determ ined solely by the depth to ground water and not by the maximum height to which the side-w alls remained free-standing. The base was covered with 15 cm of gravel and sloped at one end to an asphalt-lined sump in which a perforated casing was installed for water sampling or rem oval. After the waste was dumped voids w ere filled with shale excavated from a neighbouring trench and the top layer was com pacted to re ce iv e the capping, com posed o f 2. 5 cm o f asphalt sprayed on to a layer o f gravel.

At Savannah R iver both liquid and solid disposal have been p ra ctised for many years at a site where the soil is rather less permeable than at the other establishm ents cited owing to a higher clay content in the predom inantly granular so il. The water table lie s generally at a depth o f 13 m in deep sed im entary m ateria ls overly in g m eta - m orphic basem ent rock at a depth o f 3 00 m. The annual rain fall is roughly 125 cm /y e a r . Solid waste has accounted fo r the m ajor p o r tion of the radioactive m aterials disposed into the ground since 1953.

The burial ground covers roughly 0. 5 km2, is bare of vegetation and has a low r e lie f (m axim um cro s s fa ll 7 m ). Surface ru n -o ff is collected in three drainage ditches and because of the puddling action o f heavy equipment the sandy clay adm ixture has form ed a surface layer o f low perm eability that has in creased ru n -off and reduced infiltration [99].

8 3


Fig. 23a

View o f solid disposal trench with underdrain installed.

The solid waste is segregated into alpha and beta-gam m a types and subdivided accord in g to activity le v e ls . It is typ ica l trash , la bora tory waste, fi lte rs , ion -exch an ge res in , alum inium s cra p ,e tc . and its activity is estim ated by radiation readings, e. g. lo w -le v e l waste has le ss than 100 m rad /h at 7. 5 cm . It is packaged with m inim al covering such as cardboard boxes and buried in open unlined trenches 7 m deep. Each d a y 's burial is covered with so il to a depth o f 1 .5 -2 m. The only type of waste that rece ives m ore than nominal protection is sections of fuel rod that are norm ally embedded in concre te . Between 1953 and 1961 an estim ated 5. 77X105 Ci o f fiss ion products has been disposed of, amounting to a volum e of roughly

8 4


2.5 cm ASPHALT CAPPING

____ GROUND_WATER_ LEVEL____

Fig. 23b

Cross-section through solid disposal trench.

15 000 m3 each year. The consum ption o f land to m eet this demand has been about 9 000 m 2 per year.

Since the waste has been suspended in unsaturated so il and subjected only to m inim al in filtration , both leach ing and m igration o f rad ioactive contam inants have been low . T here has been no h o r izontal m igration , on ly a lim ited m ovem ent downward beneath the trenches to depths that are norm ally le ss than 60 cm . The so il was examined by auger and sp lit-b a rre l sam pling and only one place was found where the m igration had been larger (2. 5 m). Thi's was a local ch aracteristic that was absent in adjacent holes 13 m away. A lens o f le s s perm eable so il reduced in filtration during wet season s and caused a transient perched w ater table to develop . In dry seasons the accum ulation d ispersed and the opportunity fo r further leaching dim inished.

The su b -su rfa ce flow path from d isposa l ground to em ergen ce in surface water is 1. 75 km and the natural hydraulic gradient is

8 5


0. 8%. A tracer test with tritium indicates the ground water movement to be roughly 3 cm/day, corresponding to a travel time of more than 125 years before emergence. There is thus little possibility of leached radiocations ever emerging in public water supplies since the effect of ion-exchange retardation would probably increase the travel time by at least an order of magnitude.

Concrete is used extensively as a containment material in many establishments. In Japan solid non-combustible wastes are stored permanently in concrete trenches without treatment. At Lucas Heights, Australia, concrete is used for the disposal of intermediate-level waste where 1. 3-m-square vertical storage holes 13 m deep are formed in massive concrete. Circular vertical storage holes are similarly provided in a large range of diameters from 8 cm to 1 m, varying in depth between 60 cm and 3 m. Here the soil takes no part in shielding or heat dissipation. At Harwell, England, storage facilities include below-ground storage concrete trenches lined with asphalt and covered with removable concrete slabs. They also have vertical steel tubes set in concrete slabs and sealed with plugs of lead or concrete, all facilities being covered to exclude rain and surface water. At Saclay, France, low and intermediate solid waste is baled, inserted in concrete barrels and cast in place with more concrete. Each concrete monolith is then transported to a storage area above ground.

Experience has been gained in Czechoslovakia where wastes have been stored in an engineering cavern in limestone 100 m from a small river and 5 m above it. The formation was ascertained to be free from fissures and the portion used consisted of an access corridor (30m X 2m X 2 m) and a storage chamber (8m X 5 m X 3 m). The floor of both was lined with concrete, having a 1% crossfall to sumps in each section, and although bulkhead doors separated the compartments, adequate air flow by natural ventilation was maintained.

The waste has been stored in cans, segregated according to l i quid or solid and half-lives greater or less than 15 days, and the whole accumulated disposal does not exceed a few curies. By appropriate arrangement of cans the radiation intensity outside the storage chamber has been limited to roughly 10 mR/h. After 5 years' operation and continual monitoring of water that may have collected in the sumps, there has been no escape of radioactivity. Routine examination of aerosols showed that there was no contamination of the atmosphere, so air-filtering devices have been discarded.

8 6


A P P E N D I X IV

METHODS OF SITE INVESTIGATION

The object of this Appendix is to present a review of techniques used to determine the various physical and chemical details in the sub-soils at a proposed disposal site.

An investigation of the soil and ground water characteristics would involve, typically, the following procedures:

(i) Drilling wells and sampling for soil and water in order to define broadly the nature of the deposits, establish the level of the ground water in the upper aquifer and horizons of other waterbearing formations;

(ii) Mechanical and mineral analyses of the soil samples;(iii) Determination of the flow net of ground water and the associated

rates of movement;(iv) A determination of the physical-chemical properties of the soil

with respect to the water quality and the behaviour with certain important radioelements.

When evaluating the probable characteristics of a site, much of the general information required may be already available in agricultural or mining areas, thus simplifying the preliminary survey. The value and usefulness of any site investigation is based essentially on the precision with which the successive horizons of soil may be defined and the degree of homogeneity established. Particularly important are the measurement of variations in permeability and the recognition of narrow impermeable strata that may be of small significance in a classical hydrological survey.

I. DRILLING AND SAMPLING TECHNIQUES

The various methods of soil sampling have developed largely through soil mechanics investigations to determine the stability afforded for heavily loaded structural foundations. In general these have found greatest use in the shallower borings (to 15 m) with occasional deeper investigations for piled foundations. Much effort has been directed to the undisturbed sampling of clays whereas comparatively little attention has been paid to the collection of undisturbed cohesionless materials. The requirements for waste disposal

87


purposes are quite the reverse, but in certain cases the same tools may be used.

Well-drilling has, by contrast, been carried out with less emphasis on detailed sampling, although deeper penetrations are normally registered in the larger-diameter wells. Observations are frequently limited to the examination of bailer samples, wash-water suspension or cuttings and in particular to recognizing the intersection of water-bearing formations. For small-diameter shallow wells it is frequent practice to take no samples and continue penetration until water is struck.

There are a variety of methods for penetrating the soil. The most important of these are as follows.

(1) Jetting method of well construction

The tool and casing consists of a double concentric tube. Water is forced through the inner tube, causing violent local turbulence of water and soil at the nozzle. The return flow passes upwards in the annulus between the tubes and carries the dislodged soil in suspension. Frequently such a device will penetrate sands or similar deposits under its own weight if there is a sufficient flow of water. The rate of penetration may be rapid but the method is quite unsuitable for sampling. The only indication of the sediments penetrated is obtained by visual examination of the washings.

(2) Hand borings

Borings down to about 8 m may sometimes be carried out in the aerated zone with a hand auger. This is probably the simplest method of boring and soil-sampling and yields samples of disturbed soil to provide a good lithologic log of the sediments penetrated.

(3) Cable tool method

One of the most common methods of sinking a boring is with a cable tool or churn drill. The procedure is to drive an iron or steel casing into the soil and then churn the contained soil until it is loosened sufficiently to be extracted by a bailer. The casing may then be driven further and the process repeated but if the soil is tough and compact the material will be churned and extracted below the bottom of the casing before attempting to drive it any further.


The drilling rig is composed of a mast, hoist, walking beam and engine, all mounted on a truck. The bit may weigh up to 1500 kg (2rl0 m long) and is used, with appropriate attachments, as the drive hammer. Where a formation is sufficiently compact the boring may remain free-standing and a casing would be no longer required; boring would continue with special chopping attachments on the bit and the debris would be removed, as before, with a bailer.

The samples obtained are generally highly disturbed and poorly representative except for the most homogeneous of deposits. Sometimes small quantities of water are added to assist churning but this causes the coarse and fine fractions to separate in the samples. Where the soil is churned below the casing it is common to collect a cumulative sample of that horizon mixed with the finer fractions from higher horizons.

Drive barrel techniques are sometimes adopted to avoid these shortcomings [14], In this procedure a drive-type core barrel or tube replaces the normal bit and bailer. The barrel consists of a length of steel pipe with outside diameter close to that of the inside of the well casing (Fig. 24). It is attached to the end of the drill stem and driven into the formation at well bottom by the spudding action of the machine. The barrel is then retrieved and the relatively undisturbed sample removed. The procedure then is repeated and a continuous core sample is obtained. No water is normally added to the hole in the operation although about a litre may be added to ease sampling in some materials. Thus, samples are more representative than when bit and bailer methods are used. Accurate information also may be obtained on density, porosity, particle-size distribution, and natural moisture content. This method is successful in materials ranging from gravel to clay, but when too coarse or clean gravels or consolidated materials are encountered, or when drilling is below the water table, standard bits and bailers generally are used. The drive barrel in many instances results in faster drilling. In contaminated sediments near a disposal site the samples are more meaningful than those obtained with bit and bailer, and the spread of contamination at the surface is minimized by the absence of drilling water.

(4) Rotary drilling

The hydraulic rotary action is the fastest method for drilling in unconsolidated formations. One of its advantages is that no casing

8 9


Drive-barrel sampler used for drilling and sampling at Hanford

Fig. 24

is required. Drilling mud is circulated in suspension through the hollow drive shaft to the rotating bit and it returns in the upward flow in the borehole.

In this procedure a clay lining is deposited on the free-standing walls which, together with the hydraulic pressure, prevents the walls from caving-in. As the circulating mixture emerges at the surface it is conducted by pipe to a settling tank where the cuttings and debris are deposited before the liquid is pumped back into the hole for another circuit. Water and clay are added, as necessary, to maintain the correct consistency.

A typical drilling rig would consist of a derrick or mast, a ro tating table, a pump for the drilling mud, a hoist and the engine. Drilling bits are of various designs appropriate for the soil to be penetrated, but all have hollow shanks and one or more centrally located orifices for jetting the mud into the bottom of the hole.

This method is suitable for borings up to 45 cm diam., but for larger bores up to 1. 50 m diam. the reverse circulation system may be used. In this case the upward flow is within the shaft and the sys-

9 0


tem is more like a suction dredge where the cuttings are removed by a suction pipe and a large-capacity centrifugal pump. This method is used only in drilling unconsolidated soils.

(5) Miscellaneous

Borings are often carried out with less elaborate equipment composed typically of a tripod, hoist and a winch powered by a small- capacity two-cycle motor. Casings are driven by a weight sliding outside a guide on top of the casing and controlled by a rope slung over the pulley and looped a turn or so round the smooth drum of the winch. Soil within the casing may be removed by washing or bailing and if care is taken to remove no soil beyond the toe of the casing, high-quality samples may be withdrawn from the undisturbed soil immediately below.

II. SAMPLING

Soil samplers may be subdivided broadly into side-entry and bottom-entry types. There are numerous variations among the proprietary tools but the basic types are described in standard texts on soil mechanics. In principle, the side-entry sampler will have slots or louvres in the cylindrical wall that will encourage the entry of disturbed soil when the sampler is rotated. These may remain open or in the more elaborate models may be closed after sampling but before withdrawal.

Bottom-entry samplers are the more common since they may collect both disturbed and undisturbed samples. Some types have a bottom opening flap or finger-leaf springs that allow the soil to enter, but prevent if from falling out during withdrawal when the soil is loose and non-cohesive. Other types have an arrangement where the tube may be split longitudinally (split-spoon sampler) and an undisturbed sample may be removed in a metal liner. The sampling tube is always as thin as possible so as to cause minimal disturbance to the soil. In cohesive clays a plain steel tube may be used without risk of losing the sample, but in cohesionless sands special arrangements have to be made to ensure an air-tight seal above the contents' to prevent them slumping from the tube. The Bishop air-sampler [100] is an example of this type that may successfully withdraw undisturbed sand samples from horizons below the water table. Fig. 25

9 1


s a m p l e r b e in g l o w e r e d .AIR BELL ALREADY RESTING ON BOTTOM OF HOLE.

SAMPLER DRIVEN BEYOND END OF CASING. AIR PUMPED IN TO CLOSE VALVE ABOVE SAMPLE. EXCESS AIR EN T ER S BELL TO DISPLACE

WATER.

DRIVING RODS REMOVED.ALL WATER EXPELLED FROM BELL.SAMPLE TUBE WITHDRAWN INTO AIR BELL Si APPARATUS LIFTED TO SURFACE.

M E T H O D O F U S I N 6

C O H E S I O N L E S S S O I L

SA M P L E R

Fig. 25

Compressed air sampler for retrieving undisturbed cohesionless soil samples from beneath the water table.

shows a diagrammatic representation of the sampler, indicating how the sample tube is withdrawn into an air-bell and thus protected from erosion by turbulence. Where bottom-opening samplers are used, the best results are probably obtained if the sampler is forced into the undisturbed soil by a constant hydraulic pressure. However, since such equipment is pften available only in a rotary drilling rig for applying the required pressure on the bit, the sampler may be vibrated down [101] or driven with a drop hammer.

Samplers that extract several samples simultaneously, each from a separate horizon, have proved popular in shallow investigations (to 20 m) in cohesionless saturated soils. They are side-entry types and are housed in a string of drill rods driven into the soil. The soil sampler shown in Fig. 26a collects small disturbed samples adequate for radiochemical analysis, and the water sampler [16] collects rather smaller samples and retains them in an absorbent ma-

9 2


ANNULARSAMPLE

CHAMBER

COUPLING FOR DRILL RODS

DOUBLEPISTON

II

0

a

0

XI

g0

; *f 2mm ENTRY HOLES FOR SAM PLE

ABSORBANT MATERIAL IN ANNULAR SAM PLE CHAMBER

MULTIPLE WATER SAMPLER

Fig. 26

M ultiple samplers.A , B and C are three operations o f a soil sam pler;

D is a water sampler, in which samples are withdrawn by suction into absorbant material.

terial, shown in Fig. 26b. These samplers and other modifications of a similar type [471 have been developed specifically for the examination of detailed ground water movements where the saturated zone lies reasonably close to the surface. Water samplers can be adapted

9 3


from modified piezometers attached to the ends of drill rod [28]. While these tools are re-usable by extraction and redriving, there are other water-sampling devices that may be installed permanently in the soil. They consist typically of a porous receptacle buried in the soil, connected to the surface by two small-diameter plastic tubes. Samples are withdrawn by suction through one tube and assisted, if necessary, by applying a positive pressure to the vent tube.

Water sampling in deep formations is almost invariably carried out in a cased well and the object here is to isolate a particular section by packing devices and sampling between them. It may be possible to withdraw a sample in the same manner, but much depends on the head and permeability of the horizon. If insufficient, the applied pressure would merely force the water back into the surrounding soil. A deep well sampler [14] capable of collecting water from several levels simultaneously, consists of a string of 500-ml sample bottles separated by spacing tubes and actuated by solenoid valves. The device is installed and left in place until equilibrium is restored in the well before the samples are taken. The apparatus is suitable for any depth of water to 150 m.

Where interest is centred on the rate of penetration of radionuclides through the surface deposits, 30-cm cubes of soil may be isolated and autoradiographs taken [102]. The cubes of undisturbed soil would normally be collected by digging a hole in such a way that a central pillar of soil is left standing. This would be trimmed to 30 cm square, a metal cylinder placed round it and the intervening space filled with wax before the cube was cut free at the bottom (see Fig. 27).

III. PERMEABILITY MEASUREMENTS

The permeability of a soil is a measure of the ease with which water may pass through it. It is assessed by the "permeability coefficient" determined either from soil samples in the laboratory or by in-situ tests in the field.

(1) Laboratory, measurement

In the laboratory measurement the permeability coefficient from soil samples may be used as an indirect method of estimating ground water flow in the field by reference to the hydraulic gradient and the

9 4


Fig. 27

Soil cube sampling, for the exam ination o f surficial deposits.

thickness of aquifer. The soil is tested by passing water through an undisturbed sample, measuring the head loss across it and noting the corresponding rate of flow. The apparatus used is a permea- meter, the more usual type maintaining a constant, adjustable, head difference between the flow inlet and outlet. Low flow-rates are recommended, de-aerated water should be used, and both water and sample maintained at a known constant temperature (Fig. 28).

Owing to the vertical orientation of sampling tubes in the soil, the permeability so measured will correspond to vertical flow in the field. The permeability coefficient K is then calculated from the Darcy formula

K - QA(dh/dl)

which has dimensions [L /T ] i .e . velocity.

9 5


A I R - W A T E R IN

1111

-D IFFE R E N T IA L PRESSU RE TRANSDUCER

^>-1/2" W ELL PERFORATIONS

■INFLATABLE SEA L

■FLOW REGULATOR

-AIR CONTROL VALVE

lift

111

AIR FREE WATER

IN

TO DRAIN

fe = R A D IU S OF E Q U IV A LE N T U N C A SED W E LL

h = N E T H EAD IN S ID E P A C K E R

Cc= C O N D U C T IV IT Y C O E F F IC IE N T

h

P, —I Ath

A (area)

(a> (b )

Fig. 28

STOPWATCH t ( time)

Q (volume)

Soil perm eability measurement.(a) Apparatus for packing a section o f a well for hydraulic measurements;(b) Laboratory arrangement for measuring perm eability o f a soil sam ple.


This is an apparent velocity that ignores porosity and assumes flow through the whole cross-section under a 1/1 hydraulic gradient. It is thus no measure of a pore velocity.

The permeability coefficient is frequently expressed as cm /s in the laboratory or m etres/day in the field, but the U. S.Geological Survey expresses it as a rate of volumetric flow through unit cross- sectional area of medium under the same hydraulic impetus [103 J. It is expressed as K s= gal(U. S. )/day ft^at 60°F, under a hydraulic gradient of 1 ft/ft.

The relationship between the permeability at field temperature and that measured at laboratory temperature is identified by the viscosity of water at these temperatures. Thus

K i = Mf_Kf Mi

where /Uf and hi are the viscosities of water at field and laboratory temperatures respectively and Kf and Kj the corresponding permeabilities.

(2) Field measurement

It is sometimes possible to measure the velocity of the ground water directly and if necessary deduce the permeability coefficient from the associated hydraulic gradient and the porosity of the soil.

If the ground water velocity only is required .without reference to direction, the point-dilution method may be applied. In this technique the flow of ground water passing through a well is estimated by observing the rate at which a tracer solution in the well is diluted by this flow. A device has been constructed [104] that permits the continuous monitoring of a radioactive tracer solution introduced into an isolated horizon within the screened section of a well. The radioactive tracer (I131) is injected through a fine nozzle into a chamber which has porous sidewalls and is surmounted by a scintillating crystal, collimator and counter. The solution is thus monitored in situ where the diminishing concentration of Il3iis an indicator of the rate of dilution. A refinement of the apparatus is an incorporated electric mixer that ensures a uniform distribution of tracer within the chamber and avoids differences in concentration developing between the re gions of inflow and outflow.

9 7


Where direction also is required, more "positive" results are attainedby introducing a radioactive tracer at one point and collecting it as it passes another sampling point suitably installed downstream. When a tracer is injected into an aquifer it causes a disturbance to ground water which may be significant to the results if the measured distances are small. With large distances and deep aquifers it may be adequate to introduce large quantities of a tracer into a well and to encourage its diffusion into the surrounding soil by hydrostatic pressure. On small-scale measurements this is not possible; small quantities of tracer solution may be introduced into the soil by using a water sampler, such as that shown in Fig. 13, in reverse. If the injection pressures are low and the introduction continued over an appropriately longer period any disturbance to the natural flow may be acceptably small. If prior experiments have indicated the direction of flow, several injection points may be aligned closely at right angles to the flow path so that the plume of tracer may advance in a broad "front" toward the sampling points. In this way there is less risk that the sampling point downstream is by-passed by an unexpected tortuous streamline.

The choice of a suitable tracer will depend on the local soil and ground water. Although tritium is recognized as the most preferable owing to its low health hazard and virtually identical behaviour with that of the natural water, it may be more practicable to use another radioisotope. Sulphur-35 as sulphate, has proved a valuable beta- tracer in waters that have a naturally low sulphate content.Iodine-131 may be useful for short experiments provided that no organic material is present in the soil. Cobalt-60 is useful when present as an anion (K3Co60(CN)6); its long-life makes it adaptable for long experiments, and its detection is simple, but its use may be limited by the pollution hazard if used in concentrations exceeding the maximum permissible concentrations in drinking water. Many of these advantages are obtained with less hazardusing chromium-51-E.D.T.A. (half-life 28 days) which has been demonstrated [105] as a stable reliable tracer in concentrations down to 0.01 ppm. Other nuclides that havebeen successfully applied are rubidium-86, phosphorus-32, calcium-45, brom ine-82, and the nitro-complexes of nitrosylruthenium-106 [106].

To minimize any disruption to the streamlines of the ground water the "frozen" source [19] has beendeveloped and used experimentally only at shallow depths (2 m). A mixture of active solution and sand is frozen in a tube; it is then lowered or driven to the re

9 8


quired level and the contents are extruded or driven from the tube into the soil. As the source slowly thaws, a plume of radioactive tracer disseminates into the neighbouring soils without disrupting the streamlines, although problems of density and viscosity differences may be more pronounced. Similarly the detection of the tracer may disrupt the streamlines of flow downgradient from the injection point. For strong gamma-emitters the disruption may be negligible if the detecting counters are lowered into dry tubes previously installed in the estimated flow path or if counters are buried beforehand at appropriate points. For beta-emitters and tritium it is necessary to sample the water and the mere withdrawal encourages the movement of replacement ground water to that point. However, for shallow depths the withdrawal of 100-ml samples into evacuated flasks has not caused any noticeable loss in accuracy. The copper-rod method dispenses with liquid sampling [19], The method is based on the chemisorption reaction between the tracer and a metal. It has been successfully applied with I131 being adsorbed on copper rods driven in the soil. By installing a network of bars in the estimated path of a plume and removing them later, the boundary of the plume may be accurately delineated.

An alternative to permeability tests on samples or the direct measurement of water velocity by tracers is the pumping test in which an aquifer is subjected to an artificially induced flow pattern and measurements are taken of flow and pressure to deduce the permeability of the surrounding soil.

When water is pumped from a well the water level drops in the casing and in the surrounding annulus of soil and more water flows in radially from the neighbouring soil to replace that previously removed. The hydraulic impetus for this radial flow is observed in the profile of the water table, which in gravity aquifer may be close below the surface in the surrounding area, becoming progressively deeper towards the well. This profile is called the draw-down curve and may be plotted from the water levels in observation wells suitably placed around the pumped well. In a pumping test the discharge from the pumped well is continuously recorded and pumping is sustained, if possible, until the draw-down curve and the pumping discharge approach an apparent equilibrium. For these conditions the Thiem equation relates the permeability of the formation with the profile of the draw-down curve and the aquifer thickness. However, since "equilibrium" is seldom, if ever, achieved it is more usual to use the Theis non-equilibrium formula

9 9


e~uduu

u

where. h0 - h is the drawdown

r2S u - -r~r4?rtQ = Well discharge S = Storage coefficientT = Coefficient of transmissibility (T = Kb where b=thickness

of aquifer) t = Timer = Distance between observation and pumped well.

The integral may be expanded as a convergent series of which the first two terms only^need be considered in most cases. There are however approximate methods of solution developed by Theis, Jacobs and Chow all of which employ tables or graphs.

The Theis solution is simplified to

In standard hydrological texts the comparable values of W(u') against u are tabulated and graphical solutions enable T and S to be determined.

CLAY MINERALS - Hydrous alumino-silicate minerals having a characteristic layered structure and often a marked ion-exchange capacity. Different sequences of the layers containing the aluminium and silicon oxy-anions result in different mineral species (kaolinite, illite, e t c .). Different structures also exist because of different

h0 - h = 114. 6 X ^XW(u)

where u = 1. 87 .

GLOSSARY OF TERMS

1 0 0


cationic composition and arrangement. The term must be distinguished from CLAY sometimes used to describe the fine fraction of a soil (<0. 2 yum) which may or may not be composed primarily of clay minerals.

CRIB - A structure used to introduce liquid waste's into the ground. It may be constructed as a buried box with an open bottom or as a buried, perforated pipe laid in a long trench. The fact that it is earth-covered distinguishes it from a SEEPAGE POND which is open. The rate of seepage of wastes into the ground from these structures is termed the PERCOLATION RATE.

DEFLOCCULATION OF SOIL - The chemical action of wastes on the soil floes, whereby the latter are peptized and disintegrated into finer materials which tend to seal the formation by plugging the soil pores.

DISPERSION - The summed effect of those processes of diffusion and mixing which tend to distribute dissolved materials from wastes through an increasing volume of ground water. The ultimate effect appears as a DILUTION of the wastes by ground water.

EXCHANGE CAPACITY - The total number of equivalents of exchangeable ions contained in a unit weight of soil or minerals. The quantity found will sometimes vary with the method of analysis employed. The EQUILIBRIUM CONSTANT is the mass action constant for the equilibrium achieved when a soil is contacted by waste solution. If the ion of interest in the waste solution is of very much lower concentration in the system than that of the ion initially saturating the solid phase, its sorption behaviour may often be usefully described in terms of the EQUILIBRIUM DISTRIBUTION COEFFICIENT (Kd). This is the amount of the subject ion sorbed per gram of solid divided by the amount of ion in solution per cm? of liquid after equilibrium is achieved.

GRAVITY AQUIFER or WATER TABLE AQUIFER - A waterbearing formation in hydraulic equilibrium with the atmosphere. This is in contrast to an ARTESIAN AQUIFER, which is isolated by im permeable formations from surrounding aquifers or the atmosphere.

GROUND WATER - This term may refer strictly to any water lo cated in the ground, but generally is used with reference to the water

1 0 1


in the zone of saturation. The ZONE OF SATURATION refers to that part of the regional, unconfined water body in which the pressure head is greater than atmospheric. It is generally characterized as the zone in which the pores are completely filled with water. The CAPILLARY ZONE or the ZONE OF PARTIAL SATURATION includes the formations which contain unbound water held by surface tension or forces of capillary attraction in partly filled pores. The water pressure head is less than atmospheric. The term WATER TABLE, although not consistently defined in the same way, is usually used with reference to the level at which water stands in boreholes which just penetrate the upper zone of saturation.

HOMOGENEOUS MEDIUM - A porous medium in which the permeability is the same at every point. Formations having unequal permeabilities at different points are termed HETEROGENEOUS. A formation which, in addition to being homogeneous, has permeabilities of the same magnitude along all axes is termed an ISOTROPIC MEDIUM.

HYDRAULIC EQUILIBRIUM - The dynamic equilibrium achieved in a flowing aquifer in which the piezometric gradient and pressure head at every point is constant with time.

ICRP LIMITS - This term refers to the published recommendations of the International Commission on Radiological Protection concerning the recommended concentration limits of various radioisotopes in drinking water. The drinking water concentration limits are derived from body burden limits for radiation workers and must be interpreted in conformity with the recommendations published by ICRP as the report of Committee II (1959).

KARSTIC SYSTEM - The structure of a region with an underlying or superficial limestone formation which has been water-eroded to form sink holes, caverns, and irregular channels of importance in the transmission of water.

LEACHING - The erosion or dissolution of material from a solid. The term may be used to describe the gradual erosion of buried solid waste or the removal of sorbed material from the surface of a solid or porous bed.

LITHOSPHERE - A broad, general term which refers to the upper

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solid part of the earth. In a waste management context it is used more loosely in describing storage and disposal practices which apply to the ground as opposed to wastes discharged to the hydrosphere or atmosphere. The material composing upper parts of the lithosphere may be referred to as SUB-SOIL underlying a layer of SOIL as used in an agricultural sense. Occasionally the term soil is found in re ference to all forms of unconsolidated or semi-consolidated earth materials. An identifiable unit or stratum of material may be termed a ROCK. No standard usage exists for terms referring to earth materials of various grain sizes, e.g. the term GRAVEL refers to material which may range from 0. 2 to 7. 5 cm in average diameter.

PIEZOMETRIC GRADIENT - The first derivative in the downstream direction of the piezometric head, which is defined as:

where p is hydraulic pressure, p 1 is fluid density, g is the gravitational scaler and Z is the position head. The term enters into Darcy1 s equation for laminar flow in a porous medium, which states that the rate of flow is proportional to the piezometric gradient. In this equation the proportionality constant is termed the PERMEABILITY for the case of saturated flow systems, and the equivalent function in the case of partially saturated systems is termed the CAPILLARY CONDUCTIVITY. For partially saturated systems the term CAPILLARY PRESSURE is defined as the difference between the pressure in the adjacent air phase and p/p 1 g.

PRECIPITATION SCAVENGING - A chemical treatment whereby trace concentrations of radioactive ions may be partially removed by a co-precipitation process. The precipitate is chosen to have a high affinity for incorporating the ions of interest. The actual re moval may in some cases be described as an adsorption process on freshly formed precipitates. Occasionally a very flocculent precipitate proves to be useful for such treatment probably by trapping colloidal species; here the treatment is sometimes referred to as FLOCCULATION.

SELECTIVITY - A term that refers to the quality of a porous bed of solids and describes its ability to remove preferentially certain

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ions from waste solutions in the presence of other competing ions. It is often expressed as a comparison between the chemical behaviour of two different ions, e. g. the selectivity for caesium ion in the presence of sodium ion. Another expression sometimes used as a synonym is SPECIFICITY. The quantity referred to by these expressions differs, but is sometimes the ratio of distribution coefficients or equilibrium constants for the ion of interest relative to that of the competing ion.

SORPTION - A broad term referring to reactions taking place within pores or on the surfaces of a solid. Its use avoids the problem of technical distinction between absorption and adsorption reactions. ABSORPTION is generally used to refer to reactions taking place largely within the pores of solids, in which case the capacity of the solid is proportional to its volume. ADSORPTION refers to reactions taking place on solid surfaces so that the. capacity of a solid is proportional to its surface area. An example of the latter is ION EXCHANGE, whereby ions occupying charged sites on the surface of the solid are displaced by ions from solution.

WASTE DISPOSAL - The disposition of waste materials without specific provision for recovery. Radioactive wastes may be SOLID WASTES, which include sludges, LIQUID WASTES, or GASEOUS WASTES. The philosophy of radioactive waste disposal is to assure safety through provision of adequate, rapid dilution or dispersion (e. g. atmospheric) or provision of long decay time before reappearance of the material in a populated environment (e.g. ground disposal), or a combination of the two. The terms is also used loosely to include WASTE STORAGE, which should be applied where provision is made for recovery. The broad term WASTE MANAGEMENT is recom mended for use when all aspects of the wastes problem are considered: waste collection and handling, treatment and processing, storage, disposal, monitoring, and economics.

R E F E R E N C E S

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KNOLL, K. C . , Adsorption o f strontium by soils under saturated and unsaturated flow conditions, USAEC rep. HW-67830 (1960).MacEWAN, D. M. C . , Complexes o f clays with organic compounds. Part 1. Complex formation between montmorillonite and halloysite and certain organic liquids, Trans. Faraday Soc. 44 (1948) 349-67.MacEWAN, D. M. C . , The montmorillonite minerals, from X-ray Identification and Crystal Structures of Clay Minerals, (Brindley, G. W ., Ed.) The Mineralog. Soc. (Clay Minerals Group), Taylor and Francis Ltd., Lpndon (1951) 86-137.MARSHALL, C. E ., The Colloid Chemistry of the Silicate Minerals, Academic Press, N. Y. (1949). McHENRY, J. R ., Properties o f soils of the Hanford project, USAEC rep. HW-53218 (1957). NEWTON, T. D ., On the dispersion of fission products by ground water, Atom ic Energy of Canada Ltd. rep. AECL-909 (1959).PARAMONOVA, V. I. et a l . , The effect of pH on base exchange in chernozem , Kolloid Zhur. (USSR) 6 (1940) 249-58.REISENAUER, A. E ., Laboratory studies of Hanford waste cribs, USAEC rep. HW-63121 (1959). ROEDDER, E ., Problems in the disposal o f acid aluminium nitrate h igh-level radioactive waste solutions by injection into deep-lying permeable formations, USGS Bull. 1088 (1960). SORATHESN, A. et al. , Mineral and sediment affinity for radionuclides, CF-60-6-93 ( July 1960). SOUFFRIAU, J. et al. , "Investigations on the movement of radioactive substances in the ground. Part n, The copper-rod method for measuring ground water flow, Proc. 2nd Conf. Ground Disposal o f Radioactive Wastes, Chalk River, Canada, USAEC rep. TID-7628 (1960) 155-65.TAMURA, T . , "Strontium reactions with minerals", Proc. 2nd Conf. Ground Disposal of Radioactive Wastes, Chalk River, Canada, USAEC rep. TID-7628 (1960) 187-97.TAMURA, T. and JACOBS, D. G .. Improving cesium selectivity o f bentonite by heat treatment, Hlth Phys. 5 (1960) 149-54.UNITED STATES NATIONAL ACADEMY OF SCIENCES, Committee on Waste Disposal. The Disposal of Radioactive Waste on Land (1957).USSR MINISTER OF HEALTH, Health and Safety Regulations Governing Work with Radioactive Materials and Sources of Ionizing Radiation (1960).

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