R EM E DI AT1 NG

AT MANUFACTURED GAS TA R =C 0 N TA M I N AT E D SO I LS

rior to the widespread use of natural gas, comhusti- hle gas manufac tured P from coke, coal, and oil

1 served as the major gas- eous fuel for urban heat-

ing, cooking, and lighting in the United States for nearly 100 years ( I ) . This manufactured gas, or town gas, was produced at some 1000 to 2000 plants. Pipeline distribution of natural gas following World War I1 replaced manufactured gas as the major gaseous fuel, and as a result manufactured gas production came to an end in the 1950s (2) .

Today, soil and groundwater con- tamination problems exist at many former manufactured gas plant (MGP) sites because of prior process operations and residuals manage- ment practices (3 ) . Residuals that were produced in MGP processes are summarized in Table 1 for the three primary gas production meth- ods: coal carbonization, carbureted water gas production, and oil gas production. These process residuals are dominated by s ix primary classes of chemicals: polycyclic aro- matic hydrocarbons (PAHs), vola- tile aromatic compounds, pheno- lics, inorganic compounds of sulfur and nitrogen, and metals. Tar resid- uals were produced from the vola- tiIe component of bituminous coals in coal carbonization, from the resi- due of gasifying oils in oil gas pro- cesses, and from the cracking of en- riching oils used to increase gas Btu content in carbureted water gas pro- duction.

MGP tars are organic liquids that typically are denser than water, with a range of physical and chemi-

Gnaiienges cal properties dependent on the feedstock and operating conditions of the production process (3-5). Al- though some MGP tar was used on site or sold, during certain periods there was insufficient demand for all the tar that was produced. Fur- ther, because of changes in tar com- position owing to changes in feed- stock, problems with tar-water emulsions, and other factors, the intrinsic value of MGP tars was of- ten considered marginal. Conse- quently, MGP tars were sometimes managed off site or were deposited on site in tar wells, sewers, nearhy pits, or streams. Nuisances associ- ated with the disposal of tarry gas- plant wastes to streams and sewers were recognized early in this cen- tury (6) , and a committee on waste

R I C H A R D G . L U T H Y D A V I D A . D Z O M B A K

C A T H E R I N E A . P E T E R S S U J O Y B . R O Y

A N U R A D H A R A M A S W A M I Carnegie Mellon University

Pittsburgh, PA 15213

D A V I D V . N A K L E S Remediation Technologies, Inc.

Pittsburgh, PA 15238

B A B U R . N O T T Electric Power Research Institute

Polo Alto, CA 94303

disposal was formed in 1919 at the first annual meeting of the Ameri- can Gas Association to address the resulting water pollution problems (7, 8).

This paper discusses remediation of MGP sites and focuses on tar- contaminated soils at such sites. To- tal remediation costs for individual MGP sites are in the range of tens of millions of dollars, and the Gas Re- search Institute has estimated that nearly 70% of such costs may he at- tributed to the management of tar- contaminated soils and sediments (9) . We describe the nature and ex- tent of the tar contamination that is often present at these sites, review existing and evolving remediation strategies for management of tar contamination, and examine cur- rent understanding of tar soluhiliza- tion phenomena that determine the risk associated with tar-contami- nated soils as well as the effective- ness of water-based remediation technologies.

Tar contamination at MGP sites Each MGP site has unique as-

pects, hut common operating and waste management practices of the past have led to similar patterns of soil contamination. Evidence of these similarities has become ap- parent over the past several years as the Gas Research Institute (GRI), the Electric Power Research Institute (EPRI), and others have investigated the technical aspects of MGP site management.

A subsurface cross section of a

266 A Environ. Sci. Technot., VoI. 28, No. 6. 1994 001 3-936X194/0927-266A$04.50/0 0 1994 American Chemical Society

typical MGP site, compiled from a review of information for 25 sites (IO), is shown in Figure 1. An MGP site usually consists of a layer of fill (a mixture of soil, ash, and demoli- tion debris) underlain by a layer of sand and gravel and a layer of silty clay or other fine-grained material. Shallow unconfined aquifers USI ally exist at MGP sites and are h draulically connected to a watt body. The groundwater table is c ten shallow, 5 to 15 feet beneath tl surface.

Today, most of the structures ar the equipment at MGP sites ha1 been removed except for some su surface portions of structures th remain in the fill and the upps layer of the unconsolidated materi- als. These structures (e.& tar-water separator tanks), present at depths of up to 30 feet, often contain tars mixed with demolition debris or soil.

Contamination of soil with tar also occurred at MGP sites because of leaks and spills from on-site vessels and piping networks, in- complete separation of tar from aqueous liquids, storage of tar in unlined pits or shallow wells, and dismantling and decommissioning act ivi t ies when the p lan t was taken out of service. In addition, tar would sometimes be mixed with other site wastes and used as fill in the low-lying areas of a plant site.

Tar released into the subsurface by the above processes migrates downward as a result of gravity un- til it encounters a low permeability layer that it cannot penetrate be- cause of large capillary forces [Fig- ure I). If present in sufficient quan- tity the tar may pool on the low- permeability material or move laterally, following the geologic gra- dient (dip) of this material ( 1 1 ) . Contact of groundwater with the tar results in dissolution of tar constitu- ents and generation of contami- nated groundwater plumes.

Tar contamination along river banks and in the shallow sediments of rivers and lakes has also been found near MGP sites. These obser- vations primarily reflect the dis- charge of tars directly into the adja- cent water bodies through site sewers or ditches. Much of the tar that escaped the plants in this man- ner did so as incidental carryover of hydrocarbon-water emulsions from the tar separators. Migration from tar wells and subgrade gas holder tanks also contaminated some streams.

L Field workers at pile of coal lor

Process residuals f r o m the manufacture o f gas from coal, cok and oil

Gas manuiactUrinQ process

carbonization water gas Oil gas PhystcsI farm and principal Cqal Carbumled

chemical cowen1 I Ash

Carburetec gas tar Coal tar

Aqueous liquid: I - - Inorganics, phenolics X'

V V Solid: metals (and unburned coke or coal) Organic liquid: PAHs, B E X 4 Organic liyid: PAHs. BTEX, an phenolics

Coke and L~~~ Solid: pyrolyzed coal breeze Lampblack Sludge: elemental carbon

and oil t a r Light oils Organic liquid: BTEX

Or anic liquid: PAHs. Oil tar

Spent oxide or lime, Solid: metals. cyanide, wood chips sulfur, tar (support media) Tar sludges Solid-liquid: PAHs, BTEX Tar-oil-water Aqueous and or anic emulsions liquids: PAHs, BQEX Wastewater Solids, aqueous, and treatment sludges

="X" indicates that residual was produced;

B A X

organic liquids: inor anics, phenolics, PAHs. BYEX X X

indicates that residual was not produced in su hurlmrArhnns' RTFX = henlane toluene. ethylbenzene, and XYI

Environ. Sci. Technol.. Vol. 28. No. 6, 1994 267 A

, ."-..L ,

Generalized cross section of a typical manufactured gas plant site

300 - 500

Coal tar disposal site J- c

Management of MGP sites

While characterization of tar con- tamination problems at MGP sites has progressed, there have been only limited attempts at remedia- tion. At the same time, there has been a significant amount of inves- tigation of remediation approaches. Comprehensive reviews of potential remediation technologies for use at MGP sites have been prepared (12- 141. Most of these technologies have been proposed based on their per- formance on comparable nonaque- ous-phase liquid (NAPL) wastes from industries such as petroleum refining, wood treating, byproduct coke manufacture, and synthetic fuel production. There has been substantial laboratory and pilot- scale research on the effectiveness of the most promising technologies for application at MGP sites. Some large-scale field trials and demon- stration projects are in progress as well (see box). Such efforts are try- ing to address the now widely rec- ognized technological difficulties with remediation of dense NAPLs in the subsurface.

To date, full-scale site remedia- tion activities conducted at MGP sites have been limited in scope and have emphasized the isolation and removal of source materials with management of off-site migration via groundwater pump-and-treat systems. Source materials include free-phase liquid hydrocarbons or tars, soils contaminated with tars,

and iron oxidelcyanide wastes. Source material has been isolated largely through the use of slurry walls and, in one instance, using in situ stabilization (14) . Source re- moval has included emptying of subsurface structures, excavation of heavily contaminated soils, and di- rect pumping of liquid tars.

Removed source materials have been managed in land disposal fa- cilities or, occasionally, as raw ma- terials or fuel in the production of aggregate, hot-mix asphalt, or ce- ment. Several utilities have exam- ined limited applications of the management of source materials in utility boilers (16) . The groundwa- ter treatment systems that have been installed consist primarily of recovery wells followed by treat- ment using hydrocarbon-water sep- aration, air stripping, andlor carbon adsorption.

Research efforts have focused on the development of lower cost methods for site remediation, espe- cially alternatives to excavation and incineration, and to land disposal for treatment of tar-contaminated soils. Various ex situ treatment techniques have been investigated, including thermal desorption (1 7- 201, biological treatment in slurry reactors (21), and water-based soil washing (22). In situ treatment ap- proaches also have been investi- gated, not only because of potential cost effectiveness but also because contaminated soils cannot always he accessed easily due to the pres-

ence of surface structures, utility lines, or other physical barriers. These bench and pilot studies have emphasized in situ flushing with aqueous solvent or surfactant solu- tions to enhance the rate and extent of tar solubilization (10, 231 and in s i tu hioremediat ion for water- soluble tar contaminants (24) . Tech- niques aimed at volatilization of or- ganic contaminants from soils (e.g., steam stripping or vapor extraction) are not expected to be effective for removal of the high molecular weight, low-volatility tars encoun- tered at MGP sites.

Research on in situ methods is largely focused on removal or de- struction of tar at residual satura- tion. Although free tar may be di- rectly pumped from selected locations in the subsurface (2.51, of- ten the greatest mass of tar is that held at residual saturation by capil- lary forces. Residual saturations of dense NAPLs (DNAPLs) are typi- cally 5-25% of the pore volume ( 1 1 ) . Such tar is effectively immo- bile and fairly insoluble, but its dis- solution is sufficient to contaminate large amounts of groundwater for decades and longer. Mobilization of the residual tar phase by interfacial tension lowering (e.g., through sur- factant addition] or by viscosity re- duction [e.g., through heating) is a possible remediation approach, but the difficulty in controlling the downward movement of a mobi- lized dense organic liquid may pose an unacceptable risk (26).

268 A Environ. Sci. Technol.. Vol. 28, No. 6 , 1994

ne of the more significa

yracuse, NY, with the support RI and EPRI, the utility trade late Energy Electric Research id EPA.

asphalt that was produced I---

Limitations of water-based remediation techniques

It is generally recognized that complete groundwater restoration may be technically impracticable at DNAPL-contaminated sites. In fact, this observation was a primary fac- tor that led to the issuance of a re- cent guidance document by EPA for the evaluation of the technical im- practicability of groundwater resto- ration (271. EPA acknowledges in this document that DNAPLs “often are particularly difficult to locate and remove from the subsurface” and “very long restoration time frames (e.g., longer than 100 years) may be indicative of hydrogeologic or contaminant-related constraints to remediation.” EPA notes that res- toration to stringent levels mav not

available remediation technologies. The research efforts of EPRI, GRI,

and others have confirmed that the performance of water-based remedi- ation technologies such as extrac- tion and biodegradation with tar- contaminated soils is qu i te dependent on the specific physical and chemical characteristics of the soil-contaminant matrix. The pri- mary factors that limit the perfor- mance of these technologies are the extent and rate of contaminant dis- solution from the DNAPL into the aqueous phase (10, 21, 28). At the same time, however, if the most wa- ter-soluble contaminants in a multi- component DNAPL are substan- tially removed by contact with water, leaving only the higher mo- lecular weight, essentially insoluble comuonents, then treatment with

tamination does not pose an unac- ceptable ecological or public health risk.

Although contaminated but non- leachable residual may result from ex situ water-based treatment of tar- contaminated soils, tars and other DNAPLs at residual saturation may continue to leach contaminants af- ter treatment with water-based re- mediation technologies. Restoration goals may be established at contam- inant concentrations that are un- achievable using these technolo- gies, necessitating alternative management strategies based on source control (27).

To better understand the limits of the water-based remediation tech- nologies, researchers have studied the extent and rate of tar solubiliza- tion from contaminated soil in con- tact with water, and have examined the enhancement of tar solubiliza- tion that can be achieved using sol- vent-water and surfactant-water solutions. Collectively, the effort has provided insight into the funda- mental processes governing tar dis- solution, mobility, and lability in soil-water systems and bas permit- ted an assessment of the effects of these processes on the remediation of tar-contaminated MGP sites. In the remainder of this paper, we ad- dress dissolution phenomena in the context of in situ and ex situ reme- diation technologies involving con- taminant extraction and biodegra- dation. We d o not attempt to address the full range of remedia- tion approaches for such sites.

Tar dissolution in water: Equilibrium

Tar is a mixture of hundreds of compounds, primarily PAHs; only a portion can be identified and quan- tified through chromatographic methods (29, 30). Although repre- sentation of the tar mixture as a sin- gle system component is valid for some applications (30, 311, tar dis- solution is usually described in terms of individual compounds (32-34). Naphthalene, or benzene, is often of interest because of its rel- atively high aqueous solubility, which may provide a worst-case in- dicator of groundwater contamina- tion. The focus on individual com- pounds i s also necessary for assessing groundwater contamina- tion because cleanup standards are often specified in terms of concen- tration of individual compounds.

The aqueous solubilitv of constit- always he achi&able at some of these sites because of limitations of

wat<r;-based remedial technologies may yield soils whose residual con-

uent compounds in multicompo- nent NAPLs has been discussed in

Environ. Sci. Technol.. VoI. 28, No. 6, 1994 269 A

recent years (32-36) . Predictive methods for estimating equilibrium aqueous solubilities of compounds from complex NAPL mixtures are founded on the Raoult’s law as- sumption of ideality in the NAPL phase. If it is assumed that the mo- lecular interactions in the organic phase are similar to those in a liquid phase of the pure solute, the equi- librium relation for solute i becomes (37):

where Xi.. is the mole fraction of solute i in the water phase, X i is the mole fraction of i in the tar phase, and X ,”: is the mole fraction equiva- lent of the aqueous solubility of pure liquid i. Assuming that the aqueous phase is sufficiently dilute such that the volume of the solution is approximately equal to that of pure water (38 ) , the aqueous con- centration expressed as a mole frac- tion is proportional to mass concen- tration. An equation similar to Equation 1 can be written

where C is the mass concentration of i in the water phase (mg/L) and SF is the aqueous solubility of pure liquid i (mg/L). The tar constit- uent mole fractions are computed

where wt%j is the weight percent- age of i in the tar, MWt is the average molecular weight of the tar, and MW, is the molecular weight of compound i.

For many tar compounds, Sf is a hypothetical quantity because these compounds are solids in the pure state at ambient temperatures. An expression relating Sf. to Sf, the pure solid aqueous solubility at the system temperature, is derived from the thermodynamics of solid-liquid equilibrium where the standard state in the liquid phase is defined as the pure subcooled liquid at the temperature of the solution ( 3 9 ) . Applying this relation to Equation 2 results in

by X f = (wt%, / IOO)(MW, / MWJ,

where the term in parentheses is the ratio of the pure component fugaci- ties in the subcooled liquid and the solid states. Fugacity ratios are of- ten available in the literature (40) or can be approximated by an expres- sion that accounts for the free en-

270 A Environ. Sci. Technol., Vol. 28, No. 6, 1994

ergy change between the liquid and the solid state (39) using a constant entropy of fusion for organic com- pounds ( 4 2 ) . Raoult’s law has been shown to estimate with fair accu- racy equilibrium concentrations of organic solutes dissolving into the aqueous phase from different tars, though significant deviations have been observed for some samples

The bulk tar dissolved concentra- tion is the sum of the concentrations of all the dissolved species, Using composition and MW, data for free, pumpable tar recovered from the subsurface at an MGP site in Stroudsburg, PA, Equation 3 gives benzene and naphthalene as the most soluble constituents (2.4 and 3.9 mg/L, respectively) (30). The to- tal solubility of the Stroudsburg tar is estimated to be about 16 mg/L based on gas chromatography/mass spectroscopy estimates of constitu- ent weight percents that cumula- tively account for about half of the tar mass and the majority of the more soluble species (30) . These re- sults illustrate that MGP tars are rel- atively insoluble in water, but be- cause many of the constituents may pose human health hazards, the low aqueous concentrations may be of concern.

Tar dissolution in water: Kinetics Equilibrium NAPL solubility as

described in Equation 3 represents the maximum potential for contam- ination of pure water in contact with MGP tar. However, the rate of dissolution of tar constituents often determines the impact of tar-con- taminated soils on groundwater quality and the effectiveness of wa- ter-based treatment processes, in- cluding bioremediation. The trans- fer of these tar-derived solutes to the aqueous phase may occur slowly through a complex set of physical and chemical processes, as indi- cated in Figure 2. This schematic represents the processes of dissolu- tion, diffusion, sorption, particle- water mass transfer, and biodegra- da t ion , Assuming sequent ia l processes, the overall rate of tar mass removal will be controlled by the slowest step.

Column studies. Dissolution of NAPLs trapped in porous media is often mass transfer limited, as has been convincingly demonstrated in laboratory column experiments (42-44). This rate-limited dissolu- tion has been described using inter- phase mass transfer relationships that employ as the driving force the

(32-34).

difference in concentrations be- tween the bulk aqueous phase, C”, and the concentration that ~7ould be in equilibrium with the organic phase, C:;, as given by Equation 3 . The dissolution rate of a species (mass/time/volume), is written as a driving force times the mass trans- fer coefficient, k, (length/time), times the specific surface area, a (length2/length3) (45). Because in most contamination scenarios the specific surface area for interphase mass transfer is unknown, a lumped mass transfer coefficient, k,a (time -’), is employed. A general ex- pression describing the space and time dependence of aqueous phase concentrations of tar constituents in one-dimensional water transport through a porous medium is

where v is the average linear flow velocity (lengthhime) and D, is the longitudinal dispersion coefficient (length’/time). The value of the lumped mass transfer coefficient k,a depends on a number of physi- cal and chemical properties of the system, including the volumetric saturation of NAPL (volume of NAPL/volume of porous medium), the velocity of the flowing aqueous phase, and the diameter of the po- rous medium grains.

Various correlations have been de- veloped to relate mass transfer coeffi- cients to system properties (42-44, 46). Experimental observations and results from empirical modeling of NAPL dissolution rate data indicate that volumetric NAPL saturation (which can vary with time) has the greatest influence on the value of the lumped mass transfer coefficient (43, 44, 46). For the more soluble species, C rq in Equation 4 may decrease with time as the species mole fraction in the NAPL diminishes because of dis- solution. For tars that consist essen- tially of water-insoluble material, the volumetric fraction of tar in porous media, average molecular weight, and density will not change substan- tially owing to species dissolution into water.

In describing dissolution mass transfer from NAPLs under non- equilibrium conditions, it is com- monly assumed that the organic

k a t i c showing mass transfer and microbial degradation of solutes from nonaqueous-phase id in porus media (not to scale)

phase is homogeneous and that there are no organic-phase resis- tances to mass transfer. Laboratory experiments to assess mass transfer models such as those described above typically have utilized sim- ple one- or two-component NAPLs. Based on these models, it is often inferred that conditions of disequi- librium in field situations result from uneven distribution of NAPL in aquifer media and aqueous-phase diffusional limitations (42, 47-51). NAPL-phase mass transfer resis- tance is not addressed in most cur- rent models, though such resistance may he significant.

Slurry studies. Tar-water mass transfer of naphthalene has been s tudied in wel l -s t i r red, flow- through reactors containing mi- croporous silica beads imbibed with coal tar. Lumped mass transfer coefficients for this system can he obtained from a mass balance equa- tion for aqueous phase naphthalene in the slurry reactor:

where q is the volumetric flow rate (length3/time) through the reac- tor and V is the reactor volume [length3).

Slurrv reactor tests indicate that the rate of naphthalene mass trans- fer from tar to water decreases with

Pore water Coal tar in Biofilm macroporr /

Dissolution t

time during a test, with larger media size, or with "aging" of the tar prior to testing. The effect of aging on mass transfer may be related to the development of tar-water interfa- cial films or skins.

Recent photographic observations of the tar-water interface have re- vealed an interfacial film or skin. The interfacial film is apparent when a tar droplet is aged in water for several days or more and tar is withdrawn slowly from the interior of the droplet ( 5 2 ) . The develop- ment of an interfacial film may af- fect mass transfer because of com- positional differences between the bulk mixture and the interface, and because of the development of in- terfacial skin resistances to mass transfer. Efforts to characterize the effect of tar interfacial resistance by measuring the rate of release of naphthalene in slurry tests have shown reduced tar-water mass transfer rates upon aging of tar in water for a period of a week or longer (52).

Current research efforts are fo- cused on characterizing the film composition and quantifying its re- sistance to mass transfer. In the pe- troleum industry, oil-water interfa- cial film formation is thought to influence wettability, permeability, and flow properties of crude oil (53). Thus, in addition to influenc- ing tar-water mass transfer rates, it is possible that interfacial aging phenomena may also affect tar mi- gration in soils and sediments.

Enhanced dissolution with solvents and surfactants

Because the aqueous solubilities of MGP tar components are so low, in situ remediation of MGP tars at residual saturation is impractical with approaches based on water flushing (54) . Also, attempts to pro- mote bioremediation with nutrients alone may not be very effective be- cause most biodegradation pro- cesses in systems with organic liq- uid phases present occur in the aqueous phase. Recognition of these limitations has prompted studies of the use of solvents and surfactants to enhance the rate and extent of MGP tar solubilization.

Solvents. Water-miscible polar solvents can significantly increase the solubility of hydrophobic com- pounds whether present as solutes within tar or compounds sorbed on soil. The effect of cosolvents on the solubility of PAH compounds has been represented by the slope of a log-linear solubility relationship (55)

where the logarithm of the ratio of the mole fraction solubilities in the solvent-water solution (sw) to that in pure water (w) is proportional to the volume fraction of the cosol- vent, Z. The cosolvency power (01 is theoretically predicted and experi- mentally verified to be proportional to the logarithm of the solute's octa-

Environ. Sci. Technol., VoI. 28, No. 6. 1994 271 A

nol-water partition coefficient (55, 56). Thus, the more hydrophobic compounds exhibit a larger en- hancement of solubility in a given cosolvent solution relative to less hydrophobic compounds.

The practical significance of this result is an increase in partitioning from tar to the solvent-water phase (Le., a decrease in the partition coef- ficient K,,,, = C ' / C ""), with in- creasing solvent concentration for more hydrophobic compounds. This has been demonstrated in ex- periments with MGP tar and n-butyl- amine-water solutions with solvent concentrations of 20% or more by volume, in which K,,s,v for naphtha- lene, phenanthrene, and pyrene ap- proached a common value (30). In the same study it was shown that the weight percent distributions of 27 PAH compounds in tar were ex- tracted approximately to the same extent. For modeling purposes, this allowed the tar to be represented as a single pseudocomponent in sys- tems with appreciable solvent (31, 35). Tar dissolution in aqueous so- lutions of the solvents acetone and 2-propanol was substantial in terms of being orders of magnitude greater than bulk tar solubility in water.

Surfactants. Surfactants may fa- cilitate cleanup of NAPL contami- nants either through solubilization in surfactant micelles for enhance- ment of aqueous extraction (57, 58) or through reduction in interfacial tension to promote tar flow (26). Al- though data for MGP tars per se are lacking, the concepts appear applicable. It has been estimated that displacement of DNAPLs re- quires a substantial reduction in the DNAPL-water interfacial ten- sion, necessitating large doses of surfactant. This problem, combined wi th concern about control of downward movement of mobilized dense organic liquid, has directed research mostlv toward use of sur- factants for enhancement of the solubility of NAPL constituents (26, 57, 581.

In soil-water systems, surfactants can exist as dissolved monomers, molecules sorbed on soil or at the NAPL-water interface, or aggre- gated groups of molecules in solu- tion called micelles. With surfac- tants present, tar constituents can he solubilized in surfactant mi- celles, dissolved in surrounding so- lution, sorbed directly onto soil, or sorbed in association with sorbed surfactant (59, 60). The interiors of surfactant micelles can compete strongly with the NAPL phase as a

I W H I L E

IARACTERIZATION

OF TAR

EONTAMINATION

E ilTES HAS

compartment for the partitioning of hydrophobic solutes, but significant solubility enhancement is achieved only at surfactant doses yielding the cr i t ical micel le concentrat ion (CMC), the concentration at which micelles hegin to form in the aque- - ous phase.

The oartitionine of solute be- - tween soil and the aqueous phase in a surfactant solution is described by the solute distribution coefficient in the presence of surfactant, Kd,cmc, and the distribution of solute be- tween the micelles and the aqueous solution surrounding the micelles, K, = X "/X ,, which is the ratio of the mole fraction in micelles to the mole fraction in water (61). Non- ionic surfactant solubilization of PAHs from soil and sediment media has been modeled using these parameters and expressions for sur- factant sorption (59). A similar ap- proach can be utilized to describe enhanced solubilization by surfac- tants in NAPL-water systems.

Tar dissolution and site remediation

The rate and extent of tar dissolu- tion in water can influence the need for remediation as well as the feasi-

bility and efficiency of such opera- tions at MGP sites. When remedial operations are found to be neces- sary, tar-water mass transfer limita- tions may limit the efficiency of wa- ter-based remediation techniques. In this section, we examine the role of tar-water mass transfer phenom- ena in some water-based chemical extraction and biotreatment tech- niques under investigation for pos- sible use at MGP sites.

Chemical extraction methods. Removal of subsurface tars at or near residual saturation by injection and recovery of aqueous solutions of surfactants or solvents to en- hance solubilization of constituents may be possible, but could be per- formed only at sites where the flow and recovery of the solutions can be controlled with confidence. More- over, it is clear from bench-scale ex- periments that large concentrations of solvent or surfactant would he re- quired to achieve substantial recov- eries of tar mass by dissolution. Fairly large doses of surfactant are required to promote enhanced solu- bility of PAH compounds in the presence of soil because of sorption of surfactant on the soil (59). In the presence of an organic liquid phase, partitioning of the surfactant to the organic liquid could occur, possibly resulting in even higher required surfactant doses.

The rate-limited dissolution of NAPLs such as tar in porous media is a key issue that has emerged from small-scale studies of NAPL extrac- tion (42,44,57) and is sure to be rel- evant to field-scale applications, all the more so because of subsurface heterogeneities. In column tests with tar-contaminated porous me- dia, concentrated aqueous solutions of n-butylamine were required for effective recovery of tar (44). Even with very high solvent concentra- tions (including pure solvent), tar removal from short columns re- mained incomplete after the pas- sage of 50 pore volumes of solvent- water solution (Figure 3) , despite the tar being potentially completely miscible with the n-butylamine- water solutions at the concentra- tions employed in the column ex- periments (30). Thus, if the solvent- water solutions could contact all the tar, complete dissolution would occur in just a few pore volumes. Both physical and chemical (inter- facial) resistances are likely to con- tribute to the observed mass transfer limitation. Physical limitations to NAPL dissolution can result from fingering, in which the solvent solu-

272 A Environ. Sci. Technol., Vol. 28, No. 6, 1994

FIGURE 3

I Cumulative tar removal versus solvent volume passed through 3.5-em packed columns at two initial saturations

0.8

tion moves preferentially through particular pathways, bypassing some NAPL, and from the occur- rence of NAPL in dead-end pores, requiring diffusive transport to the flowing solution.

Similar phenomena would be ex- pected to influence recovery rates for any in situ process for tar extraction. Mass transfer limitations to soluhili- zation in ex situ solvent extraction of soil slurries are likely to be less than in situ applications, but will still oc- cur. The limited available literature on ex situ systems provides little in- formation on this topic.

The potential influence of disso- lution mass transfer limitations on the performance of a field-scale in situ solvent extraction system for tar removal from the subsurface has been examined through computer modeling (62) . A mass transfer model fitted to the tar dissolution rate data described above was incor- porated in a two-dimensional reac- tive transport model to simulate the performance of different injection- recovery well deployment schemes at a hypothetical MGP site. Simula- tions performed indicate that for

conditions typical of MGP sites (a shallow, unconfined aquifer with coarse soils: tar at residual satura- tion; and small areal extent of con- tamination), implementation of in situ solvent extraction with injec- tion and recovery wells would re- quire one to several years for signif- icant recovery of tar mass from the subsurface (62).

These results show that even for high solvent concentrations and the assumption of a small site and ho- mogeneous site conditions, slow dissolution kinetics governed by mass transfer will necessitate long peri,ods of in situ extraction to re- move tar mass at typical residual saturations. Actual recovery times in heterogeneous systems are likely to he longer. Although long periods of continuous treatment are not de- sirable, it is evident that injection of solvent-water solutions can be a significant improvement over using pump-and-treat methods with pure water flushing, provided the flow of solvent-water solutions can be ade- quately controlled.

Biotreatment. It has been demon- strated in controlled aerobic micro-

cosm studies with acclimated or- ganisms that various PAHs can be degraded readily as aqueous-phase compounds (63). However, observa- tions on above-ground bioremedia- tion of soils contaminated with tar suggest that PAH phase partitioning andlor sorption may limit the rate and extent of microbial degradation processes (64, 65). Experimental ex situ studies of biological treatment of PAHs in liquid cultures and solid matrices associated with MGP site soils have indicated that a range of 2- through &ring PAHs may biode- grade when present in aqueous so- lution, hut removal from a solid- NAPL matrix is less predictable and, generally, much less efficient (66, 67).

On the basis of such data, it is hy- pothesized that matrix effects are preventing the release of the PAHs into the aqueous phase where they may undergo biodegradation. In other words, it is mass transfer limi- tations associated with the release of the contaminants that limit the rate of removal of PAHs, rather than the explicit aqueous-phase hiodeg- radation kinetics. The use of non- ionic surfactants to enhance the rate of biodegradation of PAH com- pounds in soil slurry systems has not proved beneficial. This may be the result of the effects of surfac- tants on microbial membranes or a consequence of competitive utiliza- tion of surfactant as substrate (68).

Biotreatment studies with MGP site soil samples in slurry reactors indicate that mass transfer limita- tions can exist even in well-mixed systems where aqueous phase diffu- sional resistances are minimal (21, 67). This is inferred from the extent of biodegradation approaching a limiting value, after which there is little change in PAH content in the soil matrix. In such laboratory tests, freshly added PAHs are degraded readily, indicating that toxicity is not a limiting factor. In slurry biore- actor tests lasting more than eight months, significant reductions in aqueous phase PAH concentrations have been observed, while the con- centration of PAH compounds in soil-associated tar exhibited a dis- proportionately small decrease in concentration (211. It appears from such work that the release of PAH compounds from the tar phase is re- stricted.

Thus, data from laboratory and pilot-scale ex situ studies in soil- water-tar systems with site samples point to the problem of a threshold soil concentration of PAH com-

Environ. Sci. Technol., VoI. 28. No. 6. 1994 273 A

pounds below which biodegrada- tion becomes either very slow or ceases. Qualitatively, the results of the above studies suggest that this problem may be related to tar-water mass transfer limitations. A more quantitative approach is required to assess the effects of mass transfer phenomena on biotreatment rates and endpoints.

An integrated dissolution-bio- degradation model has been devel- oped to couple concurrent mass transfer and biokinetic phenomena occurring in tar-water slurry sys- tems (691. Consistent with recent findings on the biodegradation of sorbed hydrophobic organic com- pounds in soil-water systems (70- 72, and review in 731, the model considers primarily the aqueous- phase organic substrate to be readily available for biodegradation (see Figure 2). Concurrent dissolu- tion and biodegradation can be rep- resented as (69):

where kbi, is a pseudo-first-order biodegradation rate constant, (time -'I, assuming stable, viable microorganisms not l imited by other factors such as availability of oxygen or nutrients, or inhibited by tar constituents or degradation products. This expression recog- nizes that biotransformation in tar- water systems depends on solnbili- zation phenomena related to mass transfer of solutes from the organic phase to the bulk aqueous phase. Other important factors are bioki- netic phenomena related to toxicity, inhibition andlor the competitive utilization of tar-derived substrates, and toxic-inhibitory effects of tar solutes andlor their degradation products.

The overall rate of biotransforma- tion may be limited by either mass transfer or biokinetic phenomena. This may be described by a dimen- sionless group akin to a Damkohler number

biodegradation rate mass transfer rate

(8 )

which indicates mass transfer con- trol for values much greater than unity, and biokinetic control for values much less than unity. Naph-

- -

I

I !

1

LIMITED IN SCOPE

AND HA\

WPHASIZED THE .is ISOLATION AND

REMOVAL w r

thalene biomineralization slurry tests with small, microporous silica beads and tar from the MGP site in Stroudsburg, PA, indicate an initial value of Do << 1; thus, initially naphthalene mass transfer occurs much faster than biomineralization. After a period of time, biomineral- ization decreases, which points to the need to identify alternative rate- controlling processes as related to biological phenomena such as nu- trient supply, inhibition, and com- petitive substrate utilization, as well as experiments with larger par- ticle sizes and other PAH com- pounds to verify the utility of the dissolution-degradation model in evaluating the role of physicochem- ical mass transfer processes on the biotransformation of tar-derived or- ganic solutes. A combination of mass transfer and biokinetic phe- nomena may limit the extent of slurry biotreatment of tar-contami- nated soils. Whether this is a seri-

ous problem for in situ bioremedia- tion will depend on the established goal for the site remediation.

Future prospects The physical and chemical nature

of MGP tars, especially their den- s i ty , viscosity, and solubi l i ty , makes remediation of MGP sites a technological challenge. Of particu- lar interest is removal or destruc- tion of the residual tar remaining af- ter all "free" tar in the subsurface has been pumped out. The environ- mental impacts of MGP tars at resid- ual saturation in the subsurface and the effectiveness of water-based processes in extracting or degrading these tars are largely related to the extent and rate of tar solubilization.

Existing and emerging tecbnolo- gies that have been or may be em- ployed for remediat ion of tar- contaminated soils can be classified into the following six groups: exca- vation and off-site disposal or incin- eration, containment, groundwater pump-and-treat, bioremediation, chemical extraction, and thermal desorption. The first three have been applied to several MGP sites and other NAPL-contaminated sites, and reasonably clear guide- lines for their operation exist. The last three groups, however, are still being developed, and limited infor- mation exists on their performance at MGP andlor similar sites. Tech- nology development for remedia- tion of tar-contaminated soils is fo- cused pr imari ly on chemical extraction and biodegradation, both in situ and ex situ, which offer po- tential for source removal and de- struction at costs lower than those associated with thermal desorption.

Laboratory and pilot-scale re- search that bas been performed in support of development of solvent or surfactant extraction and biore- mediation techniques indicates clearly that the rate and extent of solubilization is a critical issue with respect to these technologies. Soln- bilization is important to the basic feasibility of bioremediation for tar- contaminated soils, as microbial degradation of tar compounds sorbed on soil or dissolved in or- ganic liquid is very much slower than degradation of the same com- pounds in the aqueous phase.

Available data indicate that mass transfer limitations to dissolution of MGP tar at residual saturation in soils can be very substantial, even with high concentrations of organic solvent in the aqueous phase. Mass transfer limitations are reduced but

2 7 4 1 Environ. Sci. Technol., VoI. 28, NO. 6 , 1994

Richard G. Luthy is professor nnd lirnd oJ civil engineering ot Curnegie rLlrllon Universify. and a iiieinher OJ the Edito- rial Advisory Board of Environmental Sciencr? and Technology. He liolds ( I

P1i.D. in civil engineering Jronz tlir I J n i - versify of California at Berkeley. Hi.< re- search Joriises on fl ip solubi1if.v and hiotransJorinrtfiort oJ Iiydropliohic or- ganic compounds in .soil-n.nter swtrnis.

David A. Dzombak (11 is an ~ i z s o ~ ~ i a t r professor in f l i t , Ilcportnirnt oJ Civil E m ginrering at Cnmegie Mellon Uiiiwrsity. He holds a Ph.D. in civil-environntrntol engineering from f l ir iMmsochosefts 111- sfitrite of Technology. His research ex- amines f h e internctions OJ uqirroiis sol- utes with solids in groundnmter syfems and in woter and soil treatment pro- cesses.

Catherine A. Peters (J i s a postdoctoral research fellow i n environriientol and wafer resources engineering at the Uni- versity ofMichigan. She holds a P1i.D. in civil engineering and engineering ond pirblic policv Jrom Camegie iMellon Uni- versity. Her research Jocuses on n o m aqueous-phase containinants. and the physical and c h r m i c d processes go". erning phose partitioning in n io l f i con- ponent systems.

n

I Ld David V. Nakles (IJ is o principal of Re- niediiifion 'Teclinologirs. Inc. He man- ages 116D p r o p r i i s reluted to the mi- ronmental nionagenient of inrmiJocfiired gas plant sifes Jor the Grrs Research Insti- tute and the Electric Power Research In- sfitufe, mnd is involved with remedial in- vesfigotion a n d des ign Jor ufilit,v conipanies. He holds a Ph.D. in climiicol engineering and engineering and puhlic policy from Camegie ,MeIlon Universit),.

Bobu R. Noff (r). a project nionager in the Mhste and Water iManagenient Program oJ the Environment Division at the Elecfric Power Research Institofe. manages air and water toxics oioniforing. wnstewafer trpaf- ment. and iMGP sife rrmediotion. He holds a P1i.D. in chemical enginerrine fmni Case Western Re.vewe liniversify.

far from eliminated when tar-con- taminated soils are slurried with wa- ter for treatment in an above-ground reactor. It is not clear which are the primary mechanisms contributing to these limitations: diffusion in the NAPL phase, resistances at the NAPL- water interface. or microporous dif- fusion in the aqueous phase.

Research to date casts doubt on the potential utility of in situ chemical

extraction or hioremediation of tar- contaminated soils at MGP sites, at least with the usual notion of in situ treatment (Le.. use of injection and recovery wells with continuous flow) and with the usual goal of restoring the site soils and groundwater to near background levels. Remediation ap- proaches involving ex situ treatment of soil slurries in reactors appear more promising, but dissolution

mass transfer limitation is an issue with these technologies as well.

All this points to the need for more funtlaniental understanding of tar- water interfacial phenomena and mass transfer, and for engineering de- velopment of ways to enhance disso- lution mass transfer rates, for both in situ and ex situ applications. Alterna- tive approaches to in situ remediation, for example the use of in situ soil mix- ing with injection and recovery of re- agent or nutrient chemicals in well- mixed soil columns (74, 75'1, need to be investigated. Soil quality criteria for remediation goals need to be devel- oped in recognition of technological feasibility and healthlecological risks based on expected land use. and al- lowance for source control (27).

Acknowledgments Prcpimtion of this paper was supported by EPRl through Contract RP 3072-2 to Carncgie Msllnn University and through Contract 3072-4 to Remediation Tech- nologies. Inc. The authors' work de- scribed i n the paper was supported by EI'RI. GRI. the U.S. Geological Survey (Award No. 14-08-0001-G1913), the U S . Department of Energy (DE-FG22- 9OPC90303). the National Science Foun- dation (Grant No. BCS-9157086). Tex- aco Research and Development (Beacon. NY). and Baltimore Gas and Electric Co. Suhhasis Ghoshal and Ashraf Ali con- tributed to the research investigation at Carnegie Mellon. Edward F. Neuhauser of thr Niagara Mohawk Power Corpora- tion provided the description of their K&D program on field testing of MGP re- mediation tcchnologies.

The views and conclusions contained in this document arc thoso of the au- thors and should not be interpreted as necessarily representing the official pol- icies, either expressed or implied, of the U S . government.

References

(1) Khodrs. E. 0. In The Ifislory of Coal Tor and Light Oil; Hoiberg. A. 1.. Ed.; Krieger: Huntington. NY. 1979 Chap- ter1.

(2) Hondhook on MonuJoclurPd Gas Plant Sites: Edisan Electric Institute. Utility Solid Waste Activities Gmup Superfund Committea: Washington, IX, 1984. Harkins. S. M. et al. US. Production of iMnnuJoctured Gases: Assessment of Past Disposal Practices: U S Envi- ronmental Protection Agency: Cincin- nati. OH, 1988: EPA/finn/2-88/012.

I41 Chemical ond Physical Chomcteristics oJ?br Samples Jmm Selected Manufac- turrd Gas Plant (MGPl Sites: final report lor EPRl contract RP 2879-12; META Environmental: Watertown. MA, 1993.

I51 >M"Monogement of MonuJocturrd Gas Plant Sites: Gas Research Institute: Chicago. IL. 1987: Vol. 1. Hansm. P. American Gos Light lour- nal 1916. April IO. 228-29. Will ien. L. J. In Proceedings of the Amrricnn Gos Association; First An-

(3 )

(fi)

(7)

Environ. Sci. Technol.. VoI. 28. No. 6 . 1994 275 A

nual Convention, Oct. 13-17, 1919; Technical Section; American Gas Aso- ciation: New York, 1919; pp. 274-80. Willien, L. J. In Proceedings of the American Gas Association; Second Annual Convention, Nov. 17-20, 1920: Technical Section; American Gas As- sociation: New York, 1920; pp. 413-64.

(9) Remediation Alternatives and Costs for the Restoration of MGP Sites; Top- ical Report No. GRI 0910098; Gas Re- search Institute: Chicago, IL. 1990.

(10) Luthy, R. G. et al. Solvent Extraction for Remediation of Manufactured Gas Plant Sites; final report, EPRI TR- 101845. Project 3072-2; Carnegie Mel- lon University: Pittsburgh, PA, 1992.

(11) Mercer, J. W.: Cohen. R. M.], Contam. Hyrol. 1990, 6 , 107-63.

(12) Management of Manufactured Gas Plant Sites; Gas Research Institute: Chicago, IL, 1987; Vol. 4.

(13) Remediation Technologies for Organ- ics Contamination; Final Report No. GS-6700; Electric Power Research In- stitute: Palo Alto. CA, 1990.

(14) Assessment of Selected Technologies for Remediation of Manufactured Gas Plant Sites; Final Report No. GS-7554: Electric Power Research Institute: Palo Alto, CA, 1991.

(15) Murarka, I. P. et al. 1. Hazard. Mater.

(16) Fuel-Use Options for the Manage- ment of MGP Site Wastes: Gas Re- search Institute: Chicago, IL, 1989; Topical Report No. GRI-8910023.

(17) Laboratory Study of Thermal Desorp- tion Treatment of Contaminated Soils from Former Manufactured Gas Plant Sites; Topical Report No. GRI-8810161; Gas Research Institute: Chicago, IL, 1988.

(18) Engineering-Scale Evaluat ion of Thermal Desorption Technology for Manufactured Gas Plant Site Soils; Topical Report No. GRI-89/0271; Gas Research Institute: Chicago. IL, 1989.

(19) Laboratory S t u d y of the Effect of Thermal Treatment on the Metal Leaching Characteristics of Soils from Manufactured Gas Plant Sites; Topi- cal Report No. GRI-8910243; Gas Re- search Institute: Chicago, IL, 1989.

(20) Investigation of Rate Processes in the Thermal Treatment of Contaminated Soils; Topical Report No. GRI-901 0112; Gas Research Institute: Chicago, IL, 1990.

(21) The GRI Accelerated Biotreatability Protocol for Assessing Conventional Biological Treatment of Soils: Devel- opment and Evaluation Using Soils from Manufactured Gas Plant Sites; Topical Report No. 9210499; Gas Re- search Institute: Chicago, IL, 1992.

(22) EPRIiAlberta Research Council Clean Soil Process; Final Report No. TR- 101802; Electric Power Research In- stitute: Palo Alto, CA, 1992.

(23) Villaume, J . F. “Recovery of Dense Nonaqueous L iqu ids by In -S i tu Flushing”; Proceedings of the Tech- nology Transfer Seminar; Electric Power Research Insti tuteiGas Re- search Institute: Atlanta, GA, 1991.

(8)

1992, 32, 6-13.

(24) Murarka, I. EPRI J , Sept. 1990. (25) Villaume, J , F. Ground Water Monit.

( 2 6 ) Fountain, J. C. In Detection andResto- Rev. 1985, 5, 60-74.

ration of DiVAPLs in Groundwater at Hazardous Waste Sites; Technical Publication; Water Environment Fed- eration: Alexandria, VA, 1992.

1271 Guidance for Evaluating the Techni- cal Impracticability of Groundwater Restoration; Directive 9234.2-25, Of- fice of Solid Waste and Emergency Response. U. S. Environmental Pro- tection Agency: Washington, DC. 1993.

(28) Rao, P.S.C.; Loehr, R. C. Estimating the Release of Polycyclic Aromatic Hydrocarbons from Coal Tar at Man- ufactured Gas Plant Sites; EPRI TR- 101060, Project 2879-07; Electric Power Research Institute: Palo Alto, CA. 1992.

(29) Taylor, B. B.; Lee, L. S. In Proceedings of the EPRIIGRI Technology Transfer Seminar on Management of Manu fac- tured Gas Plant Sites; Atlanta, GA, 1991.

(30) Peters, C. A,; Luthy, R. G. Environ. Sei. Technol. 1993, 27, 2831-43.

(31) Peters, C. A,: Luthy, R. G. Environ. Sei. Technol., in press.

(32) Groher, D. M. M.S. Thesis, Dept. of Civil Engineering, Massachusetts In- stitute of Technology. Boston, MA, 1990.

(33) Lane, W. F.; Loehr, R. C. Environ. Sei. Technol. 1992, 26, 983-90.

(34) Lee, L. S.; Rao, P.S.C.; Okuda, I. En- viron. Sei. Technol. 1992, 26, 2110- 15.

(35) Peters. C. A. Ph.D. Dissertation, Car- negie Mellon University, Pittsburgh, PA, 3992.

(36) Mackay, D. et al. I . Contam. Hydrol.

(37) Banerjee, S . Environ. Sci. Technol.

(38) Chiou, C. T. ; Schmedding, D. LV.;

1991, 8 , 23-42.

1984, 28, 587-91.

Manes, M. Environ. Sei. Technol. 1982, 26, 4-10.

(39) Prausnitz, J~ M.; Lichtenthaler, R. N.; de Azevedo, E. G. Molecular Thermo- d,vnamics of Fluid-Phase Equilibria, 2nd ed.; Prentice-Hall: Englewood Cliffs, NJ. 1986; Chapter 9.

(40) Mackay, D.; Shiu, W. Y.; Ma. K. C. Il- lustrated Handbook of Physical- Chemical Properties and Environ- mental Fate for Organic Chemicals; Lewis: Boca Raton, FL, 1992; Vols. 1-2.

(41) Yalkowsky, S. H. Ind. Eng. Chem. Fundam. 1979, 28,108-11.

(42) Powers, S . E.; Abriola, L. M.; Weber, W. J. Water Resources Res. 1992, 28,

(43) Geller, J. T.; Hunt, J . R. Water Resour. Res. 1993, 29, 833-46.

(44) Roy, S. B.; Dzombak, D. A,; Ali, M. A. Water Environ. Res., in press.

(45) Treybal, R. E. Mass Transfer Opera- t ions, 3rd ed.; McGraw Hill: New York, 1980.

(46) Miller, C. T.; Poirier-McNeill, M. M.; Mayer, A. S. Water Resour. Res. 1990,

(47) Hunt, J. R.; Sitar, N.; Udell, K. S. Wa- ter Resour. Res. 1988, 24, 1247-58.

(48) Mackay, D. M.; Roberts, P. V.; Cherry, J. A. Environ. Sci. Technol. 1985, 19 ,

(49) Wilson, J. L. et al. Laboratory Investi- gation of Residual Liquid Organics from Spills, Leaks and the Disposal of

2691-705.

26, 2783-96.

384-92.

Hazardous Wastes in Groundwater; U.S . Env i ronmen ta l Protect ion Agency: Washington, DC, 1990; EPA/

(50) Johnson, R. L.; Pankow, J. F. Environ. Sci. Technol. 1992, 26, 896-914.

(51) Anderson, M. R. ; Johnson, R . L.: Pankow, J. F. Ground Water 1992, 30.

(52) Luthy, R. G. et al. Environ. Sei. Tech- no]. 1993, 27, 2914-18.

(53) Kimbler, 0. K.; Reed, R. L.; Silberberg, I. H. SOC. Pet. Eng. I . 1966, 153-65.

(54) Mackay, D. M.; Cherry, J. A. Environ. Sei. Technol. 1989, 23, 630-36.

(55) Fu. J. K.; Luthy, R. G. I . Environ. Eng. 1986, 222, 328-45.

(36) Morris, K. R. et al . Chemosphere

(57) Pennell, K . D.; Abriola, L. M.; Weber, W. J. Environ. Sci. Technol. 1993. 27,

(58) Abriola, L. M.; Dekker, T. J.: Pennell, K. D. Environ. Sei. Technol. 1993, 27,

(59) Edwards, D. A,; Liu, Z.; Luthy, R. G. 1. Environ. Eng. 1994. 120, 5-22.

(60) McCarthy, J. F.; Wobber, F. J . , Eds. Manipulation of Groundwater Col- loids for Environmental Restoration: Lewis: Boca Raton, FL, 1993.

(61) Edwards, D. A,; Luthy, R. G.; Liu, Z. Environ. Sei. Technol. 1991, 25, 127- 33.

(62) Ali, M. A.; Dzombak, D. A,; Roy, S. B. Water Environ. Res., in press.

(63) Cerniglia, C. E. Biodegradation 1992,

(64) Erickson, D. C.; Loehr, R. C.; Neu- hauser, E. F. Water Res. 1993, 27, 931-19.

(65) Gauger, K. et al. In Proceedings of the Second International IGT Symposium on Gas, Oil, Coal and Environmental Biotechnology; New Orleans, LA, 1989.

(66) Smith, J~ R.; Weightman, R. L. Co- treatment of Manufactured Gas Plant Site Groundwaters with Municipal Wastewater; Technical report; Gas Re- search Institute: Chicago, IL, 1988.

(67) Sherman, D. F.; Loehr, R. C.; Neu- hauser, E. F. In Proceedings of Inter- national Conference on Physicochem- ical and Biological Detoxification of Hazardous Waste; Atlantic City, NJ,

(68) Laha, S.; Luthy, R. G. Biotech. Bioeng.

(69) Ramaswami, A,; Ghoshal, S.; Luthy, R. G. Water Sci. Technol., in press.

(70) Mihelcic, J. R.; Luthy, R. G. Environ. Sci. Technol. 1991, 25, 169-77.

(71) Rijnaarts, H.H.M. et al. Environ. Sei. Technol. 1990, 24, 1349-54.

(72) Ogram, A. V. et al. Appl. Environ. Mi- crobiol. 1985, 49, 582-87.

(73) Mihelcic, J . R. et al. Biodegradation

(74) Jasperse, B. H.; Ryan, C. R. In Pro- ceedings of the ASCE Specialty Con- ference on Grouting, Soil Improve- ment, and Geosynthetics; American Society of Civil Engineers: New York, 1992.

(75) Siegrist, R. L. et al. In Proceedings of Emerging Technologies in Hazardous Waste Managment IV; I&EC Division Symposium, American Chemical So- ciety, Atlanta, GA, 1992.

600-6-901004,

2 5 0-5 6.

1988, 17, 285-98.

2332-40.

2341-51.

3, 351-68.

1988, pp. 1035-45.

1992, 40,1367-80.

1993, 4, 141-55.

276 A Environ. Sci. Technol., Vol. 28, No. 6, 1994