Overbank sediments: a natural bed blending sampling medium ... Metal and RBF... · sediments...

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Chemometrics and Intelligent Laborat

Overbank sediments:

a natural bed blending sampling medium for large—scale

geochemical mappingB

B. Bblvikena,*, J. Bogenb, M. Jartuna, M. Langedalc, R.T. Ottesena, T. Voldena

aGeological Survey of Norway, NO-7491 Trondheim, NorwaybNorwegian Water Resources and Energy Administration, P.O. Box 5091 Majorstua, NO-0301 Oslo, Norway

cCity of Trondheim, NO-7004 Trondheim, Norway

Received 15 December 2003; received in revised form 28 May 2004; accepted 17 June 2004

Available online 15 September 2004

Abstract

Overbank sediments occur along rivers and streams with variable water discharge. They are deposited on floodplains and levees from

water suspension during floods, when the discharge exceeds the amounts that can be contained within the normal channel. Overbank

sediments were introduced as a sampling medium in geochemical mapping in 1989, and a number of studies have later been published on this

subject. These papers indicate:

1. Depth integrated samples of overbank sediments reflect the composition of many current and past sediment sources upstream of the

sampling point, contrary to active stream sediments, which originate in a more restricted number of presently active sediment sources

from which they move regularly along the stream channel. In many regions overbank sediments are more representative of drainage

basins than active stream sediments and can, therefore, be used to determine main regional to continental geochemical distribution

patterns with widely scattered sample sites at low cost per unit area.

2. Samples of overbank sediments can be collected in floodplains or old terraces along laterally stable or slowly migrating channels. In

some locations the surface sediments may be polluted, however, natural, pre-industrial sediments may, nevertheless, occur at depth.

Mapping of the composition of recent and pre-industrial overbank sediments can, therefore, be used (i) in a characterization of the

present state of pollution, and (ii) as a regional prospecting tool in natural as well as polluted environments.3. Vertical movements of elements in strata of overbank sediments may occur, especially in cases where the distribution of relatively mobile

elements in non-calcareous areas are heavily influenced by acid rain. However, the overall impression is that vertical migration of

chemical elements is not a major problem in the use of overbank sediments in geochemical mapping.4. The composition of overbank sediment is of great interest to society in general, since flood plains are very important for agriculture,

urbanisation, and as sources for drinking water.

Several of the above points indicate that overbank sediments represent a natural analogue to the products of bed-blending. This aspect is

mentioned here in light of the Theory of Sampling (TOS).

D 2004 Elsevier B.V. All rights reserved.

0169-7439/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.chemolab.2004.06.006

B Please note that the figures in this article appear in full colour in the

online version on www.sciencedirect.com.* Corresponding author. Tel.: + 47 6116 4709; fax: +47 6116 8236.

E-mail address: [email protected].

1. Introduction

Geochemical mapping includes (1) systematic sampling

of natural materials, such as rocks, sediments, soils and

waters; (2) chemical analysis of the samples; and (3)

illustration of the analytical results on maps. Geochemical

ory Systems 74 (2004) 183–199

B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199184

maps show significant natural distribution patterns with great

contrasts. Such distributions occur for many chemical

elements at various scales from local up to continental. This

property of geochemical data implies that geochemical maps

are of great interest to society in general, especially in fields

such as (1) environmental research, (2) exploration of mineral

deposits, (3) medical geology (geomedicine), (4) agriculture

and (5) land use planning. These aspects are discussed below.

Fig. 1. Lead content (mg/kg) in stream sediments from the Gal3a river andits tributaries, Hedmark county, Norway. After Bjbrlykke et al. [5].

1. The use of geochemical maps in environmental research

is based on the fact that pollution implies addition to the

environment of any substance at a rate that results in

higher than natural concentrations of that substance [1].

Consequently, data on the spatial variations in the

composition of uncontaminated natural materials are a

necessary prerequisite for an evaluation of the degree of

pollution within small and large areas. Geochemical

maps based on chemical analysis of natural materials

provide such information.

2. In exploration, geochemical maps may disclose geo-

chemical provinces or geochemical anomalies with

greater than normal contents of heavy metals or other

elements of economic interest. Follow-up investigations

in such environments may lead to the discovery of

workable deposits.

3. It is widely accepted that some local or regional

environments may be sub-optimal for the health of

human beings and other animals. Relations between

goitre and iodine deficiency and between caries and

fluorine deficiency are classical examples of this kind.

Geochemical maps provide information for research in

this emerging field of geomedicine, which is also called

medical geology.

4. In agriculture, information about variations in the

chemical composition of soils may be utilized in

production planning, since some chemical elements are

vital for plants and domestic animals, while others may

be harmful when present in too high concentrations.

5. In land-use planning, geochemical maps may contribute

to information about the suitability of specific areas for

specific uses.

It was previously assumed that the composition of active

sediments regularly moving along the stream channel

represents the geochemistry of large parts of the drainage

basin upstream of the sample site. As a consequence,

regional geochemical maps were often based on analysis of

active sediments collected at certain intervals along streams

of high order (usually 1–20 km2 catchments) [2].

In 1989 Ottesen et al. [3] reviewed this procedure and

claimed that active sediments in stream channels do not

reflect the chemical composition of large parts of drainage

areas, since they often originate in discrete sources of limited

extension. They suggested, however, that overbank sediment

would be a more representative type of sample. Many papers

have since been published concerning the use of overbank

sediment as a geochemical sampling medium. Our contribu-

tion summarizes the main results demonstrated in these

papers with emphasis on representativity and sampling errors

(reproducibility), which are also important aspects of the

Theory of Sampling (TOS) [4].

In order to put these results into perspective, we give three

examples of published geochemical maps at various scales, a

short summary about sampling density in geochemical

mapping and some comments on problems that may occur

in the use of chemical analysis in geochemical mapping.

2. Geochemical mapping

2.1. Examples of geochemical maps at various scales

The literature has many examples of geochemical maps

at scales ranging from local to continental. Three examples

of such maps are shown here in order to present the type of

data that may be obtained by geochemical mapping.

Fig. 1 shows a geochemical Pb anomaly in active stream

sediments disclosed during a mineral exploration project in

Southern Norway [5]. The anomaly comprises Pb contents

of 270–680 mg/kg, which is much higher than the

background levels in the surrounding 30 km2 (10–20 mg

Pb/kg). Follow-up investigations led to the discovery of an

earlier unknown Pb deposit. Even though the deposit later

proved to be sub-economic, the example shows that

geochemical mapping is a potential prospecting tool.

Fig. 2 is a geochemical map of Cr reproduced from the

geochemical atlas of Northern Fennoscandia [6]. The atlas

contains 136 single-point geochemical maps at a scale of

1:4,000,000 based on the contents of total and acid

extractable elements in four different types of sample (till,

active stream sediments, stream organic matter and stream

moss) collected at 5000–6000 sites within an area of

250,000 km2 (1 sample station per 50 km2). Systematic

distribution patterns were obtained for most elements. The

maps for the contents of an element in various sample types

are with only few exceptions similar—even after using

different chemical digestion methods on the original

samples or heavy mineral fractions of composites of

ø

Fig. 2. Chromium content in stream sediments from Northern Fennoscandia. After Bblviken et al. [6].

B.Bølviken

etal./Chem

ometrics

andIntellig

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System

s74(2004)183–199

185


geographical neighbours. This indicates that the consisten-

cies and thus the reliabilities of the maps are good. The

maps, which were originally obtained for use in mineral

exploration, are also of great interest in other fields, such as

environmental research.

Fig. 3 shows the distribution of K in the conterminous

USA. The map is based on chemical analysis of 1300 soil

samples collected across the entire country [7]. In spite of the

low sampling density (1 sample per 5000 km2), systematic

distribution patterns are disclosed for this and also for several

other elements. Taking the sampling density into account, the

reliability of the K pattern appears to be acceptable when

compared with a map of K acquired by airborne radiometric

surveys, which include millions of measurements [8], (see

Figs. 4 and 5). Some of the obtained geochemical patterns

coincide with known geological structures, while others

indicate structures, which had not previously been recog-

nized. The maps have an interesting potential use in mineral

exploration, environmental research and epidemiology (geo-

medicine, medical geology).

2.2. Sampling density in relation to survey area

The number of samples per unit area (sampling density) in

geochemical mapping has been a matter of controversy

between geochemists for many years. It has, for example,

been claimed that less than 1 sample per 25 km2 is insufficient

Fig. 3. Potassium in surface soil from the conterminous USA. The map is obtai

samples spread over the area. After Gustavsson et al. [7].

in regional geochemical mapping as the geochemical patterns

become distorted at lower sampling densities [9]. The present

authors think, however, that empirical data contradict this

viewpoint. The Li-maps in Fig. 6 were obtained in the

Nordkalott Project [6]. The original map, which is based on

nearly 6000 samples of stream sediments within a survey area

of 250,000 km2 (1 sample per 40 km2), shows systematic

distribution patterns. Moreover, most of this general pattern is

maintained in random selections down to approximately 25%

of the total number of samples. In this case 1 sample per 160

km2 appears to be a lower limit below which the geochemical

pattern may become distorted.

However, successful application of even much more

scattered sampling has been reported in the literature. The

lowest sampling density known to the authors is that used in

Northern Europe by Eden and Bjfrklund [10]. They

collected 49 samples of each of till, active stream sediments

and overbank sediments from a survey area of 1.1 million

km2 (1 sample site per 23,000 km2). Nevertheless, system-

atic patterns were obtained for the contents of several

elements in various sample types. Several of these patterns

agree with those obtained at much denser sampling, see also

the comments on p. 9.

The examples of geochemical maps given in the present

paper, as well as earlier statistical treatment of regional

geochemical data [11], point to the interesting possibility

that geochemical distributions are scale-independent, i.e.

ned by calculating the moving average (N=50) between results from 1300

Fig. 4. Contents of potassium at the surface of the conterminous USA derived from aerial gamma-ray surveys. After Duval et al. [8].

Fig. 5. Moving values for Spearman-rank correlation coefficients (N=50) between the contents of potassium in surface soils and potassium determined by air-

born gamma-ray surveys. After Gustavsson et al. [7].

B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 187

Fig. 6. Contents of acid soluble lithium in 5773 samples (100%) and random selections of 50% and 25 % of the original samples collected during the

Nordkalott Project, see Bblviken et al. [6].

Fig. 7. Example of an unpublished map from the Nordkalott project,

Northern Fennoscandia [6]. The patterns are not real showing only effects

of annual variations in analytical bias for the contents of W in stream moss.


have fractal properties (e.g. Refs. [12,13]). If this is true, the

notion of a minimal sampling density of general use

becomes meaningless. Fractal properties would indicate

that a feasible sampling density will depend on the size of

the survey area; the larger the survey area, the lower an

acceptable sampling density would be.

If the goal is to determine the main geochemical patterns

within a certain area, an optimal sample number may exist

independent of the size of the area. The examples above

suggest that a reasonable sample number would be in the

order of 1000, even at a continental scale. Such a small

number of samples would keep costs low, but require that

the composition of each of the collected samples is typical

for the sub-area it represents, in other words, it must be

representative in the sense of TOS. This points to the use of

overbank sediment from large streams as a potential

sampling medium in regional to even global geochemical

mapping.

2.3. Chemical analysis in geochemical mapping

A number of excellent methods of chemical analysis are

now available for geochemical mapping. For details, the

reader is referred to special publications, e.g. Fletcher [14].

Here we stress only two aspects, namely (1) reproducibility

and quality control, and (2) the significance of determining

the total or an extractable fraction of the elements.

(1) It is generally supposed that in applied geochemistry,

sampling errors are normally greater than the errors of

chemical analysis. This feature, which agrees with con-

clusions drawn from TOS, may be true for random

analytical errors. However, experience has also shown that

in geochemical mapping a small analytical bias may

sometimes be serious. Fig. 7 is an example taken from the

raw data of the Nordkalott project, Northern Fennoscandia

[6]. The map shows a region of low W-contents in the

central part of the survey area. This region has rectilinear

north–south striking borders juxtaposed against regions of

high W-concentrations in the eastern and western parts.

Similar patterns were not obtained for any other element.

One team did all the sampling, and all the chemical data

were obtained with the same method of analysis at the same

laboratory. Nevertheless, the patterns on the map are not

real, but reflect only variations in the analytical bias

between three different years of analysis. This map was,

therefore, not published.

The authors are aware of several examples of similar

results in other projects, where a small analytical bias has

caused apparently significant spatial patterns, because the

samples have been analysed in the geographical order of

sampling. This experience has led us to the following

conclusion: Geochemical mapping procedures require a

thorough system for quality control. In such a system

reference samples should be included at random within the

collection of samples for analysis. Ideally, all real and

reference samples should be analysed in random order and

in one batch after all the sampling has been completed.

(2) Dissolution of mineralogical samples in acids and

other solvents varies with the composition of the samples.

Minerals with low contents of Si tend to be more soluble

than those with high contents of Si. Consequently, for some

chemical elements (e.g. Mg) similar spatial patterns are

Fig. 8. Contents of total and acid soluble potassium in overbank sediments, Norway. After Ottesen et al. [26].


obtained for the total contents as for an acid soluble fraction,

while for other elements (e.g. K) the patterns for totals are

different from those of the acid soluble fraction, see Fig 8.

Many environmental surveys focus on the use of easily

extractable forms of chemical elements, since they may be

closer to a biologically available fraction than the total

concentrations.

3. Overbank sediments as a representative sampling

medium in geochemical mapping

Since Ottesen et al. [3] advocated that overbank sedi-

ments could be a potentially more representative sample

type than active stream sediments, many publications have

appeared concerning the use of overbank sediments in

Fig. 9. Water discharges during ordinary conditions with norma

geochemical mapping. The following section summarizes

the main aspects of these contributions.

3.1. Formation of overbank sediments

Overbank sediments (also called alluvial soils, floodplain

sediments or levee sediments) occur along streams with

variable water discharge. In flooding streams, the tempora-

rily enhanced discharge may exceed the amounts that can be

contained within the normal channel (Fig. 9). Material in

suspension will then be transported onto floodplains and

levees, where it may be laid down and accumulated,

especially during the latest phases of flooding. In most

streams, this process has taken place many times in the past.

Overbank deposits, therefore, consist of successive nearly

horizontal strata of young sediments overlaying older

l amounts of water (A, left), and major floods (B, right).


sediments. A vertical section through such a deposit reflects

the history of sedimentation back through time.

During floods, the great quantity of water activates many

sediments sources in the drainage area, and the material in

suspension will reflect the composition of these and earlier

developed sources. This is a reason why a composite sample

consisting of many layers of overbank sediments would

represent large parts of—or even complete—catchments.

In most cases, deposits of overbank sediment contain

mainly natural material throughout. However, in some

situations anthropogenic material may have polluted the

most recent layers. The composition of sediments at depth

may, nevertheless, still be natural.

In some occurrences of overbank sediments, the stratig-

raphy may be complex due to re-deposition of material

eroded from earlier formed upstream deposits. Young

sediments may then be intermixed with older sediments.

Various aspects of such situations are examined in the

section on sampling errors below.

3.2. Sampling errors

Since overbank sediments normally consist of individual

horizontal strata formed at different times, the variations in

chemical composition and the corresponding sampling error

would be greater in the vertical than in the horizontal

direction.

3.2.1. Vertical variations in the chemical composition of

overbank sediments

In principle vertical trends in the chemical composition

of overbank sediments have two origins, namely variations

in the composition of the original source material, and

alterations caused by secondary migration of substances

after deposition.

The first trend is caused by time variations in the

positions of the most active sediment sources as well as the

intensity of flooding, while the second trend is due to

features such as climate, pH, reduction/oxidation conditions,

amounts and type of organic material, biological activities

and time. These last factors are the same as those found to

govern the dispersion of elements in soils and lake

sediments (e.g. Refs. [15,16,17]). In many climates soil

formation processes may need hundreds of years in order to

develop significant vertical patterns. For overbank sedi-

ments the available time intervals are normally more

restricted, because new sediments are occasionally depos-

ited on top of the older ones. It appears, therefore, that

problems of vertical migration in general would be less in

overbank sediments than in other soils. However, distribu-

tions of mobile elements such as Cd could be exceptions,

because of their high solubility. The following examples

from various countries illustrate effects of these two

principles.

In Belgium, the Netherlands, Luxembourg and parts of

Germany, 34 overbank sediment profiles situated along the

banks of meandering rivers were studied [18–21]. In 30 of

these, pre-industrial sequences could be detected below

polluted surface overbank sediments. Samples were col-

lected at depth intervals of 10 cm and analysed for their

contents of major and trace elements. 14C dating was

performed from all parts of the sections where sufficient

organic material could be obtained.

Three main groups of vertical distribution patterns were

distinguished in the sections, namely (1) either low or high

metal concentrations throughout the profile. This is

thought to reflect a generally low or high natural metal

content in the catchments, (2) no variations in grain size or

lithology, but a gradual increase in heavy metal concen-

trations towards the top of the profile. Such patterns are

presumably caused by airborne pollution, (3) abrupt

changes in metal concentrations at certain depths and a

corresponding change in lithology. These patterns are

interpreted as being an effect of man-made discharges

into the catchments and a subsequent fluvial dispersion of

particle-bound pollutants.

A combination of types 2 and 3 above may occur where

the results of soil-forming processes are superimposed on

the original patterns. Signs of vertical migration of Fe and

Mn were pronounced in some profiles, but correlations

between the contents of Fe or Mn and most other heavy

metals were not found. However, in some profiles vertical

percolation was indicated for mobile elements such as As

and Cd.

Profiles that did not show pre-industrial sediments in the

lower strata, were interpreted as being a sub-group of type 2,

where the profile was not deep enough to obtain pre-

industrial sediments, or the floodplain had been reworked,

so that pre-industrial overbank sediments had been washed

away.

In a Norwegian study [22–25], overbank sediment

profiles were sampled from the Knabe3na-Kvina drainage

basin, which are influenced by Cu and Mo-rich tailings from

the now closed Knaben Molybdenum Mine. Along the

rivers, pre-industrial overbank sediments were detected

below the present inundation level in the bottom sections

of 14 out of 18 profiles. The four atypical profiles are

situated where lateral river migration has had an impact on

the sedimentary environment, or where minor river regu-

lations and influx of tailings have altered the original peat

bog and lacustrine environments into flood plains.

Most profiles show high Cu and Mo contents in the

upper section, while concentrations at depth in the bottom

section approach a lower, probably natural level similar to

those in the natural sediments above the present inundation

zone of polluted sediments (Fig. 10). In some profiles about

30% of the Cu content in the upper layers appears to be

removed by dissolution or cation exchange and re-precipi-

tated in the middle of the section possibly adhering to

organic matter. However, the sharp decrease of Mo

concentrations below the upper layers, indicates that vertical

migration of Mo is negligible.

Fig. 10. Overbank sediment profile at the polluted Knaben3a river, Southern Norway. After Langedal [22–24]. L.O.I.: Loss on ignition.


In southern Fennoscandia, Eden and Bjfrklund [10], see

also p. 4, suspected downward percolation of long-range

atmospheric pollution to be the cause of high Pb

concentrations in the lower part of overbank sediment

profiles. Acid rain and low buffer capacity in the sediments

may have contributed to the migration. However, Ottesen

et al. [26] questioned this interpretation and claimed that

the patterns of Pb enrichments in southern Norway are

natural.

In English and Welsh basins with old Pb/Zn-mines, the

vertical distributions of Pb and Zn in overbank sediments

were found to be closely related to the mining history,

suggesting that no significant vertical migration of these

metals had taken place after deposition of the sediments

[27]. Similarly, along the rivers Rio Guanajuato and Rio

Puerco, Mexico, no vertical migration was seen for any of

the elements As, Cr, Cu, Pb, Sn, and Zn [28].

Volden et al. [29] collected top and bottom samples of

overbank sediments at 43 sites within a 12,000 km2 area

around the Russian nickel mining and smelting industry on

the Kola Peninsula, and analysed the samples for 30

elements, of which results for Co, Cu and Ni are reported

in Table 1. The median values for these elements in

Table 1

Median and maximum contents (mg/kg, N=43) of aqua regia extractable

metals in top and bottom parts of overbank sediments (b0.125 mm)

compared with median values in soils (b2 mm) from the same catchments

Overbank sediments Soils

Top Bottom O-horizon C-horizon

Med. Max. Med. Max. Med. Med.

Co 5 60 8 94 2 5

Cu 10 595 20 849 13 16

Ni 9 645 17 938 14 11

Data from a 12,000 km2 area surrounding the Russian nickel mining and

smelting industry on the Kola Peninsula. After Volden et al. [29].

overbank sediments compare rather closely with those in

the O- and C-horizons of soil samples taken from the same

catchments. For most other elements, however, the O-

horizon shows concentrations different to those in other

types of sample. In regional geochemical mapping, the

median values for the composition of overbank sediments

appears thus to reflect that of the deeper soil levels, till and

bedrock. It is unclear to which extent very high metal

values in the overbank sediments mirror natural levels or

vertical percolation of elements of anthropogenic origin.

Further studies of this problem are warranted. They require

more detailed sampling of vertical overbank sediment

profiles than the 10 cm spacing used at selected sites in

this study.

Sampling of overbank sediment has also been performed

in connection with archaeological studies in mining areas in

the Hartz mountains, Germany showing that heavy metal

pollution could be detected in overbank sediments at depths

of several meters making a documentation of the natural

background difficult [30].

3.2.2. Lateral natural variations in the composition of

overbank sediment

Within floodplains the natural lateral variations in the

composition of overbank sediment seem to be small.

In a study of 49 selected floodplains across the

Fennoscandian shield, Eden and Bjfrklund [10] found that

the lateral within floodplain variations were insignificant in

relation to the between-floodplain variation, see Table 2.

Similar results were also obtained in parts of Scandinavia by

Chekushin et al. [31] and Pulkinen and Rissanen [32].

Demetriades and Volden [33] studied the reproducibility

of overbank sediment sampling in Greece and Norway, of

which the results for Greece are referred to here. Ten 60–

600 km2 drainage basins distributed over the Eastern

Macedonia and Thrace regions in Greece were selected

for sampling. After excluding the upper 5–10 cm, two

Table 2

Analysis of variance [34] [35] of the contents of aqua regia soluble

chemical elements in widely spaced duplicate samples of overbank

sediments taken at depth and near the surface in a 23,000 km2 area in

Northern Europe [10]

1 2 3 4

% F F F

Al 2.2 14.5 6.1 4.2

Ba 6.0 14.7 5.6 5.4

Ca 1.8 73.0 15.3 15.3

Co 10.0 12.0 5.8 7.8

Cr 4.7 48.0 7.1 8.3

Cu 10.7 34.7 5.6 5.3

Fe 25.5 13.0 5.1 4.4

K 3.8 33.3 10.5 10.8

La 4.3 34.0 9.3 6.1

Mg 1.8 66.0 8.6 10.3

Mn 6.2 14.1 3.7 6.0

Na 7.3 4.2 5.6 4.6

Ni 10.1 26.0 7.3 7.9

P 4.0 25.0 6.3 8.9

Pb 21.7 5.5 3.9 4.1

Sr 5.0 47.2 8.2 8.3

Th 35.8 10.7 3.9 5.6

Ti 2.2 33.3 6.7 6.9

V 4.5 7.0 6.0 5.0

Zn 5.0 30.0 6.2 7.7

Critical F

value at p=0.05

1.7 1.4 1.4

Numbers of pairs 36 36 116 116

(1)–(3) Samples at depth. (1) Combined sampling and analytical error. (2)

Ratio between total variance and combined sampling and analytical

variance. (3) Ratio of between site variance and within site lateral variance.

(4) Ratio of between site variance and within site vertical variance.


composite samples (A and B) were collected from vertical

sections 60–100 m apart at the down-stream apex of each

drainage basin. These paired samples were analysed chemi-

cally for the total contents of 21 elements.

Statistical treatments [34,35] of the analytical results

show: (1) The Spearman rank correlation coefficients

between the A and the B samples are significant at the

95% level (or better) for all elements (Al, As, Ba, Be, Ca,

Co, Cu, Fe, Li, Mn, Ni, Pb, S, Sc, Sr, Ti, V, Y, and Zn)

except Cd and Mo. For these two elements the sensitivity of

the analytical method was not adequate, and (2) The

majority of the elements (Al, As, Be, Ca, Co, Cu, Fe, Li,

Mn, Ni, Pb, Sb, Sc, Sr, Ti, V, Y) vary significantly between

sites in relation to within-site variations ( pb0.01). However,

the between-site variations for Ba, Cd, Mo and Zn are

insignificant. For Cd and Mo this is ascribed to the

analytical method, while those for Ba and Zn probably are

caused by high sampling variability.

Langedal [24] found that in floodplain surface sediments

(0–25 cm) of the polluted Knabe3na river in Norway, the

highest Cu and Mo concentrations occur in samples near the

river as well as in depressions. Enrichment of metals in

these parts of the floodplains may be an effect of differences

in the timing of the sediment transport pulse, and the timing

of floodplain inundation, see also [36]. In proximal areas

and depressions the suspended sediment transport rates are

often highest during the rising and peak stages. In polluted

streams these are the first to be inundated and receive the

largest load of particle-bound metals. Similar results were

also found along the Geul river, Belgium [37,38].

3.2.3. Complex stratigraphy due to combined vertical and

lateral processes

Complex stratigraphy in deposits of overbank sediments

may cause sampling problems downstream from mines and

other local sources of severe pollution.

According to Lewin and Macklin [39], fluvially trans-

ported mine tailings may be incorporated in alluvial

sediments in three different ways depending on the river

platform stability.

(1) Along single thread, laterally stable channels,

tailings are mainly accumulated as overbank sediments

on the floodplains. Thus, young sediments overlay older

sediments. (2) In single thread, meandering rivers, tailings

are mainly accumulated around point-bars. In this case

floodplain deposits become younger towards the margins

of the channel. As the river migrates, the tailings will be

reworked and the lateral age distribution may be

disturbed. (3) In rivers where mine tailings are discharged

into the channel, the sediment load increases to such an

extent that the alluvial plain is aggrading. The river may

then become laterally unstable with frequent channel

shifts. This results in a complex floodplain stratigraphy.

After mining ceases, erosion in the alluvial plain will

possibly rework both tailings and pre-industrial material.

This may also influence downstream overbank sediment

profiles.

Macklin et al. [27] and Ridgway et al. [28] evaluated the

use of overbank sediments in geochemical mapping within

areas of England, Wales and Mexico polluted by mining

activities. These two papers share the conclusion that

variations in the composition of overbank sediments may

be so complicated that time consuming detailed studies of

the geomorphology, history and ages of the sediments are

required at each sample station in order to distinguish

between natural and polluted patterns. A single overbank

profile very rarely spans the period from before anthro-

pogenic disturbance through the Industrial Revolution and

later. According to these authors such considerations and

associated costs may render overbank sediment non-viable

as a regional mapping medium. Instead they recommend to

use active stream sediments.

Ridgeway et al. [28] also found that in Mexico the lateral

variations of element contents in overbank sediments are

small for natural sediments, but become more complex for

deposits with mixed pristine and polluted sediments.

The conclusions drawn after the studies in Wales and

Mexico have later been questioned by other researchers (see

e.g. Ref. [40]). It is difficult to imagine that active stream

sediments would be superior to overbank sediments in

polluted environments, since active sediments (contrary to


overbank sediments) are always polluted to an unknown

degree.

3.2.4. Concluding remarks on sampling error

In both small and large catchments, the sampling error

for natural overbank sediments within a floodplain is small

in relation to the between floodplain variation. This

conclusion appears to be valid in most regions of the world

for both genuine natural deposits and in situations where

pristine sediments at depth are covered by polluted surface

sediments.

Sampling of older terraces is appropriate in order to

obtain pre-industrial material. Such sampling should be

done above the present inundation zone to avoid material

draped during recent floods. Along laterally stable river-

reaches, sampling in the bottom sections of the sediment

profiles is also adequate. Sampling along meandering

reaches, as suggested by Bogen et al. [41], may also be a

possibility if the lateral migration is slow.

Pollution of overbank sediments may be of two types: (1)

Mine wastes and other anthropogenic material may enter the

stream from local sources and then be transported down-

stream. (2) Airborne contaminants originating from distant

sources may reach the catchments. Situation (1) is often

recognizable, since the sources may be easily identified.

Situation (2) can be more difficult to recognize straight

away. In both cases (1) and (2) the contaminants may be

confined to the surface layers of the overbank sediments.

However, in some locations mixtures of old natural and

resent polluted sediments may occur, causing an intricate

stratigraphy in the overbank deposits. In addition, down-

ward percolation of soluble surface pollutants may also

contaminate the sediments at depth. In such cases detailed

investigations at each sample site may be necessary.

It is concluded that only trained personnel should be used

in order to select appropriate locations for sampling of

overbank sediments. If this prerequisite is fulfilled, high

quality subsequent chemical analysis of the samples will

produce reliable data for most chemical elements.

3.3. Representativity and regional distribution of chemical

elements in overbank sediments

Many papers have appeared in the last few years

presenting data on the spatial distribution of element

contents in overbank sediments. Selected publications with

examples from eight countries in Asia, Europe and North

America are referred below in chronological order of

appearance.

Ottesen et al. [3] pioneered the use of overbank

sediments in geochemical mapping and edited an atlas of

geochemical maps based on this type of sample [26]. Nearly

700 floodplains distributed across Norway (300,000 km2)

were selected for sampling. Each plain represents drainage

areas of between 60 and 600 km2 . The samples were

collected at distances of 2–200 m from the present-day

stream, depending on local circumstances. Where possible,

sites close to the stream were avoided in order to reduce the

possibility of collecting polluted samples. A vertical section

through the sediment was cut with a spade, and a composite

sample (5 kg) was taken from the section excluding the

upper 5–10 cm. After drying, the samples were sieved to a

minus 0.062 mm fraction, which were subsequently

analysed for the total contents of 30 elements and an acid

soluble fraction of 29 elements.

Most elements show systematic patterns with great

contrasts. In some cases these patterns agree with known

geological structures, in others they indicate structures not

known earlier. Examples of the maps are shown in Fig. 8

(see comments on p. 00) and Fig. 11. The last figure shows

that the contents of acid soluble Mo in Norwegian overbank

sediments are relatively high in most of southern Norway,

while the levels further north vary. The province of high Mo

concentrations in the south agrees with the results of earlier

prospecting, which disclosed a great number of small and

some more extensive Mo-deposits within the province,

including those mined at Knaben (see p. 9 and e.g. Bugge

[42]).

McConnell et al. [43] carried out geochemical mapping

in the Baie Verte/Springdale area of Newfoundland (2000

km2) based on several types of sample media including

overbank and stream sediments. One hundred twenty-one

samples of each of these types were collected from drainage

basins 2–10 km2 in size, sieved to minus 0.063 mm and

analysed for 38 elements. In this survey area overbank

sediments were found to be more widespread and easier to

sample than stream sediments. In general, trace element

distributions are similar in the two media, both producing

significant patterns reflecting the chemistry of the under-

lying bedrock. For most elements the concentrations are

greater in the overbank than in the stream sediments,

supposedly because overbank sediments are more fine-

grained than stream sediments. In some drainages stream

sediments are contaminated by past mining activity, while

overbank sediments, however, appear unaffected.

Eden and Bjfrklund [10] performed ultra-low density

sampling of overbank sediment and other sample types

across Finland, Norway and Sweden (1 sample station per

23,000 km2), and found that for 20 elements the within site

variation is small compared with the between site variation

(Table 2), and that one sample of overbank sediment may

substitute for 6–20 till samples depending on type of

drainage area.

Xiachu and Mingkai [44] performed an orientation

survey in a part (170,000 km2) of the Jiangxi Province of

Southern China in order to develop techniques of

implementing global ultra-low sampling in geochemical

mapping [45]. Sample sites (1 site per 1800 km2) were laid

out at the apexes of 94 drainage basins, the sizes of which

were between 100 and 800 km2. Composite samples of

overbank sediment were collected from the upper (5–40

cm) and the lower (50–120 cm) layers of sediment profiles

Fig. 11. Contents of acid soluble molybdenum in overbank sediment, Norway. After Ottesen et al. [26].


within terraces located 3–5 m above the average present

stream water level. The samples were analysed for 39

elements.

For each drainage basin the contents of five selected

elements (Cu, Pb, Sn, W and Zn) in the overbank sediments

were compared with the averages of the same elements

obtained in the National Geochemical Mapping Project of

China (1–2 samples per km2), see Fig. 12. There are good

correlations between the concentrations of these elements in

overbank sediments and averages (NN30) for the same

elements computed for earlier obtained samples, see an

example for lead in Fig. 13.

Xiachu and Mingkai [44] concluded that (1) floodplains

of 100–800 km2 catchments are suitable sample stations

for global geochemical mapping based on overbank

sediments, (2) sampling of wide-spaced lower-layer over-

bank sediment is a fast and cost-effective way to identify

geochemical provinces, (3) there is a significant correlation

Fig. 12. Illustration of how one sample of overbank sediment (left)

represents many samples of active stream sediments (right).


between the W content in the overbank sediment samples

and the presence of known W occurrences in the bedrock,

and (4) the distributions of elements such as Be, Cr, Ni,

Rb and V characterize known geological formations in the

region.

In Greece, similar data on the representativity of over-

bank sediment, as those described for China, were obtained

by Demetriades and Volden [33]. They state that the element

content in only one sample of overbank sediment represent-

ing a large drainage basin is close to the median value for

the same element in several hundred samples of stream

sediments taken from small sub-catchments within the large

basin, see also p. 00.

In Belgium and Luxembourg (survey area 33,000 km2)

Van der Sluys et al. [46], produced a geochemical atlas

based on samples of overbank sediments at 66 sites located

in the apexes of catchments, which range in size from 60 to

600 km2, see also Refs. [18–21]. At each site an overbank

Fig. 13. Lead content in single samples of overbank sediment (left) and median

province, southern China. After Xiachu and Mingkai [44].

profile was dug near the river, and bulk samples were taken

at depths of 5–25 cm and, if possible, at 1.5–1.7 m. For most

sample sections evidence such as 14C dating and the absence

or presence of anthropogenic particles were used to

determine if the samples were from pre- or post-industrial

eras. Present-time active stream sediments were also

sampled. After drying, the samples were sieved to minus

0.125 mm fractions, which were analysed for the total

contents of 10 major and 11 trace elements. An example of

their maps is reproduced in Fig. 14.

The element contents in the lower overbank sediments

indicate the natural geochemical background, which varies

in a systematic way for several elements throughout the

survey area. This background reflects the composition of the

underlying bedrock.

The active stream sediments and the upper overbank

sediment have been polluted to a varying degree, and by

comparing trace element concentrations in these media

with the concentrations in the lower overbank sediment, the

degree of pollution could be assessed. From such data it is

clear that the most severe pollution occurs in the northern

part of Belgium, where the population is denser and

industry is more developed than elsewhere in the survey

area.

Xuejing and Hangxin published the most recent results of

a pilot study for the use of overbank sediment in China [47].

They selected 500 floodplains across the entire country

(9,600,000 km2) for sampling, each plain representing a

drainage basin in the order of 1000–6000 km2. At each plain

two samples were collected at depths of 0–25 and 80–100

cm, respectively. The samples were analysed for 71

elements. Statistical parameters for the analytical results as

well as maps for the distributions of Cu, Hg and Ni are

presented in the publication. Element contents in the widely

values per catchment for lead in stream sediments (right) in the Jiangxi

Fig. 14. Al, K, Sc and Si contents in overbank sediments in Belgium and Luxembourg. After Van der Sluys et al. [46].

B.Bølviken

etal./Chem

ometrics

andIntellig

entLaboratory

System

s74(2004)183–199

196


spaced samples were compared with those from China’s

Regional Geochemical National Reconnaissance Program

(CRGNRP), which includes N1million samples of active

stream sediments. Selected results of these comparisons are

presented in Fig. 15, which shows that the geochemical data

generated from the wide-spaced sampling are strikingly

similar to those generated by the CRGNRP. A map of the

ratio between Hg contents in the surface samples and Hg in

the samples taken at depth (Fig. 16) demonstrates clearly,

that Hg (supposedly airborne) from industrial and urban

sources has polluted the eastern part of China. These results

have lead to the establishment of abatement strategies,

which will include monitoring of time trends in the pollution

by repeated sampling and analysis of overbank sediments

every 10 years.

Fig. 15. Distribution of copper in China. (A) Upper part: Averages from great n

Averages of restricted numbers of overbank sediment samples per catchment. Af

4. Summary discussion and conclusions

The characteristics of drainage regimes and sedimenta-

tion processes vary throughout the world, but it appears that

in most places overbank sediments are readily available and

very useful as a sampling medium in geochemical mapping,

even in polluted areas. In some places, such as in parts of

Britain and Mexico, as well as in heavily polluted areas in

the arctic, the geological and historical setting apparently

makes detailed studies necessary in order to obtain relevant

samples.

There is hardly an alternative sample type if the goal is to

detect both natural and polluted patterns at regional to

continental scales. Biological substances and materials such

as active stream sediments and stream waters may be

umbers of stream sediments within each catchment basin. (B) Lower part:

ter Xuejing and Hangxin [47].

Fig. 16. Mercury pollution as indicated by the ratio between the mercury

content in overbank sediments near the surface (0–25 cm) and that at depth

(80–100 cm) After Xuejing, and Hangxin [47].


polluted to an unknown degree, while soils in addition have

undergone soil forming processes, jeopardizing a compar-

ison of the pollution of ancient and recent layers in vertical

sections. Lake sediments have some of the same properties

as overbank sediments [48], but are not easily inspected

before sampling. Furthermore, lake sediments are not

available in many areas due to the lack of suitable lakes.

Depth integrated samples of overbank sediments reflect

the composition of many current and past sediment sources

upstream of the sampling point, contrary to active stream

sediments, which normally are recent deposits originating in

a more restricted number of presently active sediment

sources. In most regions where tests have been performed,

overbank sediments are more representative of large parts of

drainage basins than are active stream sediments. Overbank

sediments can, consequently, be used to disclose main

regional to continental geochemical distribution patterns

with widely scattered sampling at low cost per unit area.

The stratigraphy of overbank sediments may in some

cases be complicated due to secondary processes. However,

in flood plains or old terraces along laterally stable or slowly

migrating channels it is normally possible to obtain recent

sediments near the surface and pre-industrial sediments at

depth. Simultaneous mapping of the composition of recent

and pre-industrial overbank sediments can normally be used

(1) in a characterization of variations in the natural

geochemical background as well as in a documentation of

the present state of pollution of some elements, and (2) as a

regional prospecting tool in natural as well as polluted

environments.

The composition of overbank sediment is also of great

interest to society in general since flood plains are very

important for agriculture, urbanisation and as sources for

drinking water.

Although vertical movements of easily soluble elements

between strata of overbank sediments have been reported,

the overall impression is that such chemical migration is not

a major problem in the use of overbank sediments in

geochemical mapping. However, great care should always

be taken in the sampling of overbank sediment. This is

particularly warranted in areas with severe pollution from

local sources.

In conclusion we emphasize that overbank sediments

represent a natural analogue to a bed-blended stockpile. This

follows from the documented empirical data and from

characteristic features of overbank sediments such as: (1)

They are build up from a succession of broadly similar

stacking and layering flooding events, (2) they are formed in

closely bracketed time intervals, and (3) each flood drains

the largest possible number of source locations within the

catchments.

Gy [4] has shown that industrially laid up stockpiles are

effective averages for very large lots. In principle the only

significant difference between an industrial stockpile and a

deposit of overbank sediment is that the first is man made,

while the other is natural. The degree of success in the

averaging process of this type of natural deposits has only

been summarised in this paper, and a future more in-depth

treatment is warranted.

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