Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 www.elsevier.com/locate/chemolab

Overbank : 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, Norway bNorwegian 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 and 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 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.

1. Introduction

Geochemical mapping includes (1) systematic sampling B Please note that the figures in this article appear in full colour in the online version on www.sciencedirect.com. of natural materials, such as rocks, sediments, soils and * Corresponding author. Tel.: + 47 6116 4709; fax: +47 6116 8236. waters; (2) chemical analysis of the samples; and (3) E-mail address: [email protected]. illustration of the analytical results on maps. Geochemical

0169-7439/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.chemolab.2004.06.006 184 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 maps show significant natural distribution patterns with great sediment as a geochemical sampling medium. Our contribu- contrasts. Such distributions occur for many chemical tion summarizes the main results demonstrated in these elements at various scales from local up to continental. This papers with emphasis on representativity and sampling errors property of geochemical data implies that geochemical maps (reproducibility), which are also important aspects of the are of great interest to society in general, especially in fields Theory of Sampling (TOS) [4]. such as (1) environmental research, (2) exploration of mineral In order to put these results into perspective, we give three deposits, (3) medical (geomedicine), (4) agriculture examples of published geochemical maps at various scales, a and (5) land use planning. These aspects are discussed below. short summary about sampling density in geochemical mapping and some comments on problems that may occur 1. The use of geochemical maps in environmental research in the use of chemical analysis in geochemical mapping. 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]. 2. Geochemical mapping Consequently, data on the spatial variations in the composition of uncontaminated natural materials are a 2.1. Examples of geochemical maps at various scales necessary prerequisite for an evaluation of the degree of pollution within small and large areas. Geochemical The literature has many examples of geochemical maps maps based on chemical analysis of natural materials at scales ranging from local to continental. Three examples provide such information. of such maps are shown here in order to present the type of 2. In exploration, geochemical maps may disclose geo- data that may be obtained by geochemical mapping. chemical provinces or geochemical anomalies with Fig. 1 shows a geochemical Pb anomaly in active stream greater than normal contents of heavy metals or other sediments disclosed during a mineral exploration project in elements of economic interest. Follow-up investigations Southern Norway [5]. The anomaly comprises Pb contents in such environments may lead to the discovery of of 270–680 mg/kg, which is much higher than the workable deposits. background levels in the surrounding 30 km2 (10–20 mg 3. It is widely accepted that some local or regional Pb/kg). Follow-up investigations led to the discovery of an environments may be sub-optimal for the health of earlier unknown Pb deposit. Even though the deposit later human beings and other animals. Relations between proved to be sub-economic, the example shows that goitre and iodine deficiency and between caries and geochemical mapping is a potential prospecting tool. fluorine deficiency are classical examples of this kind. Fig. 2 is a geochemical map of Cr reproduced from the Geochemical maps provide information for research in geochemical atlas of Northern Fennoscandia [6]. The atlas this emerging field of geomedicine, which is also called contains 136 single-point geochemical maps at a scale of medical geology. 1:4,000,000 based on the contents of total and acid 4. In agriculture, information about variations in the extractable elements in four different types of sample (till, chemical composition of soils may be utilized in active stream sediments, stream organic matter and stream production planning, since some chemical elements are moss) collected at 5000–6000 sites within an area of vital for plants and domestic animals, while others may 250,000 km2 (1 sample station per 50 km2). Systematic be harmful when present in too high concentrations. distribution patterns were obtained for most elements. The 5. In land-use planning, geochemical maps may contribute maps for the contents of an element in various sample types to information about the suitability of specific areas for are with only few exceptions similar—even after using specific uses. different chemical digestion methods on the original samples or heavy mineral fractions of composites of 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 Fig. 1. Lead content (mg/kg) in stream sediments from the Gal3a river and have since been published concerning the use of overbank its tributaries, Hedmark county, Norway. After Bjbrlykke et al. [5]. .Blie ta./Ceoerc n nelgn aoaoySses7 20)183–199 (2004) 74 Systems Laboratory Intelligent and Chemometrics / al. et Bølviken B.

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Fig. 2. Chromium content in stream sediments from Northern Fennoscandia. After Bblviken et al. [6]. 185 186 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 geographical neighbours. This indicates that the consisten- in regional geochemical mapping as the geochemical patterns cies and thus the reliabilities of the maps are good. The become distorted at lower sampling densities [9]. The present maps, which were originally obtained for use in mineral authors think, however, that empirical data contradict this exploration, are also of great interest in other fields, such as viewpoint. The Li-maps in Fig. 6 were obtained in the environmental research. Nordkalott Project [6]. The original map, which is based on Fig. 3 shows the distribution of K in the conterminous nearly 6000 samples of stream sediments within a survey area USA. The map is based on chemical analysis of 1300 soil of 250,000 km2 (1 sample per 40 km2), shows systematic samples collected across the entire country [7]. In spite of the distribution patterns. Moreover, most of this general pattern is low sampling density (1 sample per 5000 km2), systematic maintained in random selections down to approximately 25% distribution patterns are disclosed for this and also for several of the total number of samples. In this case 1 sample per 160 other elements. Taking the sampling density into account, the km2 appears to be a lower limit below which the geochemical reliability of the K pattern appears to be acceptable when pattern may become distorted. compared with a map of K acquired by airborne radiometric However, successful application of even much more surveys, which include millions of measurements [8], (see scattered sampling has been reported in the literature. The Figs. 4 and 5). Some of the obtained geochemical patterns lowest sampling density known to the authors is that used in coincide with known geological structures, while others Northern Europe by Ede´n and Bjfrklund [10].They indicate structures, which had not previously been recog- collected 49 samples of each of till, active stream sediments nized. The maps have an interesting potential use in mineral and overbank sediments from a survey area of 1.1 million exploration, environmental research and epidemiology (geo- km2 (1 sample site per 23,000 km2). Nevertheless, system- medicine, medical geology). atic patterns were obtained for the contents of several elements in various sample types. Several of these patterns 2.2. Sampling density in relation to survey area agree with those obtained at much denser sampling, see also the comments on p. 9. The number of samples per unit area (sampling density) in The examples of geochemical maps given in the present geochemical mapping has been a matter of controversy paper, as well as earlier statistical treatment of regional between geochemists for many years. It has, for example, geochemical data [11], point to the interesting possibility been claimed that less than 1 sample per 25 km2 is insufficient that geochemical distributions are scale-independent, i.e.

Fig. 3. Potassium in surface soil from the conterminous USA. The map is obtained by calculating the moving average (N=50) between results from 1300 samples spread over the area. After Gustavsson et al. [7]. B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 187

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

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]. have fractal properties (e.g. Refs. [12,13]). If this is true, the were obtained with the same method of analysis at the same notion of a minimal sampling density of general use laboratory. Nevertheless, the patterns on the map are not becomes meaningless. Fractal properties would indicate real, but reflect only variations in the analytical bias that a feasible sampling density will depend on the size of between three different years of analysis. This map was, the survey area; the larger the survey area, the lower an therefore, not published. acceptable sampling density would be. The authors are aware of several examples of similar If the goal is to determine the main geochemical patterns results in other projects, where a small analytical bias has within a certain area, an optimal sample number may exist caused apparently significant spatial patterns, because the independent of the size of the area. The examples above samples have been analysed in the geographical order of suggest that a reasonable sample number would be in the sampling. This experience has led us to the following order of 1000, even at a continental scale. Such a small conclusion: Geochemical mapping procedures require a number of samples would keep costs low, but require that thorough system for quality control. In such a system the composition of each of the collected samples is typical reference samples should be included at random within the for the sub-area it represents, in other words, it must be collection of samples for analysis. Ideally, all real and representative in the sense of TOS. This points to the use of reference samples should be analysed in random order and overbank sediment from large streams as a potential in one batch after all the sampling has been completed. sampling medium in regional to even global geochemical (2) Dissolution of mineralogical samples in acids and mapping. other solvents varies with the composition of the samples. Minerals with low contents of Si tend to be more soluble 2.3. Chemical analysis in geochemical mapping than those with high contents of Si. Consequently, for some chemical elements (e.g. Mg) similar spatial patterns are 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. Fig. 7. Example of an unpublished map from the Nordkalott project, Similar patterns were not obtained for any other element. Northern Fennoscandia [6]. The patterns are not real showing only effects One team did all the sampling, and all the chemical data of annual variations in analytical bias for the contents of W in stream moss. B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 189

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, geochemical mapping. The following section summarizes while for other elements (e.g. K) the patterns for totals are the main aspects of these contributions. different from those of the acid soluble fraction, see Fig 8. Many environmental surveys focus on the use of easily 3.1. Formation of overbank sediments extractable forms of chemical elements, since they may be closer to a biologically available fraction than the total Overbank sediments (also called alluvial soils, concentrations. sediments or sediments) occur along streams with variable water discharge. In flooding streams, the tempora- rily enhanced discharge may exceed the amounts that can be 3. Overbank sediments as a representative sampling contained within the normal channel (Fig. 9). Material in medium in geochemical mapping suspension will then be transported onto floodplains and levees, where it may be laid down and accumulated, Since Ottesen et al. [3] advocated that overbank sedi- especially during the latest phases of flooding. In most ments could be a potentially more representative sample streams, this process has taken place many times in the past. type than active stream sediments, many publications have Overbank deposits, therefore, consist of successive nearly appeared concerning the use of overbank sediments in horizontal strata of young sediments overlaying older

Fig. 9. Water discharges during ordinary conditions with normal amounts of water (A, left), and major floods (B, right). 190 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 sediments. A vertical section through such a deposit reflects banks of meandering rivers were studied [18–21].In30of the history of sedimentation back through time. these, pre-industrial sequences could be detected below During floods, the great quantity of water activates many polluted surface overbank sediments. Samples were col- sediments sources in the drainage area, and the material in lected at depth intervals of 10 cm and analysed for their suspension will reflect the composition of these and earlier contents of major and trace elements. 14C dating was developed sources. This is a reason why a composite sample performed from all parts of the sections where sufficient consisting of many layers of overbank sediments would organic material could be obtained. represent large parts of—or even complete—catchments. Three main groups of vertical distribution patterns were In most cases, deposits of overbank sediment contain distinguished in the sections, namely (1) either low or high mainly natural material throughout. However, in some metal concentrations throughout the profile. This is situations anthropogenic material may have polluted the thought to reflect a generally low or high natural metal most recent layers. The composition of sediments at depth content in the catchments, (2) no variations in grain size or may, nevertheless, still be natural. lithology, but a gradual increase in heavy metal concen- In some occurrences of overbank sediments, the stratig- trations towards the top of the profile. Such patterns are raphy may be complex due to re- of material presumably caused by airborne pollution, (3) abrupt eroded from earlier formed upstream deposits. Young changes in metal concentrations at certain depths and a sediments may then be intermixed with older sediments. corresponding change in lithology. These patterns are Various aspects of such situations are examined in the interpreted as being an effect of man-made discharges section on sampling errors below. into the catchments and a subsequent fluvial dispersion of particle-bound pollutants. 3.2. Sampling errors A combination of types 2 and 3 above may occur where the results of soil-forming processes are superimposed on Since overbank sediments normally consist of individual the original patterns. Signs of vertical migration of Fe and horizontal strata formed at different times, the variations in Mn were pronounced in some profiles, but correlations chemical composition and the corresponding sampling error between the contents of Fe or Mn and most other heavy would be greater in the vertical than in the horizontal metals were not found. However, in some profiles vertical direction. percolation was indicated for mobile elements such as As and Cd. 3.2.1. Vertical variations in the chemical composition of Profiles that did not show pre-industrial sediments in the overbank sediments lower strata, were interpreted as being a sub-group of type 2, In principle vertical trends in the chemical composition where the profile was not deep enough to obtain pre- of overbank sediments have two origins, namely variations industrial sediments, or the floodplain had been reworked, in the composition of the original source material, and so that pre-industrial overbank sediments had been washed alterations caused by secondary migration of substances away. after deposition. In a Norwegian study [22–25], overbank sediment The first trend is caused by time variations in the profiles were sampled from the Knabe3na-Kvina drainage positions of the most active sediment sources as well as the basin, which are influenced by Cu and Mo-rich tailings from intensity of flooding, while the second trend is due to the now closed Knaben Molybdenum Mine. Along the features such as climate, pH, reduction/oxidation conditions, rivers, pre-industrial overbank sediments were detected amounts and type of organic material, biological activities below the present inundation level in the bottom sections and time. These last factors are the same as those found to of 14 out of 18 profiles. The four atypical profiles are govern the dispersion of elements in soils and lake situated where lateral river migration has had an impact on sediments (e.g. Refs. [15,16,17]). In many climates soil the sedimentary environment, or where minor river regu- formation processes may need hundreds of years in order to lations and influx of tailings have altered the original peat develop significant vertical patterns. For overbank sedi- bog and lacustrine environments into flood plains. mentstheavailabletimeintervalsarenormallymore Most profiles show high Cu and Mo contents in the restricted, because new sediments are occasionally depos- upper section, while concentrations at depth in the bottom ited on top of the older ones. It appears, therefore, that section approach a lower, probably natural level similar to problems of vertical migration in general would be less in those in the natural sediments above the present inundation overbank sediments than in other soils. However, distribu- zone of polluted sediments (Fig. 10). In some profiles about tions of mobile elements such as Cd could be exceptions, 30% of the Cu content in the upper layers appears to be because of their high solubility. The following examples removed by dissolution or cation exchange and re-precipi- from various countries illustrate effects of these two tated in the middle of the section possibly adhering to principles. organic matter. However, the sharp decrease of Mo In Belgium, the Netherlands, Luxembourg and parts of concentrations below the upper layers, indicates that vertical Germany, 34 overbank sediment profiles situated along the migration of Mo is negligible. B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 191

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, Ede´n and Bjfrklund [10], see overbank sediments compare rather closely with those in also p. 4, suspected downward percolation of long-range the O- and C-horizons of soil samples taken from the same atmospheric pollution to be the cause of high Pb catchments. For most other elements, however, the O- concentrations in the lower part of overbank sediment horizon shows concentrations different to those in other profiles. Acid rain and low buffer capacity in the sediments types of sample. In regional geochemical mapping, the may have contributed to the migration. However, Ottesen median values for the composition of overbank sediments et al. [26] questioned this interpretation and claimed that appears thus to reflect that of the deeper soil levels, till and the patterns of Pb enrichments in southern Norway are bedrock. It is unclear to which extent very high metal natural. values in the overbank sediments mirror natural levels or In English and Welsh basins with old Pb/Zn-mines, the vertical percolation of elements of anthropogenic origin. vertical distributions of Pb and Zn in overbank sediments Further studies of this problem are warranted. They require were found to be closely related to the mining history, more detailed sampling of vertical overbank sediment suggesting that no significant vertical migration of these profiles than the 10 cm spacing used at selected sites in metals had taken place after deposition of the sediments this study. [27]. Similarly, along the rivers Rio Guanajuato and Rio Sampling of overbank sediment has also been performed Puerco, Mexico, no vertical migration was seen for any of in connection with archaeological studies in mining areas in the elements As, Cr, Cu, Pb, Sn, and Zn [28]. the Hartz mountains, Germany showing that heavy metal Volden et al. [29] collected top and bottom samples of pollution could be detected in overbank sediments at depths overbank sediments at 43 sites within a 12,000 km2 area of several meters making a documentation of the natural around the Russian nickel mining and smelting industry on background difficult [30]. the Kola Peninsula, and analysed the samples for 30 elements, of which results for Co, Cu and Ni are reported 3.2.2. Lateral natural variations in the composition of in Table 1. The median values for these elements in overbank sediment Within floodplains the natural lateral variations in the composition of overbank sediment seem to be small. Table 1 In a study of 49 selected floodplains across the Median and maximum contents (mg/kg, N=43) of aqua regia extractable Fennoscandian shield, Ede´n and Bjfrklund [10] found that b metals in top and bottom parts of overbank sediments ( 0.125 mm) the lateral within floodplain variations were insignificant in compared with median values in soils (b2 mm) from the same catchments relation to the between-floodplain variation, see Table 2. Overbank sediments Soils Similar results were also obtained in parts of Scandinavia by Top Bottom O-horizon C-horizon Chekushin et al. [31] and Pulkinen and Rissanen [32]. Med. Max. Med. Max. Med. Med. Demetriades and Volden [33] studied the reproducibility Co 5 60 8 94 2 5 of overbank sediment sampling in Greece and Norway, of Cu 10 595 20 849 13 16 which the results for Greece are referred to here. Ten 60– Ni 9 645 17 938 14 11 600 km2 drainage basins distributed over the Eastern Data from a 12,000 km2 area surrounding the Russian nickel mining and Macedonia and Thrace regions in Greece were selected smelting industry on the Kola Peninsula. After Volden et al. [29]. for sampling. After excluding the upper 5–10 cm, two 192 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199

Table 2 and depressions the suspended sediment transport rates are Analysis of variance [34] [35] of the contents of aqua regia soluble often highest during the rising and peak stages. In polluted chemical elements in widely spaced duplicate samples of overbank sediments taken at depth and near the surface in a 23,000 km2 area in streams these are the first to be inundated and receive the Northern Europe [10] largest load of particle-bound metals. Similar results were 123 4also found along the Geul river, Belgium [37,38]. % F FF 3.2.3. Complex stratigraphy due to combined vertical and Al 2.2 14.5 6.1 4.2 lateral processes Ba 6.0 14.7 5.6 5.4 Ca 1.8 73.0 15.3 15.3 Complex stratigraphy in deposits of overbank sediments Co 10.0 12.0 5.8 7.8 may cause sampling problems downstream from mines and Cr 4.7 48.0 7.1 8.3 other local sources of severe pollution. Cu 10.7 34.7 5.6 5.3 According to Lewin and Macklin [39], fluvially trans- Fe 25.5 13.0 5.1 4.4 ported mine tailings may be incorporated in alluvial K 3.8 33.3 10.5 10.8 La 4.3 34.0 9.3 6.1 sediments in three different ways depending on the river Mg 1.8 66.0 8.6 10.3 platform stability. Mn 6.2 14.1 3.7 6.0 (1) Along single thread, laterally stable channels, Na 7.3 4.2 5.6 4.6 tailings are mainly accumulated as overbank sediments Ni 10.1 26.0 7.3 7.9 on the floodplains. Thus, young sediments overlay older P 4.0 25.0 6.3 8.9 Pb 21.7 5.5 3.9 4.1 sediments. (2) In single thread, meandering rivers, tailings Sr 5.0 47.2 8.2 8.3 are mainly accumulated around point-bars. In this case Th 35.8 10.7 3.9 5.6 floodplain deposits become younger towards the margins Ti 2.2 33.3 6.7 6.9 of the channel. As the river migrates, the tailings will be V 4.5 7.0 6.0 5.0 reworked and the lateral age distribution may be Zn 5.0 30.0 6.2 7.7 Critical F 1.7 1.4 1.4 disturbed. (3) In rivers where mine tailings are discharged value at p=0.05 into the channel, the sediment load increases to such an Numbers of pairs 36 36 116 116 extent that the alluvial plain is aggrading. The river may (1)–(3) Samples at depth. (1) Combined sampling and analytical error. (2) then become laterally unstable with frequent channel Ratio between total variance and combined sampling and analytical shifts. This results in a complex floodplain stratigraphy. variance. (3) Ratio of between site variance and within site lateral variance. After mining ceases, erosion in the alluvial plain will (4) Ratio of between site variance and within site vertical variance. possibly rework both tailings and pre-industrial material. This may also influence downstream overbank sediment composite samples (A and B) were collected from vertical profiles. sections 60–100 m apart at the down-stream apex of each Macklin et al. [27] and Ridgway et al. [28] evaluated the drainage basin. These paired samples were analysed chemi- use of overbank sediments in geochemical mapping within cally for the total contents of 21 elements. areas of England, Wales and Mexico polluted by mining Statistical treatments [34,35] of the analytical results activities. These two papers share the conclusion that show: (1) The Spearman rank correlation coefficients variations in the composition of overbank sediments may between the A and the B samples are significant at the be so complicated that time consuming detailed studies of 95% level (or better) for all elements (Al, As, Ba, Be, Ca, the geomorphology, history and ages of the sediments are Co, Cu, Fe, Li, Mn, Ni, Pb, S, Sc, Sr, Ti, V, Y, and Zn) required at each sample station in order to distinguish except Cd and Mo. For these two elements the sensitivity of between natural and polluted patterns. A single overbank the analytical method was not adequate, and (2) The profile very rarely spans the period from before anthro- majority of the elements (Al, As, Be, Ca, Co, Cu, Fe, Li, pogenic disturbance through the Industrial Revolution and Mn, Ni, Pb, Sb, Sc, Sr, Ti, V, Y) vary significantly between later. According to these authors such considerations and sites in relation to within-site variations ( pb0.01). However, associated costs may render overbank sediment non-viable the between-site variations for Ba, Cd, Mo and Zn are as a regional mapping medium. Instead they recommend to insignificant. For Cd and Mo this is ascribed to the use active stream sediments. analytical method, while those for Ba and Zn probably are Ridgeway et al. [28] also found that in Mexico the lateral caused by high sampling variability. variations of element contents in overbank sediments are Langedal [24] found that in floodplain surface sediments small for natural sediments, but become more complex for (0–25 cm) of the polluted Knabe3na river in Norway, the deposits with mixed pristine and polluted sediments. highest Cu and Mo concentrations occur in samples near the The conclusions drawn after the studies in Wales and river as well as in depressions. Enrichment of metals in Mexico have later been questioned by other researchers (see these parts of the floodplains may be an effect of differences e.g. Ref. [40]). It is difficult to imagine that active stream in the timing of the sediment transport pulse, and the timing sediments would be superior to overbank sediments in of floodplain inundation, see also [36]. In proximal areas polluted environments, since active sediments (contrary to B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 193 overbank sediments) are always polluted to an unknown stream, depending on local circumstances. Where possible, degree. sites close to the stream were avoided in order to reduce the possibility of collecting polluted samples. A vertical section 3.2.4. Concluding remarks on sampling error through the sediment was cut with a spade, and a composite In both small and large catchments, the sampling error sample (5 kg) was taken from the section excluding the for natural overbank sediments within a floodplain is small upper 5–10 cm. After drying, the samples were sieved to a in relation to the between floodplain variation. This minus 0.062 mm fraction, which were subsequently conclusion appears to be valid in most regions of the world analysed for the total contents of 30 elements and an acid for both genuine natural deposits and in situations where soluble fraction of 29 elements. pristine sediments at depth are covered by polluted surface Most elements show systematic patterns with great sediments. contrasts. In some cases these patterns agree with known Sampling of older terraces is appropriate in order to geological structures, in others they indicate structures not obtain pre-industrial material. Such sampling should be known earlier. Examples of the maps are shown in Fig. 8 done above the present inundation zone to avoid material (see comments on p. 00) and Fig. 11. The last figure shows draped during recent floods. Along laterally stable river- that the contents of acid soluble Mo in Norwegian overbank reaches, sampling in the bottom sections of the sediment sediments are relatively high in most of southern Norway, profiles is also adequate. Sampling along meandering while the levels further north vary. The province of high Mo reaches, as suggested by Bogen et al. [41], may also be a concentrations in the south agrees with the results of earlier possibility if the lateral migration is slow. prospecting, which disclosed a great number of small and Pollution of overbank sediments may be of two types: (1) some more extensive Mo-deposits within the province, Mine wastes and other anthropogenic material may enter the including those mined at Knaben (see p. 9 and e.g. Bugge stream from local sources and then be transported down- [42]). stream. (2) Airborne contaminants originating from distant McConnell et al. [43] carried out geochemical mapping sources may reach the catchments. Situation (1) is often in the Baie Verte/Springdale area of Newfoundland (2000 recognizable, since the sources may be easily identified. km2) based on several types of sample media including Situation (2) can be more difficult to recognize straight overbank and stream sediments. One hundred twenty-one away. In both cases (1) and (2) the contaminants may be samples of each of these types were collected from drainage confined to the surface layers of the overbank sediments. basins 2–10 km2 in size, sieved to minus 0.063 mm and However, in some locations mixtures of old natural and analysed for 38 elements. In this survey area overbank resent polluted sediments may occur, causing an intricate sediments were found to be more widespread and easier to stratigraphy in the overbank deposits. In addition, down- sample than stream sediments. In general, trace element ward percolation of soluble surface pollutants may also distributions are similar in the two media, both producing contaminate the sediments at depth. In such cases detailed significant patterns reflecting the chemistry of the under- investigations at each sample site may be necessary. lying bedrock. For most elements the concentrations are It is concluded that only trained personnel should be used greater in the overbank than in the stream sediments, in order to select appropriate locations for sampling of supposedly because overbank sediments are more fine- overbank sediments. If this prerequisite is fulfilled, high grained than stream sediments. In some drainages stream quality subsequent chemical analysis of the samples will sediments are contaminated by past mining activity, while produce reliable data for most chemical elements. overbank sediments, however, appear unaffected. Ede´n and Bjfrklund [10] performed ultra-low density 3.3. Representativity and regional distribution of chemical sampling of overbank sediment and other sample types elements in overbank sediments across Finland, Norway and Sweden (1 sample station per 23,000 km2), and found that for 20 elements the within site Many papers have appeared in the last few years variation is small compared with the between site variation presenting data on the spatial distribution of element (Table 2), and that one sample of overbank sediment may contents in overbank sediments. Selected publications with substitute for 6–20 till samples depending on type of examples from eight countries in Asia, Europe and North drainage area. America are referred below in chronological order of Xiachu and Mingkai [44] performed an orientation appearance. survey in a part (170,000 km2) of the Jiangxi Province of Ottesen et al. [3] pioneered the use of overbank Southern China in order to develop techniques of sediments in geochemical mapping and edited an atlas of implementing global ultra-low sampling in geochemical geochemical maps based on this type of sample [26]. Nearly mapping [45]. Sample sites (1 site per 1800 km2) were laid 700 floodplains distributed across Norway (300,000 km2) out at the apexes of 94 drainage basins, the sizes of which were selected for sampling. Each plain represents drainage were between 100 and 800 km2. Composite samples of areas of between 60 and 600 km2 . The samples were overbank sediment were collected from the upper (5–40 collected at distances of 2–200 m from the present-day cm) and the lower (50–120 cm) layers of sediment profiles 194 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199

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 overbank sediments and averages (NN30) for the same stream water level. The samples were analysed for 39 elements computed for earlier obtained samples, see an elements. example for lead in Fig. 13. For each drainage basin the contents of five selected Xiachu and Mingkai [44] concluded that (1) floodplains elements (Cu, Pb, Sn, W and Zn) in the overbank sediments of 100–800 km2 catchments are suitable sample stations were compared with the averages of the same elements for global geochemical mapping based on overbank obtained in the National Geochemical Mapping Project of sediments, (2) sampling of wide-spaced lower-layer over- China (1–2 samples per km2), see Fig. 12. There are good bank sediment is a fast and cost-effective way to identify correlations between the concentrations of these elements in geochemical provinces, (3) there is a significant correlation B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 195

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 Fig. 12. Illustration of how one sample of overbank sediment (left) survey area. This background reflects the composition of the represents many samples of active stream sediments (right). underlying bedrock. The active stream sediments and the upper overbank between the W content in the overbank sediment samples sediment have been polluted to a varying degree, and by and the presence of known W occurrences in the bedrock, comparing trace element concentrations in these media and (4) the distributions of elements such as Be, Cr, Ni, with the concentrations in the lower overbank sediment, the Rb and V characterize known geological formations in the degree of pollution could be assessed. From such data it is region. clear that the most severe pollution occurs in the northern In Greece, similar data on the representativity of over- part of Belgium, where the population is denser and bank sediment, as those described for China, were obtained industry is more developed than elsewhere in the survey by Demetriades and Volden [33]. They state that the element area. content in only one sample of overbank sediment represent- Xuejing and Hangxin published the most recent results of ing a large drainage basin is close to the median value for a pilot study for the use of overbank sediment in China [47]. the same element in several hundred samples of stream They selected 500 floodplains across the entire country sediments taken from small sub-catchments within the large (9,600,000 km2) for sampling, each plain representing a basin, see also p. 00. drainage basin in the order of 1000–6000 km2. At each plain In Belgium and Luxembourg (survey area 33,000 km2) two samples were collected at depths of 0–25 and 80–100 Van der Sluys et al. [46], produced a geochemical atlas cm, respectively. The samples were analysed for 71 based on samples of overbank sediments at 66 sites located elements. Statistical parameters for the analytical results as in the apexes of catchments, which range in size from 60 to well as maps for the distributions of Cu, Hg and Ni are 600 km2, see also Refs. [18–21]. At each site an overbank presented in the publication. Element contents in the widely

Fig. 13. Lead content in single samples of overbank sediment (left) and median values per catchment for lead in stream sediments (right) in the Jiangxi province, southern China. After Xiachu and Mingkai [44]. 196 .Blie ta./Ceoerc n nelgn aoaoySses7 20)183–199 (2004) 74 Systems Laboratory Intelligent and Chemometrics / al. et Bølviken B.

Fig. 14. Al, K, Sc and Si contents in overbank sediments in Belgium and Luxembourg. After Van der Sluys et al. [46]. B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 197 spaced samples were compared with those from China’s 4. Summary discussion and conclusions Regional Geochemical National Reconnaissance Program (CRGNRP), which includes N1million samples of active The characteristics of drainage regimes and sedimenta- stream sediments. Selected results of these comparisons are tion processes vary throughout the world, but it appears that presented in Fig. 15, which shows that the geochemical data in most places overbank sediments are readily available and generated from the wide-spaced sampling are strikingly very useful as a sampling medium in geochemical mapping, similar to those generated by the CRGNRP. A map of the even in polluted areas. In some places, such as in parts of ratio between Hg contents in the surface samples and Hg in Britain and Mexico, as well as in heavily polluted areas in the samples taken at depth (Fig. 16) demonstrates clearly, the arctic, the geological and historical setting apparently that Hg (supposedly airborne) from industrial and urban makes detailed studies necessary in order to obtain relevant sources has polluted the eastern part of China. These results samples. have lead to the establishment of abatement strategies, There is hardly an alternative sample type if the goal is to which will include monitoring of time trends in the pollution detect both natural and polluted patterns at regional to by repeated sampling and analysis of overbank sediments continental scales. Biological substances and materials such every 10 years. as active stream sediments and stream waters may be

Fig. 15. Distribution of copper in China. (A) Upper part: Averages from great numbers of stream sediments within each catchment basin. (B) Lower part: Averages of restricted numbers of overbank sediment samples per catchment. After Xuejing and Hangxin [47]. 198 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199

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 Fig. 16. Mercury pollution as indicated by the ratio between the mercury effective averages for very large lots. In principle the only content in overbank sediments near the surface (0–25 cm) and that at depth significant difference between an industrial stockpile and a (80–100 cm) After Xuejing, and Hangxin [47]. deposit of overbank sediment is that the first is man made, while the other is natural. The degree of success in the polluted to an unknown degree, while soils in addition have averaging process of this type of natural deposits has only undergone soil forming processes, jeopardizing a compar- been summarised in this paper, and a future more in-depth ison of the pollution of ancient and recent layers in vertical treatment is warranted. 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 References available in many areas due to the lack of suitable lakes. 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