Overbank sediments: a natural bed blending sampling medium for large scale geochemical mapping B

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1 Chemometrics and Intelligent Laboratory Systems 74 (2004) Overbank sediments: a natural bed blending sampling medium for large scale geochemical mapping B B. Bblviken a, *, J. Bogen b, M. Jartun a, M. Langedal c, R.T. Ottesen a, T. Volden a a Geological Survey of Norway, NO-7491 Trondheim, Norway b Norwegian Water Resources and Energy Administration, P.O. Box 5091 Majorstua, NO-0301 Oslo, Norway c City 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. 1. Introduction B Please note that the figures in this article appear in full colour in the online version on * Corresponding author. Tel.: ; fax: address: bjorn.bolviken@ngu.no. 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 /$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi: /j.chemolab

2 184 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 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. 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 geochemical 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 km 2 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 contribution 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 mg/kg, which is much higher than the background levels in the surrounding 30 km 2 (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 sites within an area of 250,000 km 2 (1 sample station per 50 km 2 ). 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 Fig. 1. Lead content (mg/kg) in stream sediments from the Gal3a river and its tributaries, Hedmark county, Norway. After Bjbrlykke et al. [5].

3 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) ø Fig. 2. Chromium content in stream sediments from Northern Fennoscandia. After Bblviken et al. [6].

4 186 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) geographical neighbours. This indicates that the consistencies 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 km 2 ), 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 recognized. The maps have an interesting potential use in mineral exploration, environmental research and epidemiology (geomedicine, medical geology) 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 km 2 is insufficient 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 km 2 (1 sample per 40 km 2 ), 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 km 2 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 Edén 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 km 2 (1 sample site per 23,000 km 2 ). Nevertheless, systematic 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. 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].

5 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 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 airborn gamma-ray surveys. After Gustavsson et al. [7].

6 188 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 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 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 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 conclusions 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. 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.

7 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 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 sediments could be a potentially more representative sample type than active stream sediments, many publications have appeared concerning the use of overbank sediments in geochemical mapping. The following section summarizes the main aspects of these contributions 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 temporarily 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 Fig. 9. Water discharges during ordinary conditions with normal amounts of water (A, left), and major floods (B, right).

8 190 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 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 stratigraphy 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 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 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 sediments the available time intervals are normally more restricted, because new sediments are occasionally deposited 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, distributions 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]. In30of these, pre-industrial sequences could be detected below polluted surface overbank sediments. Samples were collected at depth intervals of 10 cm and analysed for their contents of major and trace elements. 14 C 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 concentrations 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 preindustrial 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 regulations 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-precipitated 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.

9 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 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, Edén 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 km 2 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 Cu Ni Data from a 12,000 km 2 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] 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, Edén 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 km 2 drainage basins distributed over the Eastern Macedonia and Thrace regions in Greece were selected for sampling. After excluding the upper 5 10 cm, two

10 192 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 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 km 2 area in Northern Europe [10] % F F F Al Ba Ca Co Cr Cu Fe K La Mg Mn Na Ni P Pb Sr Th Ti V Zn Critical F value at p=0.05 Numbers of pairs (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 m apart at the down-stream apex of each drainage basin. These paired samples were analysed chemically 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] 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 transported 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 anthropogenic 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

11 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) overbank sediments) are always polluted to an unknown degree 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 riverreaches, 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 downstream. (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, downward 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 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 km 2 ) were selected for sampling. Each plain represents drainage areas of between 60 and 600 km 2. The samples were collected at distances of 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 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 km 2 ) 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 km 2 in size, sieved to minus 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 underlying bedrock. For most elements the concentrations are greater in the overbank than in the stream sediments, supposedly because overbank sediments are more finegrained than stream sediments. In some drainages stream sediments are contaminated by past mining activity, while overbank sediments, however, appear unaffected. Edén 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 km 2 ), 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 km 2 ) 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 km 2 ) were laid out at the apexes of 94 drainage basins, the sizes of which were between 100 and 800 km 2. Composite samples of overbank sediment were collected from the upper (5 40 cm) and the lower ( cm) layers of sediment profiles

12 194 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 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 km 2 ), 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 km 2 catchments are suitable sample stations for global geochemical mapping based on overbank sediments, (2) sampling of wide-spaced lower-layer overbank sediment is a fast and cost-effective way to identify geochemical provinces, (3) there is a significant correlation

13 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 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 overbank 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 representing 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 km 2 ) 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 km 2, see also Refs. [18 21]. At each site an overbank profile was dug near the river, and bulk samples were taken at depths of 5 25 cm and, if possible, at m. For most sample sections evidence such as 14 C 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 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 km 2 ) for sampling, each plain representing a drainage basin in the order of km 2. At each plain two samples were collected at depths of 0 25 and 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 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].

14 196 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) Fig. 14. Al, K, Sc and Si contents in overbank sediments in Belgium and Luxembourg. After Van der Sluys et al. [46].

15 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 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. 4. Summary discussion and conclusions The characteristics of drainage regimes and sedimentation 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 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].

16 198 B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 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 ( cm) After Xuejing, and Hangxin [47]. polluted to an unknown degree, while soils in addition have undergone soil forming processes, jeopardizing a comparison 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. References [1] The New Encyclop&dia Britannica, vol. 9, 1987, p [2] M. Hale, J.A. Plant (Eds.), Drainage Geochemistry in Mineral Exploration, Handbook of Exploration Geochemistry, vol. 6, Elsevier, Amsterdam, [3] R.T. Ottesen, J. Bogen, B. Bblviken, T. Volden, Overbank sediment: a representative sampling medium for regional geochemical mapping, J. Geochem. Explor. 32 (1989) [4] P. Gy, Sampling of heterogeneous and dynamic material systems, Theories of Heterogeneity, Sampling and Homogenizing, Elsevier, Amsterdam, 1992, p [5] A. Bjbrlykke, B. Bblviken, P. Eidsvig, S. Svinndal, Exploration of disseminated lead in southern Norway, Prospecting in Areas of Glaciated Terrain: Institution of Mining and Metallurgy, 1973, pp [6] B. Bblviken, J. Bergstrbm, A. Bjbrklund, M. Kontio, P. Lehmuspelto, T. Lindholm, J. Magnusson, R.T. Ottesen, A. Steenfelt, T. Volden, Geochemical Atlas of Northern Fennoscandia, Geological Surveys of Finland, Norway and Sweden, 1986, p. 19, 143 maps. [7] N. Gustavsson, B. Bblviken, B.D. Smith, R.C. Severson, Geochemical landscapes of the conterminous United States new map presentations for 22 elements, U. S. Geol. Surv. Prof. Pap (2001) 38. [8] J.S. Duval, W.J. Jones, F.R. Wriggle, J.A. Pitkin, Potassium and thorium maps of the conterminous United States, U. S. Geol. Surv. Open-File Rep (1990) 17. [9] F.M. Fordyce, P.M. Green, P.R. Simpson, Simulation of regional geochemical survey maps at variable sample density, J. Geochem. Explor. 49 (1993) [10] P. Edén, A. Bjfrklund, Ultra-low density sampling of overbank sediment in Fennoscandia, J. Geochem. Explor. 51 (1994) [11] B. Bblviken, P.R. Stokke, J. Feder, T. Jbssang, The fractal nature of geochemical landscapes, J. Geochem. Explor. 43 (1992) [12] B.B. Mandelbrot, The Fractal Geometry of Nature, W.H. Freeman, New York, [13] J. Feder, Fractals, Plenum Press, New York, 1988, p [14] W.K. Fletcher, Analytical methods in geochemical prospecting, in: Handbook of Exploration Geochemistry, vol. 1, Elsevier, Amsterdam, 1981, p. 255.