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Overbank Sediments: a Natural Bed Blending Sampling Medium for Large—Scale Geochemical Mappingb

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Overbank Sediments: a Natural Bed Blending Sampling Medium for Large—Scale Geochemical Mappingb Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 www.elsevier.com/locate/chemolab Overbank sediments: a natural bed blending sampling medium for large—scale geochemical mappingB B. Bblvikena,*, J. Bogenb, M. Jartuna, M. Langedalc, R.T. Ottesena, T. Voldena aGeological Survey of Norway, NO-7491 Trondheim, 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 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 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 geology (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]. B. Bølviken et al. / Chemometrics and Intelligent Laboratory Systems 74 (2004) 183–199 ø 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
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