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DISTRIBUTION OF ELEMENTS IN STREAM SEDIMENT

by W. De Vos¹, M.J. Batista², S. Pirc³, P.J. O’Connor⁴, A. Demetriades⁵, T. Tarvainen⁶, R. Salminen⁶, S. Reeder⁷, I. Salpeteur⁸, V. Gregorauskiene⁹

with contributions by

K. Lax¹⁰, J. Halamic¹¹, A. Pasieczna¹², I. Slaninka¹³, A. Mazreku¹⁴, U. Siewers¹⁵ , M. Birke¹⁵, N. Breward⁷, M. Bidovec¹⁶, B. De Vivo¹⁷, A. Lima¹⁷, M. Duris¹⁸, J. Locutura¹⁹, A. Bel-lan¹⁹

¹Geological Survey of Belgium, Brussels, Belgium;
²Geological Survey of Portugal, Alfragide, Portugal;
³Geology Department, University of Ljubljana, Ljubljana, Slovenia;
⁴Geological Survey of Ireland, Dublin, Ireland;
⁵Institute of Geology and Mineral Exploration, Athens, Greece;
⁶Geological Survey of Finland, Espoo, Finland;
⁷British Geological Survey, Keyworth, Nottingham, United Kingdom;
⁸Geological Survey of France, Orléans Cedex, France;
⁹Geological Survey of Lithuania, Vilnius, Lithuania;
¹⁰Geological Survey of Sweden, Uppsala, Sweden;
¹¹Institute of Geology, Croatia, Zagreb, Croatia;
¹²Polish Geological Institute, Warsaw, Poland;
¹³Geological Survey of Slovak Republic, Bratislava, Slovak Republic;
¹⁴Centre of Civil Geology, Tirana Albania;
¹⁵Bundesanstalt für Geowissenschaften und Rohstoffe, Hannover, Germany;
¹⁶Geological Survey of Slovenia, Slovenia;
¹⁷Dipartimento di Scienze della Terra, Universita' di Napoli "Federico II", Naples, Italy;
¹⁸Czech Geological Survey, Prague, Czech Republic;
¹⁹Geological Survey of Spain, Madrid, Spain.

Stream sediment is derived from the erosion and transport of soil and rock debris, and other materials within the catchment basin upstream of the sampling site. It is, thus, representative of the geochemistry of materials from the upstream drainage basin. Stream sediment was first used effectively in mineral exploration since the 1950’s (Lovering et al. 1950, Hawkes and Bloom 1956, Boyle 1958, Webb 1958a, 1958b). Its suitability in environmental and multidisciplinary studies was recognised with the publication of the first regional geochemical atlases by the Applied Geochemistry Research Group at Imperial College of Science and Technology of the University of London (Webb et al. 1973, 1978). Since then many national geochemical atlases have been published in Europe using stream sediment (Plant and Ridgeway 1990, Plant et al. 1996, 1997). Low sample density geochemical mapping projects using stream sediment, covering large areas, were performed by Garrett and Nichol (1967), Armour-Brown and Nichol (1970), Reedman and Gould (1970), Reedman (1973), Shacklette and Boerngen (1984). An excellent review of stream sediment case studies is given by Hale and Plant (1994).

Stream sediment samples in the present FOREGS project were collected from the small catchment basin (<100 km²), where stream water and residual soil were taken (Salminen et al. 2005a). Generally, recent active stream sediment from the stream bed was sampled, except in some dry streams in Mediterranean countries, where old stream sediment was collected.

It is noted that the description below refers to the new stream sediment maps in this volume, including the results from Sweden, and not to the maps in Part 1 of the Geochemical Atlas of Europe (Salminen et al. 2005).

In the description of element distribution in stream sediment, as for soil, the following definitions were adopted with reference to the coloured maps and the histograms in Part 1 of the Geochemical Atlas of Europe (Salminen et al. 2005):

• ·Low values group the three lowest shades of blue in the colour scale, corresponding to the range from the minimum value up to the 25th percentile, defined as “very low” and “low background” concentrations in Part 1 (Tarvainen et al. 2005, p.97), and
• ·High values group the three highest shades of red in the colour scale, corresponding to the range of values from the 75th percentile up to the maximum, defined as “high”, “very high” and “highly anomalous” concentrations in Part 1 (Tarvainen et al. 2005, p.97).

Correlation coefficients were calculated with Pearson’s product-moment linear correlation method (Table in electronic format on website) after deletion of outliers and subsequent pairwise deletion of absent data. For a given element (or oxide), outliers were defined here as values exceeding by a factor of 1.5 other nearby results, when all analytical results are ranked. They are generally visible on the histogram accompanying each map in Part 1 of the Geochemical Atlas. A maximum of four outliers were removed in this work for the calculation of linear correlation coefficients. A list of outliers is given for stream sediment (Table 5).

Table 5. Outliers of the stream sediment data. Criterion: an outlier has a value exceeding by factor of 1.5 other nearby results, when all analytical results are ranked. A maximum of four outliers were removed for the calculation of linear correlation coefficients.

Throughout the text the following notation is used for the correlation coefficients:

• Very strong correlation: >0.8;
• Strong correlation: between 0.6 and 0.8;
• Good correlation: between 0.4 and 0.6, and
• Weak correlation: between 0.3 and 0.4.

Because of the large number of samples, even the so-called weak correlations are significant at the 0.01 confidence level.

For a discussion on the merits of correlation coefficients in this large dataset, the reader is referred to the introduction to the distribution of elements in soil.

Acknowledgements

Dr Clemens Reimann from the Geological Survey of Norway gave valuable comments on this chapter.

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