Zn, Pb, Cu and As distribution patterns in overbank and medium-order stream sediment samples: their use in exploration and environmental geochemistry

Overbank and medium-order stream sediment samples were collected in Belgium and Luxembourg from 66 sampling locations (area of about 33,000 km(2)) and analysed for major and trace elements among which Zn, Pb, Cu and As. At ea ch sampling location large bulk samples were taken, namely in the lower (no rmally at greater than or equal to 1.5 m depth, over an interval of about 2 0-40 cm) and upper (normally upper 5-25 cm) parts of the overbank profiles and from the stream sediments. Furthermore, at a number of these sites, a d etailed geochemical analysis of vertical overbank sediment profiles (sampli ng intervals of 10-20 cm) was subsequently carried out to unravel element v ariations through time and to help in the overall evaluation. For most samp led sections evidences such as C-14-dating and the absence of anthropogenic particles point towards a pre-industrial and often pristine origin of the lower overbank sediment samples. From the latter bulk samples, mean backgro und concentrations were deduced. They reveal the existence of significant d ifferences between the northern and southern part of Belgium (incl. Luxembo urg) which relate to the difference in geological substrate. In the north d ominantly non-lithified Quaternary and Tertiary sands, marls and clays occu r while in the south Palaeozoic sandstones, shales and carbonate rocks outc rop. Consequently separate mean background values were calculated for the t wo areas. In the southern study area, some anomalous metal concentrations h ave been recorded in pre-industrial sediments. They are derived from minera lised Palaeozoic rocks, a feature which could be of interest for base metal exploration. In the upper overbank and stream sediments, in general, highe r heavy metal and As contents were recorded with highest values in areas wi th metal mining, metal melting and cokes treatment industries. By comparing the trace element concentrations of the upper overbank or stream sediment samples with the concentrations detected in the lower overbank samples at e ach of the sampling locations, and by evaluating the vertical distribution patterns where available, the degree of pollution of the alluvial plain and the present-day stream sediments can be assessed. From this exercise, it i s clear that highest pollution occurs in the northern part of Belgium, whic h relates to its high population density and industrial development. (C) 19 98 Elsevier Science B.V. All rights reserved.