HEAVY-METAL CONTAMINATION IN OVERBANK SEDIMENTS OF THE GEUL RIVER (EAST BELGIUM) - ITS RELATION TO FORMER PB-ZN MINING ACTIVITIES

Overbank sediments of the Geul River (East Belgium) are highly contami nated by the heavy metals Pb, Zn, and Cd due to former Pb-Zn mining ac tivities in the drainage basin. Geochemical variations in vertical ove rbank sediment profiles sampled 1 km north of the mine tailings of Plo mbieres allow metal fluxes back to the 17th century to be reconstructe d. The vertical profiles are subdivided into three major units corresp onding to different industrial periods based on sedimentological crite ria as well as on the distribution of contaminants. Alluvial sediments with the highest heavy metal concentrations correspond to the major p eriod of mining activity of the 19th century. The fact that Zn mining at the La Calamine open mine started before large-scale mining of the PbS-ZnS subsurface exploitations is reflected in the vertical profiles by an increase in Zn content before a marked increase in Pb and Cu. T he regional extent of contamination in the alluvial deposits was evalu ated on the basis of the geochemical analysis of sediments at depths o f the 0-20 cm and 80-100 cm. Most of the upper samples are extremely c ontaminated. Significant local variations in heavy metal concentration in the lower samples are interpreted in terms of which overbank sedim ent horizon has been sampled at a depth of 80-100 cm. This indicates t hat ''blind'' sampling of overbank sediments to characterize the degre e of contamination in shallow boreholes can give very erratic results.