EFFECT OF MINING AND RELATED ACTIVITIES ON THE SEDIMENT TRACE-ELEMENTGEOCHEMISTRY OF LAKE COEUR-DALENE, IDAHO, USA .2. SUBSURFACE SEDIMENTS

During the summer of 1990, 12 gravity cores were collected in Lake Coe ur d'Alene, Idaho at various depths and in a variety of depositional e nvironments. All core subsamples were analysed to determine the bulk s ediment chemistry; selected subsamples were analysed for trace element partitioning and Cs-137 activity. The purpose of these analyses was t o determine the trace element concentrations and distributions in the sediment column and to try to establish a trace element geochemical hi story of the lake in relation to mining and mining-related discharge o perations in the area. Substantial portions of the near-surface sedime nts in Lake Coeur d'Alene are markedly enriched in Ag, As, Cd, Hg, Pb, Sb and Zn, and slightly enriched in Cu, Fe and Mn. Variations in the thickness of the trace element-rich sediments, which range from more t han 119 cm to as little as 17 cm, indicate that the source of much of this material is the Coeur d'Alene River. An estimated 75 million tonn es of trace element-rich sediments have been deposited on or in the la ke bed. Estimated trace element masses in excess of those considered r epresentative of background conditions range from a high of 468 000 to nnes of Pb to a low of 260 tonnes of Hg. The similarity between the tr ace element-rich surface and subsurface sediments with respect to thei r location, their bulk chemistry, their interelement relations and the ir trace element partitioning indicate that the sources and/or concent rating mechanisms causing the trace element enrichment in the lake sed iments have probably been the same through-out their depositional hist ory. Based on a Mt St Helens' ash layer from the 1980 eruption, ages e stimated from Cs-137 activity and the presence of 80 discernible and p resumably annual layers in a core collected near the Coeur d'Alene Riv er delta indicate that deposition rates for the trace element-rich sed iments have ranges from 2.1 to 1.3 cm/year. These data also indicate t hat the deposition of trace element-rich sediments began, at least in the Coeur d'Alene River delta, some time between 1895 and 1910, dates consistent with the onset of mining and ore processing activities that began in the area in the 1880s.