Effect of mining and related activities on the sediment trace element geochemistry of lake coeur D'Alene, idaho, USA. Part I: Surface sediments

During the summer of 1990, 12 gravity cores were collected in Lake Coeur d'Alene, Idaho at various depths and in a variety of depositional environments. All core subsamples were analysed to determine the bulk sediment chemistry; selected subsamples were analysed for trace element partitioning and 137Cs activity. The purpose of these analyses was to determine the trace element concentrations and distributions in the sediment column and to try to establish a trace element geochemical history of the lake in relation to mining and mining-related discharge operations in the area. Substantial portions of the near-surface sediments 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 than 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 tonnes of trace element-rich sediments have been deposited on or in the lake bed. Estimated trace element masses in excess of those considered representative of background conditions range from a high of 468 000 tonnes of Pb to a low of 260 tonnes of Hg. The similarity between the trace element-rich surface and subsurface sediments with respect to their location, their bulk chemistry, their interelement relations and their trace element partitioning indicate that the sources and/or concentrating mechanisms causing the trace element enrichment in the lake sediments have probably been the same through-out their depositional history. Based on a Mt St Helens'ash layer from the 1980 eruption, ages estimated from 137Cs activity and the presence of 80 discernible and presumably annual layers in a core collected near the Coeur d'Alene River delta indicate that deposition rates for the trace element-rich sediments have ranges from 2.1 to 1.3 cm/year. These data also indicate that 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.