EFFECT OF MINING AND RELATED ACTIVITIES ON THE SEDIMENT TRACE-ELEMENTGEOCHEMISTRY OF LAKE COEUR-DALENE, IDAHO, USA .1. SURFACE SEDIMENTS

During the summer of 1989 surface sediment samples were collected in L ake Coeur d'Alene, the Coeur d'Alene River and the St Joe River, Idaho , at a density of approximately one sample per square kilometre. Addit ional samples were collected from the banks of the South Fork of the C oeur d'Alene and the Coeur d'Alene Rivers in 1991. All the samples wer e collected to determine trace element concentrations, partitioning an d distribution patterns, and to relate them to mining, mining related and discharge operations that have occurred in the Coeur d'Alene distr ict since the 1880s, some of which are ongoing. Most of the surface se diments in Lake Coeur d'Alene north of Conkling Point and Carey Bay ar e substantially enriched in Ag, As, Cu, Cd, Hg, Pb, Sb and Zn relative to unaffected sediments in the southern portion of the lake near the St Joe River. All the trace element enriched sediments are extremely f ine grained (mean grain sizes much less than 63 mum). Most of the enri ched trace elements, based on both the chemical analyses of separated heavy and light mineral fractions and a two step sequential extraction procedure, are associated with an operationally defined Fe oxide phas e; much smaller percentages are associated either with operationally d efined organics/sulphides or refractory phases. The presence, concentr ation and distribution of the Fe oxides and heavy minerals indicates t hat a substantial portion of the enriched trace elements are probably coming from the Coeur d'Alene River, which is serving as a point sourc e. Within the lake, this relatively simple point source pattern is com plicated by a combination of (1) the formation of trace element rich a uthigenic Fe oxides that appear to have reprecipitated from material s olubilized from anoxic bed sediments and (2) physical remobilization b y currents and wind driven waves. The processes that have caused the t race element enrichment in the surface sediments of Lake Coeur d'Alene are likely to continue for the foreseeable future.