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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 analyzed to determine bulk sediment chemistry; selected subsamples were analyzed for trace element partitioning and ^7Cs 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 somewhat enriched in Cu, Fe and Mn. Variations in the thickness of the trace element-rich sediments, which range in thickness from more than 119cm to as little as 17cm, indicate that the source of much of this material is the Coeur d'Alene River. An estimated 75 million metric tons of trace elementrich sediments have been deposited on/in the lakebed. Estimated trace element masses in excess of those considered representative of background conditions, range from a high of 468,000 metric tons of Pb to a low of 260 metric tons 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, probably have been the same throughout 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 ranged 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 1880's. INTRODUCTION Lake Coeur d'Alene (CDA) is a natural (submerged river bed) lake in the northern panhandle of Idaho (Fig. 1). The lake lies between the Selkirk and the CDA Mountains and extends northward from the St. Joe River to the headwaters of the Spokane River near the city of CDA (Meckel Engineering, et al., 1983; Bender, 1991). The main body of the lake is about 3.2 km wide by 40 km long; however, the southern part is composed of four smaller interconnected lakes that were formed in 1906 when the Post Falls Dam (10 miles downstream on the Spokane River) was completed, (Meckel Engineering, et al., 1983) (Fig. 1). The St. Joe and CDA Rivers annually account for 94% (-4.73 Gm3/yr) of the inflow to Lake CDA (Meckel Engineering, et al., 1983; Javorka, 1991). Major outflow from the lake occurs at the northern end through the Spokane River (Fig. 1). The South Fork of the CDA River, which flows into the CDA River, and thence into Lake CDA, drains a substantial part of the CDA mining district and the so-called 'Silver Valley'. The mining district has been in operation since the 1880's and was one of the major sources of Ag, Cu, Pb and Zn in the U.S. (e.g., Bender, 1991). Until 1968, when tailings ponds were established to trap/retain tailings and limit sediment transport, most of the mining and ore-processing wastes were discharged directly into the South Fork of the CDA River. These materials were highly enriched in Ag, As, Cd, Cu, Fe, Mn, Pb, Sb and Zn (Rabe and Bauer, 1977; Bender, 1991). It has been estimated that during the course of mining, processing and smelting operations in the area, about 115 million metric tons of mine tailings were produced and that 60% of this material probably entered the South Fork of the CDA and the CDA River system (Javorka, 1991). In 1983, as a result of the elevated trace element concentrations in the mining and processing wastes, the U.S. Environmental Protection Agency (EPA) established the Bunker Hill Superfund Site that encompasses 54 km2 in the Kellogg and Smelterville Flats area; some 50 km upstream of Lake CDA. (e.g., Bender, 1991; Fig. 1). Over the years, several environmental studies have indicated that large amounts of mining, milling, and ore-processing wastes, enriched in a varietry of trace elements, have been deposited throughout the lower CDA River and Lake CDA, and have had a substantial effect on the sediment geochemistry, the water quality, and the biota of the South Fork of the CDA River, the main stem of the CDA River, Lake CDA, and the Spokane River (Ellis, 1940; Galbraith, 1971; Ellsworth, 1972; Funk, et al., 1973; Bauer, 1974; Maxfield, et al. 1974a, 1974b; Norbeck, 1974; Funk, et al., 1975; Sheppard and Funk, 1975; Johnson, et al., 1976; Inoannou; 1979; Wai, et al., 1985; Savage, 1986; Neufeld, 1987; Woods, 1989; Krieger, 1990; Piske, 1990; Bender, 1991; Horowitz, et al., 1993).

u.s. geological survey open file report/open-file report

The Effect of mining and related activities on the sediment-trace element geochemistry of Lake Coeur d'Alene, Idaho, U.S.A.; Part II, Subsurface sediments