The effect of mining and related activities on the sediment-trace element geochemistry of Lake Coeur d'Alene, Idaho, USA. Part III. Downstream effects: the Spokane River Basin

During 1998/1999, surface and subsurface sediment samples were collected al ong the entire length of the Spokane River from its outlet at the northern end of Lake Coeur d'Alene (CDA), Idaho, to Lake Roosevelt on the Columbia R iver, Washington. The study was conducted to determine if the trace element enrichments observed in Lake CDA acid on the floodplain and in the CDA Riv er extend through the Spokane River Basin (SRB). As in Lake CDA, surface sediments in the SRB are enriched in Pb, Zn, As, Cd , Sb and Hg relative to local background levels. Pb, Cd and Zn are the most elevated, with maximum enrichment occurring in the upper Spokane River in close proximity to Lake CDA. On average, enrichment decreases downstream, a pparently reflecting both increased distance from the inferred source (the CDA River Basin), as well as increased dilution by locally derived but unen riched materials. Only Cd and Zn display marked enrichment throughout the S RB. Pb, Zn and Cd seem to be associated mainly with an operationally define d iron oxide phase, whereas the majority of the As and Sb seem to be matrix -held. Subsurface sediments also are enriched in Pb, Zn, As, Cd, Sb and HE: relati ve to background levels. Based on Cs-137 and excess Pb-210 dating, trace cl ement enrichment began in the middle part of the SRB (Long Lake) between 19 00 and 1920. This is contemporaneous with similar enrichments observed in L ake CDA, as well as the completion of Long Lake Dam (1913). In the most dow nstream part of the basin (Spokane River Arm of Lake Roosevelt), enrichment began substantially later, between 1930 and 1940. The temporal difference in enrichment between Long Lake and the River Arm may reflect the latter's greater distance from the presumed source of the enrichment (the CDA River Basin); however, the difference is more likely the result of the completion of Grand Coulee Dam (1934-1941), which formed Lake Roosevelt, backed up th e Spokane River, and increased water levels in the River Arm by about 30 m.