Determination of Premining Geochemical Background and Delineation of Extent of Sediment Contamination In Blue Creek Downstream from Midnite Mine, Stevens County, Washington

Geochemical and radionuclide studies of sediment recovered from cores in Blue Creek downstream from Midnite mine and Blue Creek delta in Lake Roosevelt provided a stratigraphic geochemical record of the contamination from uranium mining. Sediment recovered from deeper portions of the core in a wetland downstream from the mine site and from the fan and delta in Blue Creek cove, provided sufficient data to determine the premining geochemical background prior to mine development. Premining geochemical background from Blue Creek basin has also been determined using stream-sediment samples from parts of the Blue Creek, Oyachen Creek, and Sand Creek drainage basins not impacted by mining. Premining uranium concentrations in the Midnite mine tributary immediately downstream of the mine site were strongly elevated relative to the crustal abundance of uranium (2.3 ppm). 137Cs data and public records of production at the Midnite mine site provided age control to document timelines in the sediment from the core immediately downstream from the mine site. Contamination of the sediment by uranium during mining is documented along a small mine tributary to the confluence of Blue Creek, in Blue Creek, and in Blue Creek delta. Base metals in sediment of Blue Creek delta were elevated by as much as 4 times, Cd by 10 times, and U by more than 50 times premining geochemical background. Postmining metal concentrations in sediment are lower than during the mining period (1956-1981), but remain elevated relative to premining geochemical background. Sediment composition of surface sediment indicates that Blue Creek delta is contaminated by mining at the Midnite mine. Geochemical data show that base-metal contamination by arsenic, cadmium, lead, and zinc in sediment in the delta in Blue Creek cove is dominated by suspended sediment from the Coeur d'Alene mining district.