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Copper, Zinc, and Arsinic in Bottom Sediments of Clark Fork River Reservoirs: Preliminary Findings

Acetic acid extracts of sediments from four Clark Fork River reservoirs,--Milltown, Thompson Falls, Noxon Rapids, and Cabinet Gorge--indicate that mining and smelting operations in the upper drainage have enriched metal concentrations throughout the river. Milltown Reservoir contains metal values over 10 times the background levels for copper, zinc, and arsenic. Enrichment over background decreases downstream where the lower reservoirs contain copper and zinc concentrations 4 to 10 times over background values. In the lower reservoirs arsenic is not elevated over background. These trends suggest that contaminants have been transported over 350 miles from the major source of metals in the upper drainage and very likely reside in the sediments of the Clark Fork River delta in Lake Fend Oreille.