MONTHLY VARIATIONS IN METAL AND ARSENIC CONCENTRATIONS BEFORE AND AFTER THE DRAW-DOWN OF THE MILLTOWN RESERVOIR, MILLTOWN, MT

Milltown Reservoir, located at the confluence of the Clark Fork and Blackfoot Rivers in Milltown, MT, has been accumulating sediment from upstream mining operations since it was built in 1907. In August of 2002, the EPA conducted a draw-down of the reservoir in order to measure sediment release rates to aid in the proposed cleanup plan. This study measured the monthly change in metal and arsenic concentrations downstream of the reservoir before and after the draw-down. Samples from six sites were collected monthly by scooping the top layer of sediment into a plastic funnel and mesh sieve apparatus to attain a grain size of less than 63ìm. The slurry of sieved sediment and river water is then centrifuged, dried, ground, digested, and analyzed on an ICAPES for arsenic and metal concentrations. Grain size analyses are completed as well to determine if size fractions correlate with contaminant concentrations. Sample analysis is still currently being performed, but preliminary data shows that concentrations below the dam increase after the lowering of the reservoir. Arsenic concentrations at one downstream site increase from 22 to 56 ppm and have elevated concentrations of Zn, Cu, and Pb. Reservoir restoration goals will attempt to minimize impacts to the downstream aquatic system. Results of this study suggest modification of sediment release scenarios may be required.