CHEMICAL CHARACTERIZATION OF SEDIMENTS AND PORE-WATER FROM THE UPPER CLARK-FORK RIVER AND MILLTOWN RESERVOIR, MONTANA

The upper Clark Fork River basin in western Montana is widely contamin ated by metals from past mining, milling, and smelting activities. As part of a comprehensive ecological risk assessment for the upper Clark Fork River, we measured physical and chemical characteristics of surf icial sediment samples that were collected from depositional zones for subsequent toxicity evaluations. Sampling stations included five loca tions along the upper 200 km of the river, six locations in or near Mi lltown Reservoir (about 205 km from the river origin), and two tributa ry reference sites. Concentrations of As, Cd, Cu, Mn, Pb, and Zn decre ased from the upper stations to the downstream stations in the Clark F ork River but then increased in all Milltown Reservoir stations to lev els similar to uppermost river stations. Large percentages (50 to 90%) of the total Cd, Cu, Pb, and Zn were extractable by dilute (3 N) HCl for all samples. Copper and zinc accounted for greater than 95% of ext ractable metals on a molar basis. Acid-volatile sulfide (AVS) concentr ations were typically moderate (0.6 to 23 mu mol/g) in grab sediment s amples and appeared to regulate dissolved (filterable) concentrations of Cd, Cu, and Zn in sediment pore waters. Acid-volatile sulfide is im portant in controlling metal solubility in the depositional areas of t he Clark Fork River and should be monitored in any future studies. Spa tial variability within a sampling station was high for Cu, Zn, and AV S; therefore, the potential for toxicity to sediment-dwelling organism s may be highly localized.