USE OF BENTHIC INVERTEBRATE COMMUNITY STRUCTURE AND THE SEDIMENT QUALITY TRIAD TO EVALUATE METAL-CONTAMINATED SEDIMENT IN THE UPPER CLARK-FORK RIVER, MONTANA

The upper Clark Fork River, above Flathead River, is contaminated with large amounts of As, Cd, Cu, Pb, Mn, and Zn ores from past mining act ivities. The contaminated area extends from the Butte and Anaconda are a to at least 230 km downstream to Milltown Reservoir. Both the upper Clark Fork River and Milltown Reservoir have been designated as U.S. E nvironmental Protection Agency Superfund sites because of metal-contam inated bottom sediments. We evaluated the impacts of past mining activ ities on the Clark Fork River ecosystem using benthic invertebrate com munity assessment, residue chemistry, and toxicity testing. Oligochaet a and Chironomidae generally accounted for over 90% of the benthic inv ertebrate community in the soft sediment depositional areas. Taxa of O ligochaeta and Chironomidae were predominantly pollution tolerant. Hig her numbers of Chironomidae genera were present at stations with highe r concentrations of metals in sediment identified as toxic by the amph ipod Hyalella azteca in 28-d exposures. Frequency of mouthpart deformi ties in genera of Chironomidae was low and did not correspond to conce ntrations of metals in sediment. Total abundance of organisms/m(2) did not correspond to concentrations of metals in the sediment samples. C hemical analyses, laboratory toxicity tests, and benthic community eva luations all provide evidence of metal-induced degradation to aquatic communities in both the reservoir and the river. Using a weight-of-evi dence approach - the Sediment Quality Triad - provided good concurrenc e among measures of benthic community structure, sediment chemistry, a nd laboratory toxicity.