ScholarWorks at University of Montana

Using metal mixtures to identify site-specific risks to stream benthic macroinvertebrates in the Clark Fork River, MT

Tissue residue studies using benthic macroinvertebrates are often used to infer risk to organisms affected by metals exposure. These organisms accumulate metals from their environment and provide a direct measure of metal bioavailability. Though copper and arsenic are the primary elements of concern, a century of hard rock mining within the Clark Fork River (CFR) basin has inundated the watershed with a suite of heavy metals. Accordingly, aquatic organisms in this system are exposed to a mixture of metals that varies in concentration depending upon the relative contribution of individual or combinations of metals. To assess possible effects of remediation efforts in the most contaminated segment of the upstream reach, metal bioaccumulation patterns for standardized metals and metal mixtures were analyzed for two species of caddisflies (Hydropsyche occidentalis and H. cockerelli). Since 1997, samples were collected annually at 10 long-term monitoring stations in the CFR. Site-specific differences in individual metal distribution patterns varied for some metals, suggesting the presence of local source of metal inputs to the receiving system independent of historic mining-related sources in the headwater portions of the drainage. Piecewise regression analysis of cumulative distribution plots of individual and metal mixture concentrations in caddisflies also identified river segments of greatest relative change in bioavailable metals. Comparisons of metal mixtures and regression slopes over time provides a means to evaluate the extent to which remediation activities have affected metal-distribution patterns and relative concentrations of bioavailable metals in the CFR drainage.