ScholarWorks at University of Montana

Factors Controlling the Spatial Gradient of Metals in a Mine-impacted River

Samples of bed sediment (<63um) and aquatic insect larvae have been collected annually since 1986 at stations encompassing 380 km of the Clark Fork river, Montana. With these data, we have assessed yearly changes in the relationships between sediment contamination levels and metal bioaccumulation relative to streamflow and remediation activities in the upper Clark Fork. In the earliest years of the study (1986-94), the gradient of metal bioaccumulation showed a clear trend in concentrations from upstream to downstream. Concentrations were highest in the upper 70 Km (200-220 ppm Cu; 3-7 ppm Cd) and moderate in the middle reach (70-180 Km: 30-40 ppm Cu; 0.5-1.0 ppm Cd). Since 1995, concentrations of metals in the upper 70 Km have decreased to 80-100 ppm Cu and 1.0-1.5 ppm Cd; Cd in sediments has also declined. Bioaccumulation concentrations in the middle reach have increased almost twofold since 1995 (80-120 ppm Cu; 1.5-2.0 ppm Cd), but metal concentrations in sediments remain unchanged. The upstream decline in bioaccumulated metal may be partly due to remediation. Intensive remediation activities have occurred in the upstream segment of the river since 1992. Hydrologic factors influence metal bioaccumulation in the middle reaches and may contribute to some of the interannual differences, however, the source of the uncoupling of bioaccumulation from sediment metals remains unclear. Copper concentrations in two bioindicators, “Hydropsyche spp.” and “Arctopsyche grandis” (order Trichoptera), correlate significantly with Cu in sediment in every year, but in some years of high streamflow, there is enhanced uptake at some stations. Correlations between Cd bioaccumulation and Cd in sediment were only apparent in low-moderate flow years (1990-92; 1994).