Factors
Controlling the Spatial Gradient of Metals in a Mine-impacted River.
Hornberger,
M.I.*, Luoma, S.N., Cain, D.J. U.S. Geological Survey, Menlo Park, CA.
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).