Effects
of Floodplain Remediation on Bed
Sediment Contamination
in the Clark Fork River
Anna
B. Breuninger, Department of Geology, University of Montana, Missoula, MT 59812
The upper Clark Fork
River in western Montana is an ideal site in which to test long term effects of
remediation on the sediment quality of a large basin. Numerous EPA Superfund sites are located along the
mining-impacted river from the headwaters at Silver Bow Creek to Milltown, 240
river kilometers downstream. Between
1991 and 1998, remediation efforts chemically treated and stabilized floodplain
sources of metal-contaminated sediments. Settling
ponds, first constructed in the 1950’s, were upgraded
at the head of the Clark Fork River, immediately downstream of Silver Bow
Creek, a major source of As and trace metals to the Clark Fork River.
To determine the effect these actions had on bed sediment composition,
paired basin surveys in August 1991 and August 1998 were conducted, collecting
bed sediments at 44 sites on the upper Clark Fork River. Samples were dissolved
and analyzed for major and trace elements using ICP-ES. This study found that
major element concentrations appear to have remained constant or increased
slightly between 1991 and 1998. The
more mobile trace elements, such as Cd and Zn, slightly decreased in
concentration in the middle part of the basin.
Less mobile trace elements, such as Pb and Cu, showed
no apparent change between 1991 and 1998.
These data suggest that
response to remediation may be a slow process or that stabilization of
floodplain sources may not have a measurable effect on metal contamination in
fine-grained bed sediments.