ScholarWorks at University of Montana

Effects of Floodplain Remediation on Bed Sediment Contamination in the Clark Fork River

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.