TRACKING RELEASE OF RESERVOIR SEDIMENT CONTAMINATED BY HEAVY METAL MINING WASTE: A STUDY OF LARGE-SCALE DAM REMOVAL AT MILLTOWN, MONTANA

The removal of Milltown Dam at the confluence of the Clark Fork and Blackfoot rivers, near Missoula, Montana, caused an elevation in heavy metals concentration in downstream sediment. The primary source of contamination came from base metal extraction mining waste which had collected in reservoir sediments for more than a century. Health and environmental concerns associated with high concentrations of arsenic and copper in the sediment and nearby groundwater prompted the removal of the dam and its toxic sediments. Milltown Dam was breached on March 28, 2008. Fine-grain (\u3c63 µm) bed sediment was collected between May 3 and August 21, 2008, over a 254-km stretch in the Clark Fork River downstream of the dam and analyzed for concentrations of heavy metals (As, Cd, Cr, Cu, Pb, Zn and Hg). The highest metals concentration occurred in early May, before the peak in stream discharge began transporting massive amounts of sediment. Metals concentration peaked at 290 ppm As, 8 ppm Cd, 2200 ppm Cu, 180 ppm Pb, 2400 ppm Zn, and 2 ppm Hg. Dam removal did not affect Cr, but the other metals were enriched well above background conditions: As was 42x higher and Cu was 103x higher in the lower CFR than its tributaries. Elevated metals concentration extended over the entire study area, decreasing exponentially with distance downstream and returning to near pre-breach conditions by the end of the study period. Supplemental data from the USGS for suspended-sediment was combined with bed sediment results to track the source, sequence and transport mechanisms of sediment release. Contaminants in the sediment acted as tracers, showing that heavily contaminated, fine-grain sediment was translated downstream immediately after dam removal, then nearly two months later the bulk of the sediment was rapidly dispersed during maximum stream discharge and diluted by uncontaminated bank sediment