Stratigraphy and Chemistry of Metal-Contaminated
Floodplain Sediments, Upper Clark Fork River Valley

David A. Nimick*
University of Montana

     Mine tailings deposited by historic floods contaminate large areas of the upper Clark Fork River floodplain. Figure 1. The metals which are highly enriched over background levels in floodplain sediments include Cu (up to 1800 times) and As, Pb. and Zn (up to 80 times). Based on floodplain mapping of tailings, over 704,000 m3 of mine wastes are spread over 275 ha along a 10 kilometer reach between Warm Springs and Racetrack. Figure 2. Tailings are up to 1.2 m thick and occur primarily in fine-grained overbank deposits and in point bars as reworked mixtures of tailings and cleaner sediment. Although most flood-contaminated areas have less than 30 cm of tailings, the thickest tailings generally are near the river and have the highest probability of being eroded into the river. Metal concentrations in total, acid, soluble, and water-soluble extracts of floodplain sediments indicate that metals released by oxidation of sulfides in tailings move either to the ground surface and precipitate as hydrated metal sulfates or move downward to be concentrated in acid-extractable phases such as diagenetic sulfides and organic complexes in reduced tailings or pre-mining floodplain deposits Bioavailable (acid-extractable) concentrations of As, Cu, Fe, Pb, and Zn are very high in bank sediments. When eroded into the river, concentrations of these metals can exceed EPA aquatic-life standards. Cattle grazing has a deleterious effect on streambank vegetation and increases the extent of bank erosion and, therefore, the amount of metal-rich sediment in the river. The percentage of actively eroding streambanks increases from 2.5% in ungrazed reaches to 16-21% in grazed reaches.

    Crusts of sulfate precipitates on streamside tailings dissolve readily in rainwater and release high concentrations of As, Al, Cd, Cu, Fe, Mn, Zn, and acid. A pollution index is proposed to quantify the average enrichment over aquatic-hazard levels of water-soluble metals, which occur in floodplain surface sediments, Figure 3. The pollution index correlates strongly with pH because much of the acidity produced during dissolution of surface salts is caused by the natural acidity of the transition metal (Cu, Cd, Mn, and Zn) ions released from metal sulfates. Because the pollution index can be predicted from pH, reconnaissance mapping of metal contamination in surface sediments on the Clark Fork River floodplain can he conducted quickly.
 

*Current address: U.S. Geological Survey Helena, MT 59626