A combined flood surface and geochemical analysis of metal fluxes in a historically mined region: a case study from the New World Mining District, Montana

Abstract.

Remediation of environmental damage due to mining activities requires distinguishing anthropogenic alteration from natural baseline conditions. Through ICP-OES analyses of the <63 µm fraction of overbank sediments dating from ~8,000 years to the present, the development of a regional flood probability model, traditional topographic surveying, and the use of flood surface modeling, it is shown that the combination of hydraulic modeling and geochemical analysis of sediments is an effective tool for determining remediation strategies. This combination provides a means of distinguishing natural from anthropogenic sediments in a naturally metal-rich region, delineating zones of sediments with elevated metal concentrations, and creating an accurate estimate for appropriate remediation. Specifically, the distribution of metals in Fisher Creek of the New World Mining District, Montana, suggests the following: (1) The Glengarry adit is currently a point source of metal contamination (the present surface distribution of metals in the <63 µm fraction of overbank deposits is related to the probability of a surface being covered by water during a flood of a given recurrence probability); (2) sediments with elevated metal concentrations pre-date mining activity; (3) pre-mining overbank sediments contain metal concentrations as great or greater than post-mining sediments; (4) remediation of sediments with the highest concentration of metals requires assessment of sediments deposited above the 100-year flood surface.