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Bioremediation of Pit Lakes (Gilt Edge Mine)
Primary Issue Addressed: Pit Lakes
Secondary Issue Addressed: Biological Treatment, Characterization, Modeling
Project Site: Gilt Edge Mine Superfund site near Deadwood, South Dakota
Collaborating Entities: MSE Technology Application, Inc.; U. S. Environmental Protection Agency (EPA) Region VIII; CDM Federal Services; Arcadis-U.S.; South Dakota Department of the Environment and Natural Resources
Cost Share: Site logistical support and laboratory analytical support by CDM under EPA Region 8 Remedial Action Contract.
Project Description
This project focuses on an in situ treatment of the Anchor Hill Pit, an open pit at the Gilt Edge site originally containing approximately 70 million gallons of acidic water containing high levels of metals, sulfate, and nitrate. EPA Region 8’s interest in this project is to conduct a treatability study as part of the site Remedial Investigation/Feasibility Study (RI/FS) process, while the Mine Waste Technology Program’s (MWTP) interest is to develop data applicable to other similar sites. The treatment consisted of two steps. The initial step was to neutralize the pit lake with lime using a Neutra-Mill, essentially a floating lime slaker developed in Australia. The second step was to add nutrients to the pit to stimulate biological activity, which was intended to further improve water quality and create a long-term, stable system. After the two-step treatment, accomplished between March and May 2001, the project entered a monitoring mode where the pit lake was regularly physically and chemically characterized. The purpose was to see how well the treatments work and how stable the pit lake water becomes, e.g., if metal sulfides are produced, does the system reoxidize and remobilize those metals.
Status
Project accomplishments in fiscal year (FY) 2005 included continuation of monitoring the pit water chemistry via obtaining analytical samples regularly as well as vertical profiles of physical measurements. In 2002 to 2003, the pit lake had become strongly meromictic, i.e., stratified such that the vertical water column does not mix during the year. The surface zone was well-aerated, while the deeper zone was very anoxic. This meromictic condition remained throughout FY05, and the only noticeable change in pit water chemistry was the slow continuation of sulfate reduction in the deep portion of the pit due to the presence of excess carbon and nutrients.
In FY05, a continuing effort was devoted toward discharging treated water from the pit. During the winter, while the pit was frozen over, approximately four million gallons of water was pumped from immediately beneath the ice, aerated via a riprap, and successfully discharged, meeting all applicable South Dakota state water quality criteria. An additional ten million gallons of surface water were discharged in August 2005. Also in August 2005, concentrated hydrogen peroxide (H2O2) was added to the deep, anoxic zone with the goal of eliminating excess hydrogen sulfide, which would hopefully lead toward discharge of more water. The remainder of FY05 was spent monitoring the outcome of the hydrogen peroxide addition. It is expected that monitoring will continue through the winter of 2005-2006, with the project being considered complete at that point. The final report is expected to be completed in FY06.