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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, 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
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. An in situ treatment of the Anchor Hill Pit, an open pit at the Gilt Edge site containing approximately 70 million gallons of acidic water containing high levels of metals, sulfate, and nitrate, is being performed. The treatment consisted of an initial neutralization step, followed by a biological treatment to further improve water quality and create a long-term, stable system. After the two-step treatment, the project entered a monitoring mode where the pit lake was physically and chemically characterized on a quarterly basis for several years. 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 FY04 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 FY04, 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.
Also in FY04, significant effort was devoted toward discharging treated water from the pit. It appeared that use of the technology for long-term water treatment would probably be to feed raw or partially neutralized acid rock drainage to the deeper anoxic zone, and after treatment, remove water from that zone for discharge. It was thought that the water would probably require filtration to remove residual suspended metal sulfides, then aeration to bring dissolved oxygen and biochemical oxygen demand to desired levels for discharge. Tests were performed determining the degree of filtration required. An existing pond on-site was modified to serve as an aeration pond. A batch of approximately 150,000 gallons was pumped from the pit, filtered, and aerated in the aeration pond. There were small anticipated releases of hydrogen sulfide gas due to the presence of excess dissolved hydrogen sulfide in the deep zone of the pit. Remaining excess hydrogen sulfide was oxidized to elemental sulfur during aeration, resulting in a slow-to-settle colloidal precipitate. Eventually, the bulk of this water was found to be of dischargeable quality and was discharged from the site. This indicates, that in principle, the technology could be used on a batch basis to successfully produce dischargeable water, though issues with hydrogen sulfide and the colloidal sulfur precipitate would need to be addressed. The MWTP will continue involvement in this project through FY05, and the final report is expected to be completed in FY06.