Anchor Hill pit lake two-stage in situ treatment, Gilt Edge Mine Superfund site, South Dakota, USA

The EPA Region VIII Superfund office and the EPA National Risk Management Research Laboratory (NRMRL) Mine Waste Technology Program (MWTP) are conducting a field-scale technology demonstration of a two-stage in situ treatment of the Anchor Hill Pit lake at the Gilt Edge Mine Superfund site near Deadwood, South Dakota. The project goal is to develop cost and performance data of the treatment approach for potential application in long-term water treatment/management activities at the Gilt Edge site, as well as potential application at other similar sites. The Anchor Hill Pit initially contained approximately 265 000 m 3 of acidic mine water, with elevated metals, sulfate, and nitrate content and a pH of approximately 3. The first step of the two-stage in situ treatment consisted of using a Neutra-Mill (developed in Australia, licensed to Shepherd-Miller, Inc. in U.S.) to neutralize the pit pH to approximately 7 using lime, accomplished between late March and early May 2001. After a stabilization period, a patented process for in situ pit lake treatment using an organic formulation of molasses, methanol, and proprietary ingredients was implemented in the pit by Green World Science, Inc. in late May 2001 to create reducing conditions; stimulate bacterial activity for nitrate, selenium, and sulfate reduction; improve water quality; and create a stable system. Additional neutralizing reagent was added in September 2002. Additional organic dosages have occurred in October 2001, April 2002, and September 2002. As of June 2003, bacterial activity has reduced nitrate concentrations to non-detectable levels and bacterial sulfate reduction has been initiated, with associated decreases in metals concentrations. The pit lake has become a meromictic lake with a chemocline at a depth of approximately 10 meters. Quarterly monitoring of pit water quality and annual monitoring of pit sediment is ongoing, and is planned to continue through the summer of 2003. This paper will describe project activities, results, and lessons-learned to-date as well as future plans. The MWTP is funded by the U.S. EPA and is jointly administered by the EPA and the U.S. Department of Energy (DOE) under DOE contract number DEAC22-96EW96405.