The U.S. Environmental Protection Agency (EPA) Region 8 Superfund Office and the EPA National Risk Management Research Laboratory (NRMRL) Mine Waste Technology Program (MWTP) conducted a field-scale treatability study demonstrating an in situ bio/geochemical treatment technology for decontaminating acid/metal-toxic water within the Anchor Hill Pit lake at the Gilt Edge Mine Superfund site near Deadwood, South Dakota. The purpose of the project, carried out between March 2001 and May 2006, was to develop performance data of the treatment approach for potential use in long-term water treatment/management activities at the Gilt Edge site, as well as other similar sites. The treatment process was applied to approximately 72 million gallons of acidic water, with high concentrations of metals (including iron, aluminum, arsenic, selenium, copper, cadmium, and zinc), sulfate, and nitrate, and the pH was approximately 3. The treatment process involved pit neutralization, then application of nutrients to stimulate biological activity. The treatment process was successful and approximately 40 million gallons of treated water that met the State of South Dakotas strict surface water discharge standards were discharged from the Anchor Hill Pit during the demonstration. All project objectives were met, and considerable experience and insight was gained into how operational aspects of such a remediation technique would have to be designed for future efforts.