ASSESSMENT AND CLOSURE OF THE GLENGARRY ADIT, NEW WORLD MINING DISTRICT, COOKE CITY, MONTANA 1

The Glengarry adit is an underground gold mine developed between 1925 and 1934 in the New World Mining District of south-central Montana. The adit has been a historic source of metals-laden, acidic water that discharged into Fisher Creek, a major tributary to the headwaters of the Yellowstone River. The adit discharge contributed more than 30 percent of the metals load to Fisher Creek, had an average historical flow of about 363 L/min (80 gpm), Cu and Fe concentrations near 7 and 78 ppm respectively, and a pH of 2.2. The USDA- Forest Service undertook a rehabilitation and closure project to address this adit discharge in 2000, with a goal of minimizing or eliminating this discharge. Assessment included reconditioning 915 meters of underground workings and characterizing water inflows. Four principal sources of water inflows were identified. Chemical mass loading analysis allowed quantification of the impact of each of these sources to surface water quality, as well as a means of evaluating effectiveness of potential closure options. Closure alternatives considered included plugging, containing, or diverting water-flows. Engineering design work on the selected alternative was completed in 2002 and a contract awarded in 2003. Closure consisted of a combination of surface and underground grouting of water- bearing fractures and faults, selective backfilling of three segments of the underground workings, and strategic placement of five water-tight underground adit and raise plugs. A non-water tight portal plug was also constructed. Closure work was completed in 2005 and resulted in an average metals concentration reduction of 83 percent, a metal load reduction of 99.8 percent and an adit discharge flow reduction of 97 percent. The closure method used at the Glengarry Mine offers an effective approach considered to be a walk-away solution to handling sources of contamination from adit outflows. No future operational, maintenance, or treatment costs are anticipated. A long-term district-wide program to monitor water quality and aquatic health will be implemented once other reclamation construction projects in the District are complete.