The State of New Mexico requires mining sites to meet state ground-water quality standards on closure unless it can be demonstrated that the ground-water quality was worse before mining. An interdisciplinary team of geologists, geochemists, hydrogeologists, and geophysicists from the USGS are attempting to infer the pre-mining ground-water quality at the Molycorp’s Questa molybdenum mine. The area is near the southern boundary of the Questa caldera, along the Red River, that contains Tertiary volcanics overlying preCambrian rocks and intruded by granitic plutons. The basic strategy is to determine the hydrologic and geochemical processes in a similar setting that has not been mined, a proximal analog site, and apply that information, appropriately scaled, to the mine site. A wide variety of tools are being applied that include lithological and mineralogical characterization of both sites, AVIRIS characterization, geophysical characterization of aquifer contacts, tracer and water quality studies of the Red River, and ground-water hydrology and geochemistry of both sites. Preliminary chemical data from the Straight Creek analog site indicate that ground water is acidic (pH 3.5-4) in the shallow colluvial/alluvial aquifer but circumneutral (pH 6-8) in the deeper bedrock aquifer. Acidity is produced from the oxidation of pyrite in tuffaceous volcanics that weather rapidly, forming “scar” areas, devoid of vegetation. Gypsum is abundant in the aquifers and ground waters are, therefore, characteristically high in calcium and sulfate concentrations. Surprisingly, iron concentrations in the bedrock aquifers can be as high as in the acidic aquifers, up to 53 mg/L. Trace metal and major solute concentrations also indicate important differences between alluvial and bedrock aquifers. Al, Zn, Cd, and Cu are higher in concentration for the acidic alluvial aquifer as expected but concentrations of B, Sr, and Ba are higher in the bedrock aquifer. Several constituents exceed the New Mexico standards for the ground-water quality at the Straight Creek analog site.