PRE-MINING GROUND-WATER QUALITY IN DEBRIS FANS DEVELOPED FROM PYRITE-MINERALIZED EROSIONAL "SCARS" IN THE RED RIVER VALLEY, NEW MEXICO: COMPARISON BETWEEN MINED AND UNMINED AREAS

The USGS is concluding a baseline and pre-mining ground-water quality study in the Red River Valley, New Mexico to assist the State of New Mexico and Molycorp, Inc. in determining one aspect of mine closure requirements for Molycorp's Questa molybdenum mine. Expertise in economic geology, environmental geology, mineralogy, structure, geomorphology, geochemistry, geophysics, and hydrology was integrated to achieve the overall objective. The key to inferring pre-mining ground-water quality was a detailed study of an unmined proximal analog site, the Straight Creek catchment. The Red River Valley contains numerous erosional scar areas and associated debris fans both on and off the mine site, which developed through chemical weathering and erosion of pyrite-mineralized rocks. Ground waters in debris fans developed from “scar” areas and in mine-site waste-rock piles are acidic (ranging in pH from 2 to 4), and have high concentrations of Fe, Al, Mn, SO4, Zn, Cd, Cu, Ni, Co, Li, Be, and F. Source minerals for these elements were identified and ground-water flowpaths were characterized. Ground waters in bedrock are of neutral pH, buffered by dissolution of carbonate minerals, but usually highly mineralized as well. After delineating the concentration ranges of elements from the analog and other unmined sites, using a combination of empirical correlations and mineral solubility constraints, the range of likely pre-mining ground-water quality parameters was estimated for each catchment on the mine site. The results demonstrate that ground-water concentrations of elements of concern for unmined or pre-mining conditions range from slightly less than the New Mexico ground-water quality standards to as much as 250 times the standards. Many constituents are 10-20 times the standard. This example shows that unmined mineralized areas can mobilize high concentrations of elements potentially harmful to the environment and that state laws allowing for that contingency are appropriate if pre-mining conditions can be defined.