What was the water quality before mining? Inferring pre-mining water-quality at hard-rock mines as a goal for remediation

摘要

Environmental restoration of hard-rock mining sites, whether active or inactive, is complex,expensive, and necessary. A reasonable goal would be to restore sites to pre-mining conditions; however, premining condifons have rarely been adequately characterized and must be inferred. Water quality in mineralized areas containing metallic ores can be acidic and contain high metal concentrations before mining, especially for deposits with high sulfide content and little neutralizing potential. Inferences about pre-mining conditions can be fraught with uncertainties. Possible approaches include remote analog sites; proximal analog sites; equilibriumbased geochemical modeling; kinetic-based geochemical modeling; statistical analyses; stable isotopes; dating gossan through paleomagnetic reversals; and mass balances. Research in these areas has been pursued and examples are provided from Iron Mountain, California, the San Juan Mountains, Colorado, and Questa/Red River,New Mexico. At Iron Mountain, a paleomagnetic study provided an age date that constrained the time period of massive-sulfide weathering. In the San Juans, several mineralized but unmined areas have been characterized and are known to produce acid waters with high metal concentrations. A detailed proximal analog study was used for the Questa project to determine the pre-mining ground-water quality for regulatory requirements.The results from the Questa project demonstrated that mineral solubilities, solute correlations, mineralogy, and mineral chemistry along with geologic and hydrologic considerations can be used to constrain the likely range of concentrations for pre-mining conditions at an active mine site.