SAN MATEO CREEK BASIN NEW MEXICO, LEGACY URANIUM MINES SITE 2018 GROUNDWATER INVESTIGATION OF MINE WATER DISCHARGE

The Grants Mining District (GMD), New Mexico contains 85 legacy uranium mines and four inactive milling facilities within the 321 square miles of San Mateo Creek (SMC) Basin. Legacy mining and milling operations discharged substantial quantities of solid waste, mill effluent, treated and non-treated mine water into the SMC Basin. To assess the potential impacts by legacy uranium operations on water resources, a groundwater investigation was completed in 2018 as part of the U.S. Environmental Protection Agency Region 6 GMD Five-Year Plan (EPA, 2018). Uranium deposits located in saturated zones required discharging billions of gallons of mine water to surface drainages. Mine water discharge (MWD) originated as natural groundwater in bedrock formations with variable water quality that evolved through oxidation, mineral dissolution, and groundwater mixing due to chemical changes resulting from legacy mining operations. Geochemical indicators of MWD include elevated concentrations of total dissolved solids (TDS), sulfate (SO₄), chloride, and uranium. Unique light δ³⁴S-SO₄ ratios result from oxidation of pyrite associated with uranium ore. Indicator values for MWD were established for TDS (>1,500 milligrams per liter (mg/L), sulfate (>1,000 mg/L), chloride (>30 mg/L), and uranium (>0.030 mg/L) for Upper Basin (UB) groundwater, but not for Lower Basin (LB) alluvial groundwater. δ³⁴S-SO₄ ratios less than -8.0‰ suggest the presence of MWD in alluvial groundwater. Geochemical data and information demonstrate that MWD is a mixture of groundwater sourced from three major hydrogeologic units (Mancos Shale, Dakota Sandstone and Morrison Formation) that evolved during mining operations. After MWD was released to SMC Basin drainages as surface flow, it provided recharge to the alluvium and underlying bedrock hydrostratigraphic units and continued to undergo more physicochemical changes in response to mineral dissolution-precipitation, groundwater mixing, and evaporation in the vadose zone. The chemical variability of MWD are included in the conceptualization, methodology, and approaches used during site investigation and data analysis to characterize the impacts from MWD on UB and LB water resources.