Is Treatment of Mine Dewatering Water Necessary Prior to Rapid Infiltration Basin Recharge? A Case Study

In Nevada, USA, the return of mine dewatering water (MDW) to the subsurface through rapid infiltration basins (RIBs) requires treatment if the quality exceeds the state’s Division of Environmental Protection (NDEP) Profile I reference values. However, a 2019 change to the Nevada Administrative Code allows discharge without treatment if the natural background groundwater solute concentrations are not exceeded. We developed a novel approach to demonstrate that groundwater will not be adversely affected by the untreated discharge of MDW. At the Cortez Gold Mine, dewatering will discharge water to the Grass Valley RIBs with 0.045 mg/L of As, exceeding the NV Profile I reference value (0.010 mg/L) and natural background (0.015 mg/L). A MODFLOW-SURFACT groundwater model incorporated empirical hydraulic conductivities to evaluate the extent to which changes in mound water depth and quality would occur. Modeling inputs were determined using column tests to assess SO₄ leaching from the alluvium and batch tests to quantify As partition coefficients (Kd_As?=?8.9 L/kg) to the alluvium. The results indicated that Profile I will be met at all compliance wells due to dispersion and attenuation. The Kd_As was also used to calculate the attenuation capacity of the alluvial mound water by adsorption (100-years), well beyond the 13-year operational span of the RIBs. Based on this analysis, and in conjunction with a similar analysis in the adjacent Crescent Valley, where 25 years infiltration of 0.045 mg/L As had not affected groundwater quality, the NDEP concurred that a dewatering water treatment plant was unnecessary. Overall, our analysis obviated construction of four treatment plants in the area and permitted direct discharge of mine dewatering water to the subsurface.