EVALUATING MINE-WASTE SEEPAGE WATER AND SOLUTE SOURCES USING STABLE ISOTOPES AND INVERSE GEOCHEMICAL MODELING

The Big Ledge mine is a former barite mine located in a mountainous area of northeastern Nevada. The site contains several mine-impacted waters including a small pit lake and an area of apparent waste rock seepage. The Big Ledge pit lake has displayed variable geochemical characteristics over time, ranging from circumneutral with relatively low concentrations of constituents of concern to acidic (pH~4) with elevated concentrations of Al, Cd, SO₄, and Zn. The area of waste rock seepage produces highly concentrated and acidic waters on a seasonal basis, with blooms of efflorescent sulfate salts.

In order to evaluate potential water and solute sources in waste rock seepage, samples of seepage, groundwater, and the pit lake were analyzed for isotopes of water (δ²H, δ¹⁸O) and sulfate (δ³⁴S) on several occasions in summer 2017, in addition to standard water-quality monitoring. Results of isotope analyses were paired with inverse geochemical modeling, mixing calculations, and multivariate statistical analysis to investigate site hydrology and potential flow paths.

Results of isotope analysis indicate that seepage is enriched in heavy isotopes of H and O, similar to the Big Ledge pit lake. In contrast to seepage and pit-lake samples, groundwater samples, even in close proximity to the seeps, appear to be representative of meteoric recharge over the entire study period. Evaporative enrichment of seepage compared to surrounding groundwater suggests partial sourcing of seepage in the upgradient pit lake, although fault compartmentalization had been previously hypothesized as precluding pit-lake outflow. Sulfur isotopes of regional groundwater show a wide range in values while pit-lake and seepage waters display similar values to one another, with the latter potentially influenced by additional sulfide oxidation. Mixing calculations indicate that a majority of seepage water is likely sourced from the pit lake, although influence of multiple water sources is required for solute and isotopic mass balance. Inverse geochemical modeling shows that seepage water could be derived from several distinct source waters mixed in various proportions, but that dissolution of minerals in the waste rock facility is required to account for observed metal concentrations in seep waters.