Application of Stable Isotopes and Geochemical Analysis to Characterize Sulfate, Nitrate, and Trace Element Contamination of Groundwater and Its Remediation at a Former Uranium Mining Site

Abstract

Sulfate, nitrate, and certain trace elements are common groundwater contaminants observed at mining sites. Their source, fate, and remediation were investigated at a former uranium mining site. First, groundwater samples collected across the site were analyzed for geochemistry, stable isotopes, and trace elements. Then, two pilot-scale ethanol injection tests were conducted for biostimulation of nitrate and sulfate reduction. Groundwater was monitored in the test area before and after the tests. The results showed a mixing of two discrete sources of sulfate. Quantification of these two sources using two methods showed that sulfide-mineral oxidation of the mine tailings served as a steady but low-discharge source while sulfuric acid (applied during ore processing in the 1960s) served as a variable, strong source. It appears that sulfuric acid served as a sustained source of sulfate for approximately 40 years. This source may be from accumulation of sulfate salts (formed from sulfuric acid) in the source zone due to the arid climate of the site. Results showing correspondence of isotopic compositions of ammonium and nitrate confirmed the generation of nitrate via nitrification. Moreover, it was observed that ammonium concentration is closely related to concentrations of uranium and a series of other trace elements including chromium, selenium, vanadium, iron, and manganese. It is hypothesized that ammonium-nitrate transformation processes influence the disposition of the trace elements through mediation of redox potential, pH, and possibly aqueous complexation and solid-phase sorption. As for the biostimulation, sulfate reduction condition has been maintained for a period of approximately 3 years after a single input. Atypical fractionation behavior of the delta34S in sulfate was hypothesized to be caused by release of sulfate from sulfate minerals associated with the sediments. Elevated hydrogen sulfide concentrations were not observed until approximately four months after the start of the test. This behavior, in concert with the observed changes in aqueous iron and manganese species, suggests that hydrogen sulfide produced from sulfate reduction was precipitated, presumably in the form of iron sulfides, until the exhaustion of readily reducible iron oxides. Hydrogen sulfide produced thereafter appears to have been in part re-oxidized.