Isotope geochemistry of drainage from an acid mine impaired watershed,Oakland, California

Oxidation of sulfides at the Leona Heights Sulfur Mine has resulted in the liberation of acid, SO₄ and metals to Leona Creek. Previous research at the site has indicated Fe(II) oxidation at rates faster than would be predicted by abiotic oxidation alone, particularly in the segment of stream between the Adit and Leona Street sample stations. In order to assess the mechanisms responsible for sulfide oxidation, samples were collected for isotopic analysis of water and SO₄, the results of which were used to develop a stoichiometric isotope-balance model. This exercise indicated that the percentage of water-derived oxygen in SO₄ increased spatially from between 56% and 64% at the Adit to between 71% and 72% at Leona Street, illustrating that increased sulfide oxidation via Fe(III) was occurring within, or as water flows over, the waste rock, relative to water emanating directly from the former mine. The incorporation of water-derived oxygen in SO₄ during pyrite oxidation is a process controlled by Fe oxidizing bacteria such as A. ferrooxidans at low pH. The role of bacteria was further supported by estimates of the rate constant for Fe oxidation between sampling stations, yielding values that were approximately 10⁶ faster than abiotic Fe oxidation alone. Stable isotopic analysis of water further indicates a close correlation of adit spring water to the local meteoric water line, while ³H data indicate a groundwater apparent age, or time of travel from its primary zone of recharge, of <5–18 a. Additionally, the δ³⁴S data, in conjunction with reported albitized feldspars within the Leona Rhyolite host rock, indicate a magmatic origin of ore sulfur, contrary to previous interpretations at the site.