Geochemistry and Isotopic Composition of H2S-rich Water in Flooded Underground Mine Workings, Butte, Montana, USA

Abstract

Groundwater being pumped from the flooded West Camp mine workings of Butte, Montana, is elevated in hydrogen sulfide (H2S), has a circum-neutral pH, and has high arsenic but otherwise low metal concentrations. The daily flux of H2S and As pumped from the extraction well are each estimated at roughly 0.1 kg. Isotopic analysis of coexisting aqueous sulfide and sulfate confirms that the H2S was produced by bacterial sulfate reduction. the mine waters are close to equilibrium saturation with amorphous FeS, amorphous ZnS, siderite, rhodochrosite, calcite, and goethite, but are undersaturated with orpiment (As2S3). The higher solubility of orpiment relative to other mental sulfides allows concentrations of dissolved arsenic (~ 100 μg/L) that are well above human health standards. The West Camp waters differ markedly from the acidic and heavy metal-rich mine waters of the nearby Berkeley pit-lake. These differences are partly attributed to geology, and partly to mining history.