Influence of Reduction Reactions and Solid-Phase Composition on Porewater Concentrations of Arsenic

An increasing amount of attention is being focused on the effects of arsenic on health and the factors that control arsenic mobility in groundwater. The release and transport of arsenic below the water table in the current tailings impoundment at Campbell Mine in Balmertown, ON, Canada, are controlled by the reduction of ferric oxide phases within the saturated tailings. The activity of chemolithotrophic bacteria is indicated by enriched δ 34 S−SO 4 ratios, high alkalinity, depleted δ 13 C−CO 3 ratios, and low SO 4 concentrations, all of which are consistent with bacterially mediated Fe and SO 4 reduction. The concentrations of Fe and As in the tailings porewater increase with depth. This increase is attributed to the bacterially mediated reduction of As-rich hematite and maghemite. The maximum concentrations of Fe and As are 20 and 100 mg/L, respectively. At the bottom of the impoundment, As concentrations decrease sharply to 0.3 mg/L. This decrease in concentration is attributed to reprecipitation of the As as a diagenetic As-bearing sulfide phase.