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 arseni c on health and the factors that control arsenic mobility in groundwater. T he release and transport of arsenic below the water table in the current ta ilings impoundment at Campbell Mine in Balmertown, ON, Canada, are controll ed by the reduction of ferric oxide phases within the saturated tailings. T he activity of chemolithotrophic bacteria is indicated by enriched delta(34 )S-SO4 ratios, high alkalinity, depleted delta(13)C-CO3 ratios, and low SO4 concentrations, all of which are consistent with bacterially mediated Fe a nd SO4 reduction. The concentrations of Fe and As in the tailings porewater increase with depth. This increase is attributed to the bacterially mediat ed 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 impoun dment, As concentrations decrease sharply to 0.3 mg/L. This decrease in con centration is attributed to reprecipitation of the As as a diagenetic As-be aring sulfide phase.