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A field-scale experiment was conducted to evaluate the potential for inducing microbial sulfate reduction as a passive in situ technique for managing water quality in mine tailings deposits. Sulfide- and carbonate-rich minetailings, characterized by near-neutral pH pore water, were amended with < 1 dry wt. % organic carbon. The geochemical evolution of pore water was monitored for four years. The results demonstrate that organic carbon supported dissimilatory sulfate reduction (DSR) in the vadose zone. Decreases in dissolved SO4 and S2O3 were accompanied by H2S production, increased populations of sulfate-reducing bacteria (SRB), 34S-SO4 enrichment, and undersaturation of pore water with respect to gypsum [CaSO4 x 2H2O]. The mass of dissolved S decreased by > 45% during the monitoring period, which coincided with the removal of Zn, Sb, and Tl. Mobilization of Fe and As occurred initially; however, subsequent decreases in aqueous concentrations were observed. Mineralogical investigation confirmed the presence of secondary Fe-S and Zn-Fe-S phases. Amendment of tailings with a small and dispersed mass of organic carbon resulted in a general decrease in mass transport of sulfide oxidation products.

The Greens Creek Mine's 3.5M tonne dry-stack tailings pile receives about 800 tonnes of filter-pressed tailings daily. The silt-sized tailings are placed in thin, compacted lifts using a bulldozer and vibratory roller. Carbonate minerals produce a long lag time to acid generation despite a net-neutralization potential of-210 tonnes CaCO3/ktonne. Key influences on pore water compositions include near-surface pyrite and thiosulfate oxidation, vadose zone iron and manganese reduction and saturated zone sulfate reduction. Annual precipitation in excess of 1450 mm poses the largest challenge to achieving design placement densities in this seismically active region. Despite higher operational costs relative to slurry-tailings disposal, the decision to produce dewatered tailings provided economic and environmental advantages. It reduced the ultimate footprint of the facility, lowered closure and water treatment costs, improved pile stability and reduced environmental liability by allowing half of the tailings to be returned underground for use as structural backfill.

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