Geochemistry of a permeable reactive barrier for metals and acid mine drainage

A permeable reactive barrier, designed to remove metals and generate alkali nity by promoting sulfate reduction and metal sulfide precipitation, was in stalled in August 1995 into an aquifer containing effluent from mine tailin gs. Passage of groundwater through the barrier results in striking improvem ent in water quality. Dramatic changes in concentrations of SO4 (decrease o f 2000-3000 mg/L), Fe (decrease of 270-1300 mg/L), trace metals (e.g., Ni d ecreases 30 mg/L), and alkalinity (increase of (800-2700 mg/L) are observed . Populations of sulfate reducing bacteria are 10 000 times greater, and ba cterial activity, as measured by dehydrogenase activity, is 10 rimes higher within the barrier compared to the up-gradient aquifer. Dissolved sulfide concentrations increase by 0.2-120 mg/ L, and the isotope S-34 is enriched relative to S-32 in the dissolved phase SO42- within the barrier. Water che mistry, coupled with geochemical speciation modeling, indicates the pore wa ter in the barrier becomes supersaturated with respect to amorphous Fe sulf ide. Solid phase analysis of the reactive mixture indicates the accumulatio n of Fe monosulfide precipitates. Shifts in the saturation states of carbon ate, sulfate, and sulfide minerals and most of the observed changes in wate r chemistry in the barrier and down-gradient aquifer can be attributed, eit her directly or indirectly, to bacterially mediated sulfate reduction.