A permeable reactive barrier, designed to remove metals and generate alkalinity by promoting sulfate reduction and metal sulfide precipitation, was installed in August 1995 into an aquifer containing effluent from mine tailings. Passage of groundwater through the barrier results in striking improvement in water quality. Dramatic changes in concentrations of SO 4 (decrease of 2000-3000 mg/L), Fe (decrease of 270-1300 mg/L), trace metals (e.g., Ni decreases 30 mg/L), and alkalinity (increase of 800-2700 mg/L) are observed. Populations of sulfate reducing bacteria are 10 000 times greater, and bacterial activity, as measured by dehydrogenase activity, is 10 times higher within the barrier compared to the up-gradient aquifer. Dissolved sulfide concentrations increase by 0.2-120 mg/ L, and the isotope 34 S is enriched relative to 32 S in the dissolved phase SO 4 2within the barrier. Water chemistry, coupled with geochemical speciation modeling, indicates the pore water in the barrier becomes supersaturated with respect to amorphous Fe sulfide. Solid phase analysis of the reactive mixture indicates the accumulation of Fe monosulfide precipitates. Shifts in the saturation states of carbonate, sulfate, and sulfide minerals and most of the observed changes in water chemistry in the barrier and downgradient aquifer can be attributed, either directly or indirectly, to bacterially mediated sulfate reduction.

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The contribution of the milling, smelting, and refining of sulfide ores to Hg emissions and to Hg byproduction is not adequately quantified in a global context. In this study, we estimate Hg emissions from the pyrometallurgical treatment of Cu, Pb, and Zn sulfide ores. We base our calculations on quantities processed and Hg content in Cu, Pb, and Zn concentrates, derived from unique global databases on smelter feed and production. In 2005, about 275 tons of Hg were emitted globally to the atmosphere from Cu, Pb, and Zn smelters. Nearly one-half was emitted from Zn smelters and the other half equally divided between Cu and Pb smelters. Most Hg was emitted in China, followed by the Russian Federation, India, and South Korea. Global emission factors were 5.81, 15.71, and 12.09 g of Hg ton(-1) of metal for Cu, Pb, and Zn smelters, respectively. Calculations indicate that Hg abatementtechnologies applied to flue gases may have recovered 8.8 tons and 228 tons Hg from Pb and Zn smelters, respectively, most of which was probably sold as a byproduct. In conclusion, Hg emitted from processing copper, lead, and zinc ores has been largely underestimated in Hg emission inventories. Reducing these emissions may be one of the most economical measures to reduce global Hg emissions.