BACTERIAL SULFATE REDUCTION EFFICIENCY AND ITS EFFECT ON WATER QUALITY IN NATURAL WETLANDS RECEIVING MINE DRAINAGE FROM THE PIKE HILL COPPER MINE SUPERFUND SITE, VERMONT (Invited Presentation)

Acid-mine drainage (pH 2.9 – 3.7) from the Pike Hill Copper Mine Superfund Site was rapidly neutralized by the receiving brook (pH 6.6 – 7.6) before flowing 3.5 km and entering a series of four natural wetlands that range in size from 0.2 to 13.8 ha. As the brook flowed through the first two wetlands, the total discharge increased between 150 to 280 percent depending upon season, presumably reflecting groundwater inputs. Iron and Cu entering the wetlands (0.2 to 0.6 mg/L total and 0.02 to 0.06 mg/L total, respectively) were dominantly as particulates. In contrast, Zn was dominantly dissolved (0.07 to 0.15 mg/L total), consistent with expected adsorption behavior. Dissolved sulfate concentrations above the wetlands for two different seasons (43.8 and 65.5 mg/L) were higher than those below (8.0 and 16.2 mg/L). The dissolved sulfate load through the wetlands showed a decrease of 25 percent in July 2007 and 30 percent in October 2007. Shallow coring (< 1 m) within the wetlands revealed a vertical redox zonation. Iron oxyhydroxides are the dominant iron phases in the oxic upper zone, and authigenic sulfide minerals (pyrrhotite, pyrite, chalcopyrite, and sphalerite) are dominant in the lower anoxic zone.

The δ³⁴S and δ¹⁸O values of dissolved sulfate increase from an average of 4.1 and -5.3 permil upstream of the wetlands to 12.9 and 2.5 permil, respectively, downstream of the upper two wetlands. These increases are consistent with bacterial sulfate reduction within the wetlands. Rayleigh modeling of the dissolved sulfate δ³⁴S values implies between 15 to 20 percent bacterial sulfate reduction, which is in distinct contrast to the greater than 85 percent of reduction implied by the δ³⁴S of the authigenic sulfides (-6.9 to 3.7 permil). The discrepancy between these two estimates is explained by the fact that the dissolved sulfate downstream of the wetlands was a mixture of at least two components: dissolved sulfate that traversed the wetlands in oxic surface flow, and residual dissolved sulfate that traversed the anoxic lower part of the wetlands. Thus, the δ³⁴S data from the authigenic sulfides indicate that the natural wetlands downstream of the abandoned copper mines are highly effective at removing sulfate and associated trace metal contaminants (Fe, Cu, and Zn).