Metal Cycling and Anomalous Accumulation of Sulfide Minerals In Wetlands Downstream of the Pike Hill Copper Mine Superfund Site, Vermont

Mine drainage from the Pike Hill Copper Mine Superfund Site flows 3.5 km to a series of three wetlands (< 15 hectares each). Samples collected in August and October 2007 showed an increase in streamflow of 40 and 60%, respectively, through the first wetland, presumably reflecting seasonal variation in ground-water inputs. The pH was unchanged above (7.5 to 7.7) and below (7.6 to 7.9) the first wetland. Dissolved sulfate above the wetland (43.8 and 65.5 mg/L) was higher than below (8.0 and 16.2 mg/L); the dissolved sulfate load through the wetland decreased 25 % (July 2007) and 30 % (October 2007). The δ³⁴S of dissolved sulfate shows a 9 permil increase below the wetland. Iron entering the wetland (0.2 to 0.3 mg/L total) was dominantly in particulate form, but was mainly dissolved (0.2 to 0.3 mg/L total) exiting it. Likewise, copper above the wetland (0.02 mg/L total) was dominantly in particulate form, but dissolved (0.007 to 0.008 mg/L total) below. In contrast, zinc was dominantly dissolved both above (0.04 to 0.06 mg/L total) and below (0.01 mg/L total). Shallow coring (< 1 m) within the wetlands revealed a zone of sulfide mineral precipitation. Elemental concentrations in solids reached a maximum of 15.7 wt. % S, 21.6 wt. % Fe, 1.5 wt. % Cu, and 0.8 wt.% Zn. X-ray diffraction identified significant amounts of authigenic pyrrhotite, pyrite, chalcopyrite, and sphalerite in the cores. The data suggest that important processes contributing to metal cycling and sulfide mineral precipitation in wetlands include transport of metals as hydrated ferric oxides with adsorbed base metals, reductive dissolution of these particles and bacterial sulfate reduction in the wetlands. Unlike many other wetlands, a significant portion of the resulting biogenic sulfide was captured as sulfide minerals because of the significant flux of metals entering the wetlands.