Sulfur isotopes as indicators of remediation efficacy in constructed wetlands receiving acid mine drainage
Acid Mine Drainage (AMD) containing high levels of sulfate, heavy metals, and acidity can be effectively remediated by constructed wetlands. One process contributing to remediation success is bacterial sulfate reduction. This wetland process raises pH and removes dissolved metals and sulfate from AMD. This paper documents the utility of sulfur isotope measurements as indicators of the extent of bacterial sulfate reduction (BSR), and hence of the effectiveness of wetland performance and design for this removal mechanism. Four constructed wetland systems, Midwestern and Tecumseh in Indiana and Simco and Wills Creek in Ohio were studied to determine sulfate reduction trends and remediation success. Influent, effluent and intra-wetland water and soil samples were collected and analyzed for stable sulfur isotope ratios. The difference between inlet and outlet δ 34S for dissolved sulfate, δ 34Sin-out, is a measure of the extent of BSR in the wetland system, with more negative numbers indicating more extensive reaction. Values from summer measurements at Tecumseh (-2.3‰) and Wills Creek (-1.1‰) show relatively minor BSR activity, and are similar to winter values of -1.9 ‰ and +0.1 ‰. In contrast, Simco (δ 34Sin-out = -9.8‰) and Midwestern (-5.7‰) show that a significant removal of dissolved sulfate by BSR in summer, but not in winter (δ 34Sin-out = -0.7 ‰ and -0.2‰). Within-system trends for concentration of dissolved sulfate, δ 34S of sulfate in water, and of sulfide in soil confirm these conclusions. Abiotic sulfate, iron, and aluminum removal continue throughout the year by precipitation of minerals like gypsum and the formation of amorphous iron and aluminum hydroxides or sulfates. On a yearly basis, about twice as much sulfate is removed by abiotic precipitation as by BSR, although in the summer season, removal by the bacterial process predominates in successful wetlands.