SULFUR ISOTOPES AS A GUIDE TO SULFATE REDUCTION IN WETLANDS RECEIVING ACID MINE DRAINAGE

Bacterial sulfate reduction is a potentially important remediation process that occurs in wetlands receiving acid mine drainage (AMD). It not only removes sulfate, but also immobilizes Fe as the sulfide and at the same time generates alkalinity. How effective is this process in different wetlands? We used sulfur isotopes to study three AMD wetlands in Ohio (Simco, Will's Creek, Lake Essington) and three in Indiana (Midwest, Tecumseh, Friar Tuck). Wetlands with significant sulfate reduction show a rise in d³⁴S with passage through the wetland system produced by the preferential removal of the light isotope by the bacteria. Only two of the wetlands showed significant bacterial reduction. Simco shows a progressive rise of 9.8 permil in d³⁴S in the summer months, while Midwest shows a rise of 4.5 permil. The controlling factors are input alkalinity and wildlife control. The starting alkalinity is high for the two successful wetlands, but low for the others, and beaver and muskrat have degraded several of the wetlands by flooding out the cattails or by grazing them, which stops the generation of the organic matter needed to fuel the reduction process. For the two successful wetlands, sulfate reduction rates are still too slow for complete sulfate removal. No bacterial sulfate reduction takes place in the winter months, when sulfate removal is by CaSO₄ precipitation only. In summer, Simco has significant excess removal by bacteria, 55% of input compared to only 10% in the winter. Midwest is also removing 55% in the summer, but still removes 45% in winter, reflecting a 2-fold higher initial sulfate concentration compared to Simco.