THE CONSEQUENCES OF SULFATE REDUCTION DOWNSTREAM OF AMD, EVEN WHEN THE ACID HAS BEEN NEUTRALIZED

In a mesocosm experiment, a range of sulfate additions to artificial wetlands was positively correlated with increased nutrient and mercury concentrations, as well as with increased porewater sulfide. Dissolved sulfate acts as a terminal electron acceptor for the decomposition of organic matter, so an enhanced sulfate flux to freshwater ecosystems enables additional decomposition beyond the point when all dissolved oxygen has been consumed. Mineralization releases dissolved organic carbon, nitrogen, phosphorus, and mercury, all of which would otherwise be sequestered in the anoxic sedimentary column, and sulfate reduction generates alkalinity. Furthermore, sulfate reducing bacteria may methylate mercury, leading to higher methylmercury in the food web. The biogeochemical significance of sulfate reduction is often considered to be minimal in freshwater systems, but the observed ecosystem consequences are substantial not just for lakes and wetlands receiving neutral mine drainage, but in cases where groundwater supporting wetlands is supplemented or supplanted by higher-sulfate surface (e.g., river) water, where subsidence or sea level rise lead to intrusion of sulfate-rich water into freshwater ecosystems, sites receiving wastewater, etc. Even in cases where porewater sulfide is sequestered by iron or other metals and thus remains below concentrations toxic to aquatic plants, the byproducts of sulfate reduction could lead to eutrophication, decreased water transparency, increased alkalinity, and higher total mercury and methylmercury.