Evaluation of kinetic controls on sulfate reduction in a contaminated wetland-aquifer system

Our ability to understand and predict the fate and transport of contaminants in natural systems is vital if we are to be successful in protecting our water resources. One important aspect of understanding chemical fate and transport in natural systems is identifying key kinetic controls on important redox reactions such as sulfate reduction. Anaerobic microbial activities like sulfate reduction are of particular interest because of the important role they play in the degradation of contaminants in the subsurface. However, current rate estimates for sulfate reduction have a wide range in the literature making it difficult to determine representative rates for a given system. These differences in rate data may be explained by varying kinetic controls on reactions. Push-pull tests were used to evaluate sulfate reduction rates at the wetland-aquifer interface. Anaerobic aquifer water containing abundant sulfate was injected into sulfate-depleted wetland porewater. The injected water was subsequently withdrawn and analyzed for geochemical indicators of sulfate reduction. Complexities in rate data, such as presence of a lag phase, changing rate order and spatial variability, were observed and are hypothesized to be linked to activities of the native microbial population. Subsequent experiments explored the response of native microorganisms to geochemical perturbations using a novel approach to measure directly the effects of a geochemical perturbation on an in situ microbial population and measure rates of resulting reactions. In situ experiments involved colonization of a substrate by microorganisms native to the wetland sediments followed by introductions of native water amended with sulfate and tracer. Experimental results showed that higher sulfate concentrations and warmer seasonal temperatures result in faster sulfate reduction rates and corresponding increases in sulfate reducing bacteria. Findings from this research provide quantitative evidence of how geochemical and microbiological processes are linked in a system not at equilibrium.