Predicting Mercury Methylation: Testing the Neutral Sulfide Speciation Model in a Groundwater-Dominated Wetland

Our proposal addresses the groundwater quality issue of methylmercury (MeHg) formation in the hyporheic zone of wetlands. Because the geochemical conditions that support mercury methylation in hyporheic zones are tied to water availability, they are affected by fluctuations such as flooding and drought, streamflow levels, and groundwater withdrawals. Our objective is to improve the ability of mathematical models to predict the conditions under which MeHg forms in these environments. We will use a diagenetic transport-reaction model that extends the Everglades Mercury Cycling Model to better approximate mercury cycling in wetland sediments. Our experiments will calibrate the model extension to a northern temperate wetland ecosystem (Allequash Creek Wetland, northern WI) and test its ability to predict observed depth profiles of MeHg. Using the calibrated model, we will explicitly test the hypothesis that the concentration of dissolved neutral Hg-S complexes controls mercury bioavailability to sulfate-reducing bacteria. Our project approach will be to construct bioreactors using geochemically well-characterized wetland sediments and a range of sulfate, sulfide, and inorganic mercury amendments. The amendment concentrations will cause shifts in chemical speciation, allowing us to test the mathematical model against experimental results under a range of environmentally relevant input conditions. We will then measure a suite of biogeochemical and microbiological parameters to determine the effects of the amendments on mercury methylation and the abundance, diversity, and activity of sulfate-reducing bacteria. Methodology used to quantify these parameters will include: mercury and MeHg by cold vapor atomic fluorescence spectrometry, microbial activity and terminal electron accepting processes by 14C-acetate uptake, bacterial density by MPN-PCR, bacterial diversity by construction of clone libraries, and net methylation rate measurements by ICP-MS. Our improved ability to predict where and when MeHg will form in the hyporheic zone will benefit many sectors including water quality managers, public health officials, and scientists studying mercury biogeochemistry.