Modeling hydrologic controls on sulfur processes in sulfate-impacted wetland and stream sediments

Recent studies show sulfur redox processes in terrestrial settings are more important than previously considered, but much remains uncertain about how these processes respond to dynamic hydrologic conditions in natural field settings. We used field observations from a sulfate-impacted wetland and stream in the mining region of Minnesota (USA) to calibrate a reactive transport model and evaluate sulfur and coupled geochemical processes under contrasting hydrogeochemical scenarios. Simulations of different hydrological conditions showed that flux and chemistry differences between surface water and deeper groundwater strongly control hyporheic zone geochemical profiles. However, model results for the stream channel versus wetlands indicate sediment organic carbon content to be the more important driver of sulfate reduction rates. A complex nonlinear relationship between sulfate reduction rates and geochemical conditions is apparent from the model's higher sensitivity to sulfate concentrations in settings with higher organic content. Across all scenarios, simulated e(-) balance results unexpectedly showed that sulfate reduction dominates iron reduction, which is contrary to the traditional thermodynamic ladder but corroborates recent experimental findings by Hansel et al. (2015) that cryptic sulfur cycling could drive sulfate reduction in preference over iron reduction. Following the thermodynamic ladder, our models shows that high surface water sulfate slows methanogenesis in shallow sediments, but field observations suggest that sulfate reduction may not entirely suppress methane. Overall, our results show that sulfate reduction may serve as a major component making up and influencing terrestrial redox processes, with dynamic hyporheic fluxes controlling sulfate concentrations and reaction rates, especially in high organic content settings. Plain Language Summary Unlike in oceans, sulfur reactions have not been considered to play a prominent role in the biogeochemistry of terrestrial environments because of much lower concentrations, but recent studies have been showing terrestrial sulfur reactions to be more important than previously thought. These reactions often take place in wetland, stream, and lake sediments, which can contain a mix of both surface water and underlying groundwater. Our study investigated how water fluxes through these sediments can affect sulfur reactions when the surface water and groundwater chemistry differ significantly, such as through influxes of surface water sulfur from mining activities. Using field measurements and a computer model that simulates chemical reactions and water flow, we found that water flux drives water chemistry in wetland and stream sediments. This affects how rapidly sulfur reactions occur, especially in organic-rich wetland sediments. The model showed sulfur reactions to dominate over iron reactions, which contradicts classic chemical thermodynamics. Also, when water fluxes carry high sulfur concentrations into wetland sediments, sulfur reactions out compete production of methane, a strong greenhouse gas. Our results indicate that sulfur reactions can play a prominent role in terrestrial biogeochemistry, and the degree to which it does is affected by hydrological fluxes.