Perez, Samantha2023-11-282023-11-282023-06https://hdl.handle.net/11299/258601University of Minnesota M.S. thesis. June 2023. Major: Earth Sciences. Advisors: Gene-Hua Ng, Cara Santelli. 1 computer file (PDF); viii, 71 pages.Tims Branch is a riparian wetland in the Savannah River Site, SC with uranium (U)contamination. U mobility is affected by iron (Fe) redox chemistry, which depends on interactions with carbon (C) and sulfur (S) in the hyporheic zone. Predicting Fe-S-C cycling is difficult not only because of variable redox gradients that develop in response to dynamic fluxes, but we hypothesize that it is further complicated by “cryptic” S cycle reactions not usually represented in reactive transport models. Our goals were to develop a reactive transport modeling framework with PFLOTRAN that incorporates “cryptic” S cycle reactions and to evaluate how these reactions control Fe-S-C processes under upwelling and downwelling hydrological conditions. First, we identified key “cryptic” S cycle reactions by comparing model simulations with standard redox reactions (no “cryptic” S reactions) against geochemical field observations. Surface and porewater pH, aqueous Fe, sulfate (SO 42- ), and methane (CH 4 ) served as the main model constraints. This model without “cryptic” S reactions acts as the base case. The base case struggled to capture the persistence of low porewater SO 42- concentrations, which were observed under highly reducing conditions that support methanogenesis. These results provided evidence of anaerobic sulfide reoxidation through intermediate-valence S forms that replenish porewater SO 42- and moderate rates of AOM coupled to SO 42- reduction. In addition, intermediate-valence S forms measured with X-ray absorption spectroscopy and microbially-mediated S-intermediate oxidation and disproportionation reactions observed in the metagenomes support anaerobic oxidation of sulfide. We incorporate the key “cryptic” S cycle reactions into the reactive transport model to simulate the chemical concentrations of aqueous Fe, SO 42- , and CH 4 for upwelling and downwelling conditions and compare these results to the base case. These results show that modeling the “cryptic” sulfur cycle is more important for the upwelling scenario than viiidownwelling because it provides a mechanism to replenish sulfate under anaerobic conditions while at the same time preventing the over simulation of methane. Our new modeling framework can be used to evaluate the role of these “cryptic” S reactions in driving Fe-S-C processes that have critical implications for U mobilization.enhyporheic zonemicrobial matsPFLOTRANreactive transport modelingredox reactionssulfur cyclingThesis or Dissertation