Reactive Transport Modeling of Iron-Sulfur-Carbon Cycling: Investigating The Impacts Of Dynamic Hydrologic Conditions at a Riparian Wetland
Authors
Zixuan Chen1* ([email protected]), Samantha Perez1, Crystal Ng1, Cara Santelli1, Daniel I. Kaplan2, Shreya Srivastava1, Carla Rosenfeld3, Kenneth M. Kemner4
Institutions
1University of Minnesota–Minneapolis/Twin Cities, MN; 2University of Georgia, GA; 3Carnegie Museum of Natural History, PA; 4Argonne National Laboratory, IL
Abstract
Wetlands play a crucial role in enhancing water quality by transforming nutrients and organic substances, influencing the global carbon (C) cycle and greenhouse gas emissions, and retaining metals. Iron (Fe) flocs in the wetland can immobilize uranium (U), and the team hypothesizes that the presence of these flocs highly depends on Fe interactions with C and sulfur (S) in the hyporheic zone. Because spatiotemporally dynamic water fluxes promote fast redox cycling, net changes in S species are challenging to observe. Even when active, S cycling often remains hidden and thus overlooked in freshwater systems. The team’s previous work has emphasized the importance of cryptic S reactions in Fe-S-C cycles under groundwater upwelling via two pathways: (1) anaerobic sulfide reoxidation (ASR) likely coupled to Fe3+ reduction replenishes porewater sulfate (SO42-); and (2) anaerobic oxidation of methane (AOM) coupled to sulfate (SO42-) reduction moderates porewater methane (CH4), while methanogenesis buffers H+-consumption by Fe3+ reduction. The team’s current research further investigates the importance of cryptic S reactions under more complex hydrologic settings. Based on the observations at Tim’s Branch, researchers implemented and compared reactive transport simulations with PFLOTRAN under three hydrologic flux scenarios: (1) constant flux; (2) regularly alternating flux direction; and (3) field-based dynamic flux. This model highlights the crucial role of cryptic S cycling in wetlands by increasing Fe2+ and moderating CH4 porewater concentrations. Moreover, this model indicates that dynamic hydrologic flux with alternating direction can facilitate increased Fe2+ concentrations at different depths. When such an increase in Fe2+ occurs at the sediment surface under upwelling conditions, it leads to the production of iron flocs in surface waters. These findings can help illuminate the impacts of dynamic hydrological settings on Fe-S-C cycles critical to carbon budgets and heavy metal mobilization.