Reactive Transport Modeling of Iron-Sulfur-Carbon Cycling: Investigating The Impacts Of Dynamic Hydrologic Conditions at a Riparian Wetland

Authors

Zixuan Chen¹* (chen7452@umn.edu), 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 (SO₄^2-); and (2) anaerobic oxidation of methane (AOM) coupled to sulfate (SO₄^2-) 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.