Modeling Hyporheic Zone Fe-S-C Cycling with Cryptic Sulfur Reactions at the Tims Branch Riparian Wetland (Argonne Wetland Hydro-biogeochemistry Science Focus Area)

Authors

Samantha Pérez¹, G.-H. Crystal Ng¹* (gcng@umn.edu), Cara Santelli¹, Shreya Srivastava¹, Carla Rosenfeld², Daniel Kaplan³, Kenneth M. Kemner⁴

Institutions

¹University of Minnesota–Twin Cities, Minneapolis/St. Paul, MN ; ²Carnegie Museum of Natural History, Pittsburgh, PA; ³University of Georgia, Athens, GA; ⁴Argonne National Laboratory, Lemont, IL

Abstract

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 researchers hypothesize that it is further complicated by cryptic S cycle reactions not usually represented in reactive transport models. Researchers have found that model simulations at Tims Branch with standard redox reactions (no cryptic S reactions) struggle to capture the observed persistence of low porewater sulfate (SO₄^2-) concentrations under highly reducing (methanogenic) conditions. This supports the occurrence of anaerobic sulfide reoxidation (ASR) through intermediate-valence S forms that replenish porewater SO₄^2-. The goals were to develop a reactive transport modeling framework with the PFLOTRAN code that incorporates cryptic S cycle reactions and to evaluate how these reactions control Fe-S-C processes under variable hydrologic conditions that researchers measure at the site. The team constructed an ASR reaction network based on X-ray absorption spectroscopy and metagenomic analyses. Metagenomic results revealed the presence of microbes corresponding to multiple reactions involving sulfur intermediates, with the highest functional S gene abundance throughout the vertical soil profile of those capable of anaerobically oxidizing thiosulfate (S₂O₃^2-). Sulfur XANES confirmed the presence of various intermediate valence S forms. These outcomes prompted incorporation into PFLOTRAN of anaerobic sulfide oxidation to sulfate through thiosulfate and other intermediary sulfur forms, coupled to iron reduction. Monod kinetic reaction rate parameters were manually calibrated to simulate redox activity through thiosulfate pathways as well as to match observed aqueous SO₄^2-, Fe, and methane concentrations. Preliminary model scenarios show the dominance of ASR under upwelling conditions compared to downwelling for driving Fe-S-C cycling.