Coupled abiotic and biotic processes in the hyporheic zone, where surface water and groundwater mix, play a critical role in the biogeochemical cycling of carbon, nutrients, and trace elements in streams and wetlands. Dynamic hydrologic conditions and anthropogenic pollution can impact redox gradients and biogeochemical response, although few studies examine the resulting hydrobiogeochemical interactions generated within the hyporheic zone. This study examines the effect of hyporheic flux dynamics and anthropogenic sulfate loading on the biogeochemistry of a riparian wetland and stream system. The hydrologic gradient as well as sediment, surface water, and porewater geochemistry chemistry was characterized at multiple points throughout the 2017 spring-summer-fall season at a sulfate-impacted stream flanked by wetlands in northern Minnesota. Results show that organic-rich sediments largely buffer the geochemical responses to brief or low magnitude changes in hydrologic gradient, but sustained or higher magnitude fluxes may variably alter the redox regime and, ultimately, the environmental geochemistry. This has implications for a changing climate that is expected to dramatically alter the hydrological cycle. Further, increased sulfate loading and dissolved or adsorbed ferric iron complexes in the hyporheic zone may induce a cryptic sulfur cycle linked to iron and carbon cycling, as indicated by the abundance of intermediate valence sulfur compounds (e.g., polysulfide, elemental sulfur, thiosulfate) throughout the anoxic wetland and stream-channel sediment column. The observed deviation from a classical redox tower coupled with potential changes in hydraulic gradient in these organic-rich wetland and stream hyporheic zones has implications for nutrient, trace element, and greenhouse gas fluxes into surface water and groundwater, ultimately influencing water quality and global climate.
Bibliographical noteFunding Information:
Research was supported by MnDRIVE Environment (CMS) and the College of Science and Engineering (GHCN) from the University of Minnesota-Twin Cities as well as by a United States Department of Energy grant (#DE-SC0019439) to CMS and GHCN. We are grateful for all the assistance with fieldwork, hydrogeology data analysis, and geochemistry analysis from many current and former researchers and students in the Santelli and Ng research groups: Ella Fadely, Jordan Loy, Jacqueline Mejia, Elizabeth Roepke, Mary Sabuda, Amanda Yourd, Alexander Waheed, Tingying Xu, and Christopher Schuler. This research used resources of the Advanced Photon Source, a U.S. Department of Energy Office of Science User Facility operated by Argonne National Laboratory under contract DE-AC02-06CH11357. We thank beamline scientists Carlo Segre, John Katsoudas, Yujia Ding, and Tianpin Wu for assistance with XAS data collection. We are grateful to Brandy Toner for guidance on S XANES analysis and for providing reference standards, as well as to Josh Feinberg for providing Fe EXAFS standards.
© 2022 The Royal Society of Chemistry
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