Exploring sulfate cycling in a mineral-soil wetland restored with wastewater

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Denny, Mariya
University of Lethbridge. Faculty of Arts and Science
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Lethbridge, Alta. : University of Lethbridge, Dept. of Biological Sciences
Sulfate (SO42-) pollution is contributing to the salinization of surface waters worldwide. Wetlands are natural filters on the landscape that remediate surface water by retaining and processing pollutants. However, the capacity for wetlands to process excess SO42- from wastewater is poorly understood, especially for natural (as opposed to constructed) wetlands. Here, I explore the SO42- remediation capacity of Frank Lake, a restored, multi-basin wetland complex in southern Alberta, Canada, that is used to treat effluent from municipal and beef slaughterhouse sources. Using a combination of approaches, I show that there was limited SO42- processing throughout the wetland. Mass balances constructed for two distinct hydrologic periods showed that Frank Lake shifted from a SO42- source during wet years (2013 - 2015) to a sink during drought years (2021 - 2022). Yet I found little evidence of active SO42- processing in surveys conducted during drought years. SO42- remained the dominant form of sulfur (S) among all three basins (>95% of total S), implying little net change in the S pool. Similarly, dual stable isotope ([delta]34S and [delta]18O) analysis showed limited isotopic enrichment among wetland basins, implying limited transformation of SO42- via microbial reduction. Sediment incubations confirmed the patterns observed with stable isotopes, showing little net removal of SO42- throughout the wetland. The preferential reduction of nitrate (NO3-) and other more energetically favourable constituents of the effluent may restrict the extent of microbial SO42- reduction throughout Frank Lake. The limited extent of emergent and submerged vegetation may also limit SO42- uptake by plants. Given the limited SO42- processing in Frank Lake, and the headwater position of this wetland complex in the broader aquatic network, my work provides context for previous reports of increasing salt concentrations documented in rivers of the South Saskatchewan River watershed.
sulfate cycling , restored wetland , wastewater , Frank Lake, Alberta