Biogenic methane (CH4) from wetlands plays a crucial role in the carbon cycle, but the dynamics of dissolved methane flux across the surface water-ground water interface remain poorly understood. This study focused on the effects of spatial transformation of dissolved methane and the role of ground-water recharge in the distribution of dissolved methane across the surface water-ground water interface. Here we present carbon isotopic measurements of biogenic methane and dissolved inorganic carbon (DIC) from the Sarita Wetland, on the St. Paul Campus of the University of Minnesota, and also in six monitoring wells located down gradient from the wetland. The δ13C values of CH4 vary between −10.6 and −58.4‰, and the δ13C values of DIC vary between 0.8 and −14.1‰ across the study site. Based on dissolved methane concentrations during the growing season, we estimate that ground water methane represents 8%–38% of total methane dissolved in the wetland. Using the carbon isotopic composition of methane and knowledge of the site hydrology, we found that the degree of methane oxidation increased as methane moved away from the wetland along the ground water flowpath. The proportion of methane oxidized ranged between 4% and 99% with most of the methane oxidation occurring within the first 120 m from the wetland. The degree of oxidation within the wetland itself varied from 81% in the spring to 99% during the winter, suggesting that oxidation of dissolved methane occurs more rapidly in surface waters than in ground water recharge. This study shows that ground water flow paths are a primary control on the export of dissolved methane produced in wetlands. This study also demonstrates that C stable isotopes can be used to study transport of dissolved methane across the surface water-ground water interface, accounting for methane oxidation during transport.
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Vol. 28 • No. 4