Wetlands remove many dissolved pollutants from surface waters byvarious mechanisms. Stable isotope ratio measurements may provide a means of detecting and possibly quantifying certain removal processes, such as reduction of SeO4(2-), Cr(VI), NO3-, and HClO4-, that fractionate isotopes. However, the magnitude of the isotopic fractionation for a given reaction depends on the setting in which it occurs. We explore the case where isotope ratio shifts in surface waters are used to detect or quantify reactions occurring in pore waters of underlying sediments. A series of SeO4(2-) reduction experiments reveals that the effective isotopic fractionation, observed in the water column as a result of SeO4(2-) diffusion into underlying, Se-reducing sediments, is weakerthan the intrinsic fractionation induced by the same reduction reactions in well-mixed systems in which reaction sites are not separated from measured SeO4(2-). An intact sediment core yielded an effective epsilon (approximately delta(react) - delta(instantaneous prod)) of 0.20% per hundred, whereas the intrinsic epsilon was 0.61% per hundred. These results are consistent with previously published reactive transport models. Isotopic studies of sediment-hosted reactions in wetlands and other surface water systems should use the smaller effective fractionation values, which can be estimated using the models.

We present a comprehensive set of Se concentration and isotope ratio data collected over a 3-yr period from dissolved, sediment-hosted, and organically bound Se in a Se-contaminated lake and littoral wetland. Median isotope ratios of these various pools of Se spanned a narrow isotopic range (delta80/76Se(SRM-3149)) = 1.14-2.40 per thousand). Selenium (VI) reduction in the sediments is an important process in this system, but its isotopic impact is muted by the lack of direct contact between surface waters and reduction sites within sediments. This indicates that using Se isotope data as an indicator of microbial or abiotic Se oxyanion reduction is not effective in this or other similar systems. Isotopic data suggest that most Se(IV) in the lake originates from oxidation of organically bound Se rather than directly through Se(VI) reduction. Mobilization of Se(VI) from bedrock involves only a slight isotopic shift. Temporally constant isotopic differences observed in Se(VI) from two catchment areas suggest the potential for tracing Se(VI) from different source areas. Phytoplankton isotope ratios are close to those of the water, with a small depletion in heavy isotopes (0.56 per thousand). Fish tissues nearly match the phytoplankton, being only slightly depleted in the heavier isotopes. This suggests the potential for Se isotopes as migration indicators. Volatile, presumably methylated Se was isotopically very close to median values for phytoplankton and macrophytes, indicating a lack of isotopic fractionation during methylation.

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