Effective isotopic fractionation factors for solute removal by reactive sediments: a laboratory microcosm and slurry study.

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.