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

Wetlands remove many dissolved pollutants from surface waters by various 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 HClO ₄^-, 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 weaker than 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 ε (≈ δ_react - δ _{instantaneous prod}) of 0.20‰, whereas the intrinsic ε was 0.61‰. 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.