Smith Lecture: Tracing Groundwater Contaminant Transformation, Immobilization, and Transport Using Cr, U, and Se Isotope Ratios
Chromium (Cr), selenium (Se), and uranium (U) are all mobile and toxic in their hexavalent, oxidized states but immobile and much less dangerous in more reduced valences. Accordingly, detecting and perhaps quantifying reduction reactions can be critically important in determining impacts of these contaminants on surface water and groundwater resources and/or monitoring remedial actions that involve reduction. Isotopic fractionation accompanies Cr, Se, and U reduction reactions, and 53Cr/52Cr, 82Se/76Se, and 238U/235U measurements appear to provide direct evidence for progress of reduction reactions in laboratory and real-world systems: Isotope ratios in the remaining contaminant change predictably with increasing extent of reduction.
This general relationship between isotope ratios and extent of reduction allows qualitative or semi-quantitative interpretation of field data, but several challenges must be overcome to enable more quantitative interpretations. First, the isotopic fractionation is a kinetic phenomenon and isotopic fractionation factors show considerable variability, depending on reaction mechanism and conditions. Second, natural systems are heterogeneous and reduction is likely confined to certain subdomains of the system. Simple models for heterogeneous systems suggest that the effective fractionation, observed at the field scale, may be a factor of two or more smaller than the intrinsic fractionation (dproduct - dreactant) occurring at the micro scale. A further complication may arise if isotopic exchange between oxidized and reduced species occurs and alters isotopic compositions after the initial reduction.
The talk will cover the general concepts behind these new tools, recent work addressing some of the challenges, and some examples of field applications.