Modeling effective isotopic fractionation in groundwater aquifers with heterogeneous reaction rates
Isotope ratio measurements are an important tool in evaluating the remediation process in contaminated aquifers. The chemical indicators provided by certain isotope ratio shifts help to resolve complications from the physical and chemical heterogeneities unique to each site. For a number of redox-active contaminants (e.g., chromate, nitrate, selenate, etc.) reduction reactions transform toxic, soluble compounds into non-toxic forms. Reduction induces isotopic fractionation, which may thus be used to determine the amount of reaction that has occurred. However, when the Rayleigh distillation equation is used to relate the isotope ratio shifts to the amount of reaction, known flaws in this approach lead to a considerable underestimation in results. Observed isotope shifts are less than expected based the assumption of a homogeneous system with no mixing, and this causes a substantial underestimation of the extent of reaction in natural aquifers. The goal of this study is to develop a reactive-transport model that may be used to determine the magnitude of isotopic shifts when reduction is unevenly distributed in aquifers (e.g., restricted to organic-rich lenses). This model incorporates heterogeneity and determines effective isotopic fractionation values for various systems. Thus, it improves our ability to understand the controls on the effective fractionation and ultimately to estimate the amount of reaction at contaminated sites. The widely used flow and reactive transport codes MODFLOW and MT3D were used to construct a set of simple 2-dimensional simulations, with reaction occurring only in certain restricted domains. The results indicate strongly decreased effective isotopic fractionation relative to the homogeneous case, and show that the fractionation is muted to a greater extent when reaction is stronger, or when the interiors of the reactive domains are less well connected to the rest of the aquifer.
heterogeneity, modeling, isotope fractionation, contaminant transport, redox reactions, groundwater quality
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign