Paper No. 3
Presentation Time: 9:30 AM
MODELING ISOTOPE FRACTIONATION IN HETEROGENEOUS GROUNDWATER AQUIFERS
The physical and chemical heterogeneities of natural aquifer systems complicate efforts to use measured isotope ratios to quantify the extent of chemical reactions. Due to their distinct signature, isotope ratios can provide accurate data for calculating the amount of natural attenuation or active remediation at a contaminated field site. For a number of redox-active contaminants (e.g., chromate, nitrate, selenate, etc.) reduction reactions transform toxic, soluble compounds into analogous non-toxic forms. Since reduction is associated with significant isotopic fractionation, it may be used to determine the amount of reaction that has occurred. However, when the Rayleigh distillation equation is used to determine the isotope ratio shift relative to the amount of reaction, previous studies report a considerable underestimation in results [Bender, 1990; Brandes and Devol, 1997; Abe and Hunkeler, 2006; Clark and Johnson, 2008; Green et al., 2010]. Observed isotope shifts are less than expected based on closed homogeneous systems and this causes a substantial underestimation of the extent of reaction.
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). MODFLOW was used to develop a two-dimensional groundwater flow model, and chemical reduction was incorporated into the model using MT3D. Preliminary simulations with reaction occurring only in certain restricted domains show strongly decreased effective isotopic shifts for any given extent of reaction, relative to that produced by Rayleigh models for the same extent of reaction in a homogeneous aquifer. Further work will analyze a variety of aquifer configurations with the goal of determining the effective isotopic fractionation expected for a variety of contaminated field sites. We expect results to greatly improve estimates of the amount of natural attenuation or active remediation.