Influence of Aquifer Heterogeneity on Contaminant Transport at the Hanford Site

Aquifer heterogeneities defined by mapping and correlating lithofacies derived from existing geologic and borehole geophysical data at the U.S. Department of Energy's (DOE) Hanford Site (south–central Washington) were compared to tritium migration trends within the unconfined aquifer. Lithofacies percentage maps proved useful in defining large–scale (on the order of miles) tritium migration pathways at Hanford but were not sufficiently detailed to serve as unambiguous predictors of these pathways. Results suggest that: (1) incorporation of geologic parameters more closely related to hydraulic properties (e.g., quantified grain size and sorting, degree of cementation, packing arrangement, sedimentary structures, and grain shape), and (2) conscientious integration of all hydrogeologic data at the Hanford Site (and elsewhere) will allow definition of three-dimensional aquifer elements termed “hydrofacies” and enhance reliable prediction of contaminant migration pathways. Strict reliance on standard fluvial models in the evaluation of lithofacies at Hanford is not appropriate for predicting plume migration because of the complex depositional and erosional history of the sediments comprising the unconfined aquifer. Use of “hydrofacies” ultimately may serve as the appropriate conceptual framework for predicting plume migration.