Surface complexation models are often parameterized for use in predicting contaminant migration in the field, but few studies have evaluated the merit and challenges of this approach in complex hydrochemical systems. Here we discuss a multiscale study of kinetic uranium (U) surface complexation performed with the goal to predict seasonal and long term concentration trends for a persistent 2 km2 U(VI) groundwater plume influenced by groundwater-surface water exchange. The water composition is relatively dilute with a mixed cation suite (Na+, Ca2+, Mg2+), milli-molar concentrations of bicarbonate, and pH ~ 7.5. Uranyl carbonate complexes dominate aqueous speciation. Aquifer sediments consist of coarse river cobble and lithic fragments of granitic and basaltic provenance, and a reactive fines fraction (~5-10 mass %) of complex mineralogic composition including phyllosilicates. Experimental and modeling results are presented for laboratory batch, column, and field systems of increasing scale and decreasing surface site/sediment volume ratios. Generally, the effects of surface complexation were considerable at small scales, but were small at the plume scale. Consistent changes were observed in parameters needed to describe the experimental data. Successful upscaling relationships based on grain size were observed for surface complexation parameters in laboratory systems; but these were not easily applied in the field because of inadequate data on the in-situ physical and geochemical properties of the coarse-textured sediment. While many field cores (6” diameter) were taken of aquifer sediments hosting the plume for laboratory characterization and study, these did not retrieve dominant very coarse river cobble. In general, field predictability was poor even after considerable study. The primary uncertainties that limited plume predictability were: poorly quantified in-situ site concentrations and scaling relationships, heterogeneity in sorbent distributions and adsorbed U, and complex macropore flow paths through coarse gravel.

Surface complexation from the grain to plume scale in a gravel aquifer: Considerations and challenges for predictability

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.