Coupled Hydrology and Uranium Geochemistry of the Hanford Caliche Layer

Subsurface disposal and dissemination of uranium has occurred at the US Department of Energy Hanford Reservation in Richland, WA. A caliche layer forms the approximate boundary between the vadose and saturated zone in places, and uranium contamination is observed. The cemented, calcium-rich caliche is geologically distinct from the proximal unconsolidated sediments, which suggests that the transport of uranium may differ from the surrounding subsurface environment. Further, it has recently been shown that calcium exerts a major influence upon uranium speciation at neutral pH. Our objectives were to quantify the coupled hydrology and geochemistry of uranium transport, considering the high calcium content within the caliche. The hydrology of the layer was characterized by performing multiple nonreactive tracer experiments on two undisturbed cores under saturated and unsaturated conditions. Physical nonequilibrium was minimal, as evidenced by the co-elution of the nonreactive tracers, which contrasts with results from the surrounding unconsolidated sediments. The geochemistry was quantified by performing kinetic batch, equilibrium isotherms, and repacked miscible displacement experiments, and it was observed to be influenced by the presence of natural and added Ca. X-Ray Absorption Spectroscopy will be utilized to quantify the geochemistry of the adsorbed and/or precipitated complex. Coupled uranium transport processes were investigated under unsaturated conditions in the undisturbed cores, and less retardation was observed in comparison to repacked and batch experiments. This was attributed to differences in reactive surface area in undisturbed versus disturbed rock samples. Our results suggest uranium transport in the Hanford subsurface will be influenced by the distinct hydrological and geochemical characteristics of the caliche layer which contrast with that of the surrounding sediments.