Quantifying the effects of small-scale heterogeneities on flow and transport in undisturbed cores from the Hanford Formation

Accelerated migration of contaminants in the vadose zone has been observed beneath tank farms at the U.S. Department of Energy's Hanford Reservation, Richland, WA. This paper focuses on quantifying hydrologic processes that control the fate and transport of contaminants in the unsaturated sediments beneath the Hanford tank farms. The experimental approach involved the use of field relevant, long-term unsaturated nonreactive transport experiments in undisturbed sediments from the Hanford Formation. Undisturbed sediment cores were collected from a laminated fine-grained sand unit within the Hanford Formation in both the vertical direction (flow cross bedding) and the horizontal direction (flow bedding parallel). Laboratory-scale saturated and unsaturated flow experiments were conducted using multiple nonreactive tracers to investigate hydrologic processes controlling the vertical and lateral spread of contaminants. The nonreactive tracers differ in their free-water molecular diffusion coefficients, thus providing a quantitative measure of diffusional processes and the presence of immobile water. Asymmetric breakthrough curves (BTCs) and coelution of tracers were observed during saturated flow in both horizontal and vertical cores, indicating advection enhanced solute dispersion with no accompanying immobile water. Unsaturated tracer transport in the vertical and horizontal cores resulted in earlier breakthrough, asymmetric BTCs, and differential breakthrough of tracers where the elution of piperazine-1-4-bis(2-ethanesulfonic acid) (PIPES) preceded that of pentafluorobenzoic acid (PFBA), which preceded that of Br-. These results suggest that physical nonequilibrium processes (PNE) such as preferential finger flow coupled with immobile water may control the unsaturated movement of contaminants in the Hanford Formation.