Two-region flow and decreased sorption of uranium (VI) during transport in Hanford groundwater and unsaturated sands

Abstract

Uranium, U(VI), sorption and transport in unsaturated coarse- and fine-textured sands were evaluated using a centrifuge method; batch incubation and saturated column experiments were conducted to isolate the effect of flow from that of water content. At higher water contents (≥66% saturation), decreases in U(VI) sorption were due to rate limitations. These breakthrough curves (BTCs) were well characterized with a two-site model for sorption (linear distribution coefficient Kd and first-order kinetic term); BTC characteristics were not captured with an equilibrium sorption model. At lower water contents (<30% saturation), two-region flow (mobile-immobile water) was apparent and had a significant impact on U(VI) sorption and transport. Uranium BTCs were characterized with two-region mass transfer parameters that were determined in experiments with conservative tracers. Decreases in uranium sorption at 22% and 13% saturation were greater than could be explained by velocity or rate-limited effects. Conservative tracer experiments indicated that solutes were excluded from a fraction of the pore volume; however, the decrease in uranium sorption was greater than predicted from this fraction. The results suggest that a greater propensity to sorb uranium was associated with sediment particles in contact with the excluded immobile water domain.