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.