Effect of Particle-Scale Heterogeneity on Uranium(VI) Transport in Unsaturated Porous Media

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Uranium(VI) sorption and transport was evaluated in mixtures of silt loam and coarse sand sediments using traditional static batch sorption, saturated column, and unsaturated centrifugation experiments to evaluate the association of mobile and immobile water domains with particles of different size and surface reactivity. Exclusion of conservative tracers and a decrease in U sorption compared with what was predicted by the mass-averaged equilibrium distribution coefficient (K_d-mass-avg) was observed in sediment mixtures where the mass fraction of silt loam was 10%. This is consistent with behavior that was previously reported for coarse and fine sand separates. No exclusion of the conservative tracer, as predicted for the moderate water content range, was measured during unsaturated transport in sediment mixtures that contained 30% or more silt loam by mass. Sorption under unsaturated conditions was greater than predicted based on the batch sorption measurement of K_d-mass-avg value, however, which suggests that the fine-textured silt was in contact with the mobile water domain. This is the first evidence linking sorption to transport in a particular water domain. Results of this investigation demonstrate that the interaction between the geochemical and hydrodynamic processes has a profound effect on transport in unsaturated sediments. Definition of the fraction of mobile water was especially important for defining the front of the breakthrough curve, which is integral to predicting the arrival time of solutes at a particular depth and location in the sediment.