Multirate sorption of Uranium(VI) under field scale conditions

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1D Simulation of uranium(VI) transport under field scale conditions at the Hanford 300A area, Washington State. The model domain extends from the Columbia River (right boundary) 500m into the aquifer. Uranium(VI) is initially present in the adsorbed and dissolved state between 200m and 450m. River water from the adjecent Columbia River (located at 500m) intrudes and recedes into and from the aquifer up to a few times per day. The average flow direction is towards the river, thereby carrying U(VI) from the aquifer into the river. During higher river water levels the river intrudes far into the aquifer (between Day 200 and 280) bringing a typical river water quality of higher pH and lower carbonate concentration than the ambient groundwater which is displaced during intrusion. The different water quality of the river water leads to a higher adsoprtion of U(VI) due to surface complexation (SC) reactions, lowering the dissolved U(VI) concentrations in the aquifer. Apparent ad- and desorption rates were found to occur on many time scales for the Hanford 300A sediment. This is a result of diffusional masstransfer within immobile intra grain pore water at the micrometer scale, limiting the mass exchange between mobile pore water and intra grain surface sites. Different masstransfer models can describe this process, e.g., a multi-rate kinetic SC model (chemical model) or a physically based multi-continuum (multi-porosity) SC model (physical model). The latter explicity accounds for the diffusional solute exchange between mobile and immobile porewater and treats surface complexation in the intra grain pores as an equilibrium process. The models produce the same results if the water quality does not change (here under ambient ground water conditions). They start to deviate when two conditions apply at the same time: (i) the water quality (most importantly pH, carbonate and Ca concentration) change and (ii) when mobile and immobile pore water are not in equilibrium, most notably at the plume fringes and the intruding river water front. For further information see Greskowiak et al. 2011, Water Resources Research