Reactive Transport of Hexavalent Uranium in Ground Water with Variable Geochemical Conditions

Uranium is a contaminant of concern to the US Federal Government at many sites where it has been released to ground water or where it remains in vulnerable storage facilities. Leaching of uranium from mill tailings has resulted in contamination of soils and aquifers with high concentrations of uranium and other species such as dissolved inorganic carbon and heavy metals. Under oxidizing conditions, the most stable valence of uranium is U(VI) which is often present as aqueous uranium-carbonate or calcium-uranium-carbonate complexes in ground water. U(VI) can also be adsorbed to sediment surfaces causing the overall migration of U(VI) to be controlled by the competition between the formation of aqueous and the adsorbed complexes. A mechanistic description of U(VI) adsorption by natural sediments similar to that achieved in laboratory studies with reference materials is difficult to obtain because of the inherent heterogeneity and poorly defined electrical properties of natural sediment surfaces and because of competing adsorption reactions with major ions in solution. As an alternative, we have developed two semi-mechanistic surface complexation models (SCMs) to describe U(VI) adsorption as a function of experimental conditions for sediments collected near a former uranium mill site near Naturita CO and for sediments collected from the alluvial aquifer near Forty Mile Wash in Nye County, NV. Reactive transport simulations at the Naturita site using the semi-mechanistic SCM gave good predictions of U(VI) transport in small-scale tracer tests and reproduced the observed plume scale U(VI) transport. Hypothetical simulations using the two semi-mechanistic SCMs demonstrated that the simulated aqueous U(VI) concentrations increased significantly in response to an increase in the alkalinity caused by changes in influent boundary conditions, recharge properties or mixing caused by variable pumping rates.