Rate‐limited U(VI) desorption during a small‐scale tracer test in a heterogeneous uranium‐contaminated aquifer

Abstract

A tracer test was performed at the Rifle Integrated Field Research Challenge site to assess the effect of addition of bicarbonate on U(VI) desorption from contaminated sediments in the aquifer and to compare equilibrium and rate‐limited reactive transport model descriptions of mass transfer limitations on desorption. The tracer test consisted of injection of a 37 mM NaHCO3solution containing conservative tracers followed by down‐gradient sampling of groundwater at various elevations and distances from the point of injection. Breakthrough curves show that dissolved U(VI) concentrations increased 1.2–2.6‐fold above background levels, resulting from increases in bicarbonate alkalinity (from injectate solution) and Ca concentrations (from cation exchange). In general, more U(VI) was mobilized in shallower zones of the aquifer, where finer‐grained sediments and higher solid phase U content were found compared to deeper zones. An equilibrium‐based reactive transport model incorporating a laboratory‐based surface complexation model derived from the same location predicted the general trends in dissolved U(VI) during the tracer test but greatly overpredicted the concentrations of U(VI), indicating that the system was not at equilibrium. Inclusion of a multirate mass transfer model successfully simulated the nonequilibrium desorption behavior of U(VI). Local sediment properties such as sediment texture (weight percent <2 mm), surface area, cation exchange capacity, and adsorbed U(VI) were heterogeneous at the meter scale, and it was important to incorporate these values into model parameters in order to produce accurate simulations.