COMPARISON OF REACTIVE TRANSPORT MODELING WITH CONSTANT-KD AND SURFACE COMPLEXATION MODELS FOR URANIUM(VI) SORPTION

Applying the surface complexation concept in the natural environment requires a simplified model, characterized by mass laws that are coupled with aqueous speciation, while lumping parameters that are difficult to determine with other parameters. In order to be applied at the field scale, the complexity of a surface complexation model (SCM) needs to be balanced with the goal of using the simplest model possible that is still consistent with observed data. This can be achieved with the semi-mechanistic, generalized composite (GC) modeling approach. The approach is a compromise between a constant-Kd model and more complex SCM that are difficult to apply in natural systems. A GC model is preferable to empirical approaches because the important linkage between surface and aqueous species can be retained.

A study was conducted of uranium(VI) sorption on natural and U-contaminated sediments in the laboratory and in the field environment. The location of the study was a uranium mill tailings site near Naturita, Colorado, which has a well characterized uranium(VI) contaminant plume in a shallow alluvial aquifer. It was shown in laboratory batch and column experiments with uncontaminated Naturita sediments that the sorption and retardation of U(VI) transport by the Naturita sediments was strongly influenced by the dissolved bicarbonate concentration. An SCM was developed to describe U(VI) sorption measured in the laboratory on the uncontaminated sediments. For the range of chemical conditions observed in the Naturita aquifer, variable bicarbonate was more important than either variable pH or U(VI) concentration in influencing U(VI) mobility. Transport simulations were conducted for the field scale to compare simulations with the constant-Kd and GC modeling approaches.

Methods were also investigated to measure U(VI) sorption on the sediments under field conditions. Such methods are needed for: 1) validation of SCM model parameters in transport simulations, and 2) estimation of initial conditions for sorbed metals at contaminated sites for predictive transport simulations. It was shown that isotopic exchange and desorption extraction methods can be an important part of a field characterization and modeling program.