MODELING URANIUM TRANSPORT IN UNSATURATED ZONE AT PEÑA BLANCA, MEXICO

Current models utilizing U- and Th-series disequilibria for radioisotope transport in groundwater systems are mostly stationary in nature. These models are of limited use for the unsaturated vadose zone where the concentration and migratory behavior of radioisotopes are often time dependent. We present here a model simulating the non-steady state situation. Applicable to vadose as well as phreatic (saturated) layers, the model provides constraints on in-situ radioisotope migration in dissolved and colloidal phases in terms of retardation factor and rock-water interaction (or water transit) time. For the mobility of uranium, the model shows that water percolating through the vadose zone will have its U concentration and ratio ²³⁴U/²³⁸U linearly correlated under the ideal condition of a constant removal of the two U isotopes from vadose-zone solids. By knowing the non-flushing period between two major rain events, the intercept and slope of the linearity serve to constrain removal rates of the two uranium isotopes from solids to the solution through dissolution and alpha recoil. At Peña Blanca, the uneven distribution of rainfall between winter/spring (dry) and summer/fall (wet) results in a seasonal variability of uranium removal rate within the unsaturated zone. Data on ²³⁴U/²³⁸U indicate a higher uranium dissolution rate relative to alpha-recoil rate during drier seasons, possibly reflecting increased oxygenation in vadose layers.