| Abstract
| - Assessing the quantity of U(VI) that participates insorption/desorption processes in a contaminated aquiferis an important task when investigating U migration behavior.U-contaminated aquifer sediments were obtained from16 different locations at a former U mill tailings site atNaturita, CO (U.S.A.) and were extracted with an artificialgroundwater, a high pH sodium bicarbonate solution,hydroxylamine hydrochloride solution, and concentratednitric acid. With an isotopic exchange method, both a KDvalue for the specific experimental conditions as well as thetotal exchangeable mass of U(VI) was determined.Except for one sample, KD values determined by isotopicexchange with U-contaminated sediments that were inequilibrium with atmospheric CO2 agreed within a factorof 2 with KD values predicted from a nonelectrostatic surfacecomplexation model (NEM) developed from U(VI) adsorptionexperiments with uncontaminated sediments. The labilefraction of U(VI) and U extracted by the bicarbonate solutionwere highly correlated (r2 = 0.997), with a slope of 0.96± 0.01. The proximity of the slope to one suggests that bothmethods likely access the same reservoir of U(VI)associated with the sediments. The results indicate thatthe bicarbonate extraction method is useful for estimatingthe mass of labile U(VI) in sediments that do not containU(IV). In-situ KD values calculated from the measured labileU(VI) and the dissolved U(VI) in the Naturita alluvialaquifer agreed within a factor of 3 with in-situ KD valuespredicted with the NEM and groundwater chemistry at eachwell.
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