UPSCALING OF URANIUM(VI) TRANSPORT IN CONTAMINATED SOIL AND GROUNDWATER

With few exceptions, mathematical descriptions of non-conservative (sorbing) contaminants have invoked distribution coefficients (e.g., K_d values) to account for contaminant solid / solution partitioning. Distribution coefficients though, are conditional, ‘non-chemical' parameters that do not allow for variations in system chemistry. As an alternative, reactive transport models (RTMs) couple mechanistic chemistry with water flow models; this coupling allows for the incorporation of the explicit chemical behavior of target contaminants in transport simulations. A major barrier for RTMs, however, is how to extrapolate information collected at the bench scale up to a field scale setting. In this presentation, we report on experiments which are taking place at the intermediate level (between bench and field scales) and are designed to ‘link' the bench scale to the field scale.

Approximately 3m³ of uranium contaminated aquifer sediments were collected from the Naturita UMTRA site in southwestern Colorado. Two intermediate scale tanks (2.44 m x 1.22 m x 7.6 cm, and 2.44m x 0.61m x 7.6cm) were constructed and filled with size sorted aquifer material. The larger tank was filled in a homogenous manner using only the <2mm fraction. For the smaller tank, the <2mm fraction was re-sieved into 0-0.250mm and 0.250-2mm and these two fractions were packed in a heterogeneous fashion. Bulkhead fittings were installed through the side of the tank wall to allow for spatial measurements of pressure head, as well as withdrawal of water samples to determine pH, [U(VI)]_aq, [Ca(II)]_aq, and alkalinity.

Uranium distribution within the tank was found to vary with pH, alkalinity, dissolved calcium and the rate of release of U from the different particle size categories and the nature of the heterogeneity distribution. In the larger tank, effluent uranium concentrations ranged from 7.26mM at early time points, and decreased to ~1.5mM as the tank began to exhibit tailing behavior. For the aquifer material and ground water composition under study, it is suspected that the major control on both pH and alkalinity is the dissolution/precipitation of calcite. Modeling is underway using the code CRUNCH (Steefel, 2005) to simulate the intermediate-scale tank behavior based on the experimental data of U desorption at the bench-scale.