Connecting Soil Pores to the Field: Uranium Transport Experiments at the Intermediate Scale

An overlying goal of reactive transport modeling (RTM) is to explicitly couple the chemical behavior of a specific contaminant with the hydrology and the geochemical conditions in a given aquifer. Chemical behaviors can include dissolution/precipitation, microbial degradation, redox transformation and sorption/desorption. By combining the chemistry with the hydrology, a more accurate and robust model of what is occurring in the sub- surface should emerge. However, a major obstacle for RTM is how to 'up-scale' the chemical information collected in laboratories to a field setting. In this presentation we report on experiments that are taking place at the intermediate scale (between lab and field scales) which are designed to elucidate methodologies to 'link' pore- and field-scale processes. Two intermediate scale tanks (2.44 m x 1.22 m x 7.6 cm, and 2.44m x 0.61m x 7.6cm) have been constructed and two separate packings of uranium contaminated sediment from the Naturita Uranium Mill Tailings Remedial Action (UMTRA) site have been completed. The first packing, in the larger of the two tanks, was physically homogenous using only the <2mm fraction of sediment. In the second, smaller tank, physical heterogeneity was introduced by splitting the <2mm fraction into 0-0.250mm and 0.250mm-2mm fractions. These two size fractions were packed in alternating layers of different thicknesses. In both tanks, samples for uranium and water quality analysis as well as measurements of pressure head were taken through bulkhead fittings installed through the wall of each tank. In the larger tank a bromide tracer test has been completed to determine hyrdrologic parameters for this sediment. Uranium distribution within the tanks 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 of the two tanks, effluent uranium concentrations ranged from 7.26microM at early time points, and decreased to ~1.5microM as the tank began to exhibit tailing behavior. The spatial gradients of the major chemical constituents were generally smooth, but were variable both as a function of time and space. Kinetic hindrances to uranium desorption were exhibited during stop flow events; these hindrances may be either physical or chemical in nature.