USING SURFACE WATER INJECTIONS IN LABORATORY COLUMN AND FIELD TRACER TESTS TO DETERMINE URANIUM SORPTION PARAMETERS FOR REACTIVE TRANSPORT MODELING

Reactive transport models can simulate the mobility of uranium (and other contaminants) under a variety of geochemical conditions. Thus, improving on the traditional approach of using a constant sorption distribution coefficient (K_d, solid-phase concentration divided by the water-phase concentration). However, obtaining the uranium sorption parameters required for reactive transport modeling is not straightforward. Laboratory column and field tracer testing were performed at a former uranium mill site, located in Grand Junction, Colorado, to determine uranium sorption parameters for future use in site reactive transport modeling of remedial scenarios. The contaminated aquifer at this site is an alluvial sand and gravel point bar deposit with groundwater flow controlled by the stage of the nearby Gunnison River. All groundwater at this former uranium mill site ultimately discharges to the Gunnison River.

The column influents were uncontaminated Gunnison River water followed by contaminated groundwater to desorb and then resorb uranium to the column sediments. The field push-pull tracer tests were similar; uncontaminated (0.0067 mg/L uranium) traced river water was injected into and then pumped out of a single well to desorb and then resorb uranium with contaminated groundwater (0.37 mg/L uranium). The laboratory and field data included measurements of major cations, anions, and trace metals (including uranium). Graphically, a qualitative amount of uranium sorption is seen in the columns that is more difficult to recognize in the field data due to groundwater mixing during pumping. For both the column and field tracer tests, deriving quantitative sorption parameters requires initial reactive transport models (PHREEQC based) that are calibrated to the measured data. Final calibrated uranium sorption parameters from the column data are similar to those from the field tracer data. These parameters can be used to evaluate remedial scenarios, such as carbonate injections, that change the site groundwater geochemical conditions to desorb solid phase uranium and enhance natural flushing.