EXAMINING THE DEPENDENCE OF RADIONUCLIDE TRANSPORT ON MICROBIAL ACTIVITY THROUGH EXPERIMENTS WITH A MESO-SCALE UNSATURATED FLOW COLUMN

Radionuclide transport within the unsaturated zone is controlled by moisture content and moisture flux, water and soil chemistry, gas composition, and microbial activity, as well as a host of other factors. These controls are largely interdependent because of their effects on, and ensuing feedbacks, with microbiological activity. We are examining these interactions, and their depth dependence, by conducting a variety of transport experiments in a 2.5-m high x 1-m diameter unsaturated, homogeneous, soil column. Gas and conservative-solute tracer experiments within the column demonstrate dominantly 1-D transport behavior.  CO₂ and O₂ concentrations in the soil, monitored for over a year, indicate that microbial CO₂ production is relatively uniform with depth, though concentrations vary dramatically in response to perturbations in gas-phase composition. At quasi-steady state, CO₂ concentrations increase from atmospheric levels, near the surface, to >15% by volume at depth. The system is thus a well-constrained model of a waste disposal site in which organic matter, degrading with time, alters the gas-phase composition and aqueous geochemistry through CO₂ production. Contaminant transport experiments in the column will include injections of ¹⁴C, tritium and uranium. We designed the uranium injection experiments by conducting a series of reactive transport simulations with the geochemical transport code, PHREEQC. These simulations illustrate how uranium mobility in such a system depends on microbiological activity and also how transient events in the production rate of CO₂ affect uranium transport. The predictions, which will be tested in the actual column experiments, should provide significant insight as to how actinide transport in waste sites may be affected by long-term changes in the distribution of organic carbon within the subsurface.