Numerical Modeling of Gas Flow in the Hanford Vadose Zone

Gas flow behavior in the unsaturated subsurface (vadose zone) has implications on environmental work, and is investigated here in the context of waste sites at the U.S. Department of Energys Hanford site. Past operations at the Hanford site have resulted in vadose zone contamination with radionuclide, organic, metal, and inorganic pollutants. Soil vapor extraction (SVE) and gaseous nutrient amendments for biostimulation are two technologies relevant to vadose zone remediation. SVE treatment of carbon tetrachloride contamination at the Hanford 200 West area produced pressure data for pumping wells and surrounding monitoring wells. These gas pumping tests were modeled with GASSOLVE, which uses an analytical solution to identify horizontal air permeability under both steady-state and transient conditions. The pump tests were also modeled with the HYDROBIOGEOCHEM finite element numerical code to evaluate how gas pump tests are affected by barometric pressure effects. Model comparisons were made using Hanford SVE pump test data to validate the model and assess air permeability by accounting for barometric pressure fluctuations. The model was also used for gas injection predictions to examine the efficacy of gas injection in a bioreduction remediation technology. Injection of gaseous nutrients into the vadose zone is an approach for changing the redox conditions and facilitating immobilization or degradation of contaminants like uranium, nitrate, technetium-99, and hexavalent chromium. HYDROBIOGEOCHEM simulations of gaseous amendment delivery were analyzed to understand how porous media and organic gas properties affect delivery and distribution of amendment mass in the vadose zone.