The ORFRC is located in Bear Creek Valley, on the U.S. Department of Energy's Oak Ridge Reservation in eastern Tennessee. Operation of the S-3 Waste Disposal Ponds resulted in extensive areas of subsurface inorganic, organic, and radioactive contamination from historical and secondary sources. The Ponds received 3.2 x 108 liters of acidic, nitrate and uranium-bearing waste for 32 years until the Ponds contents were "neutralized", "denitrified", and capped in 1988. Although the Ponds are capped, the vast majority of contaminant mass has migrated away from the Ponds into the underlying geologic media where it has precipitated or adsorbed onto the solid phase or migrated into the matrix via diffusion creating an extensive secondary source of contamination. Ground water interaction with this secondary source of contamination has resulted in a relatively stable ground water plume that extends over 4 kilometers down Bear Creek Valley. Contaminants in the ground water plume include uranium (U), technetium-99 (Tc), nitrate, thorium, and volatile organic compounds such as acetone, methylene chloride, toluene, and tetrachloroethylene (DOE 1997). The ground water pH, which can range from 3.2 close to the Ponds to over 7.0 in wells farther down-gradient, is postulated to have a tremendous impact on subsurface processes and contaminant fate and transport. Elevated levels of dissolved organic carbon (DOC >200 ppm), primarily acetate leftover from Pond treatment, and hydrogen (>40,000 ppmv) are also associated with the plume. Other fixed gases recently detected in ground water include significant quantities of CO2, CO, N2O, N2 and CH4. Dissolved oxygen is typically low (<1 ppm) in deeper more contaminated ground water zones but can be higher (2 – 4 ppm) in areas subject to shallow recharge.