Waste material buried in drums in the shallow subsurface at the Radioactive Waste Management Facility (RWMC) of the Idaho National Engineering and Environmental Laboratory (INEEL) contained significant amounts of organic compounds, including lubricating oils and chlorinated solvents. Carbon dioxide concentrations in pore gas samples from monitoring wells in the vicinity of the disposal pits are three to five times higher than the concentrations in nearby background wells. The stable C isotope ratios (δ13C values) of CO2 from the disposal pits averaged 2.4‰ less than CO2 from the background wells, indicating that the elevated CO2 concentrations around the pits were derived from source materials with δ13C values in the range of −24 to −29‰. These δ13C values are typical of lubricating oils, but higher than most solvents. The radiocarbon (14C) contents of CO2 across most of the site were significantly elevated above modern concentrations due to reactor blocks buried in a subsurface vault at the site. However, several samples collected from the high-CO2 zone on the far side of the RWMC from the reactor blocks had very low 14C contents (<0.13 times modern), confirming production from lubricating oils manufactured from fossil hydrocarbons. The magnitude of the CO2 anomaly observed at the site is consistent with intrinsic biodegradation rates on the order of 0.5 to 3.0 t C yr−1.