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Carbon Isotopic Evidence for Biodegradation of Organic Contaminants in the Shallow Vadose Zone of the Radioactive Waste Management Complex

^a Earth Sciences Division, MS 70A-4418, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720
^b Earth Sciences Division, MS 90R-1116, E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720

Correspondence: ^* Corresponding author (MSConrad{at}lbl.gov).

Received for publication 20 May 2003. 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 (¹³C 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 ¹³C values in the range of –24 to –29. These ¹³C 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.

Abbreviations: CAMS, Center for Accelerator Mass Spectrometry • INEEL, Idaho National Engineering and Environmental Laboratory • RWMC, Radioactive Waste Management Facility • SDA, Subsurface Disposal Area • TSA, Transuranic Storage Area