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Vadose Zone Journal
Vol. 3, No. xx, 2004; Pages: 143–153

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

Mark E. Conrad* and Donald J. DePaolo

Earth Sciences Division, MS 70A-4418, E.O. LawrenceBerkeley National Laboratory, Berkeley, CA 94720.


Waste material buried in drums in the shallow subsurface at theRadioactive 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 inpore gas samples from monitoring wells in the vicinity of the disposalpits are three to five times higher than the concentrations in nearbybackground wells. The stable C isotope ratios (13C values) of CO2from the disposal pits averaged 2.4‰ less than CO2 from the back ground wells, indicating that the elevated CO2 concentrations aroundthe pits were derived from source materials with 13C values in therange of-24 to-29‰. These 13C values are typical of lubricating oils,but higher than most solvents. The radiocarbon (14C) contents of CO2across most of the site were significantly elevated above modernconcentrations due to reactor blocks buried in a subsurface vault atthe site. However, several samples collected from the high-CO2 zoneon the far side of the RWMC from the reactor blocks had verylow 14C contents (<0.13 times modern), confirming production fromlubricating oils manufactured from fossil hydrocarbons. The magnitude of theCO2 anomaly observed at the site is consistent with intrinsic biodegradation rates on the order of 0.5 to 3.0 t C yr-1.

Keywords:lubricating oils,Carbon dioxide,radiocarbon,bioremediation,Radionuclides.

Corresponding author:

E-mail: MSConrad@lbl.gov


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