A Field Test of delta13C as a Tracer of Aerobic Hydrocarbon Degradation (POSTPRINT)

The controlled release of a mixture simulating jet fuel was conducted to determine the effects of transport and degradation on the delta13C of reactants and products in a field setting. Benzene, toluene, naphthalene, xylenes, and decane were mixed with native soil and placed 4 m below ground into a surficial aquifer. The delta13C values of the hydrocarbons prior to and after emplacement were measured and no significant isotopic fractionation was observed following migration and partial degradation. The delta13C values of the compounds within the hydrocarbon mixture prior to addition and following 40 and 164 days in the field ranged from -26.6 to-30.0, -26.3 to -30.4 and -26.5 to -29.4%, respectively. Also measured were the concentrations and delta13C values of respiration end products CO2 and CH4 in groundwater samples prior to and 164, 278, 468, 642 and 831 days after the contamination of the surficial aquifer. End product concentrations were clearly elevated relative to pre-emplacement values indicating microbial respiration of the added hydrocarbon mixture. Isotope mass balance calculations yielded similar delta13C values for the sum of the respiration products and the added hydrocarbon mixture. Our results indicate that the products of hydrocarbon respiration reflect the delta13C of the substrates, and in less controlled contaminated field settings DIC isotopic values may be useful for estimating hydrocarbon degradation when DIC up-gradient of the spill has a delta13C value different from that of the contaminants. Published in Organic Geochemistry, v35 p123-135, 2004. Prepared in collaboration with the Department of Geological Sciences, University of Missouri, Columbia, MO; Airbase Technologies Division, Air Force Research Laboratory, Tyndall AFB, FL; and Virginia Institute of Marine Sciences, Gloucester Point, VA.