Significant challenges remain in developing reliable techniques to monitor in situ biodegradation. Stable carbon and oxygen isotope analyses of the contaminants, products of degradation, and electron acceptor(s) may provide robust means for monitoring the occurrence, pathways, and rates of intrinsic or enhanced in situ biodegradation. Results of a laboratory study using diesel fuel and a mixed microbial culture show that combined stable carbon isotope analyses of carbon dioxide and stable oxygen isotope analyses of molecular oxygen allow monitoring of the occurrence and pathways of degradation. The first-order rate constants for contaminant degradation (about -0.04 day -1 ) obtained from oxygen and contaminant concentrations are in excellent agreement with those obtained from isotopic data for oxygen (-0.04 to -0.05 day -1 ), indicating that oxygen isotope analyses of molecular oxygen can be used for quantifying the rate of contaminant degradation. Based on our results and a review of the published literature on oxygen isotope systematics of molecular oxygen and other common electron acceptors (nitrate and sulfate), it is suggested that combined carbon and oxygen isotope analyses of carbon dioxide and the electron acceptors provide effective tools for monitoring intrinsic and enhanced in situ biodegradation of fuel or chlorinated hydrocarbons under aerobic and anaerobic conditions.

The metabolism of various explosive compounds-1,3,5-trinitrobenzene (TNB), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetraazocine (HMX)-by a sulfate-reducing bacterial consortium, Desulfovibrio spp., was studied. The results indicated that the Desulfovibrio spp. used all of the explosive compounds studied as their sole source of nitrogen for growth. The concentrations of TNB, RDX, and HMX in the culture media dropped to below the detection limit (<0.5 ppm) within 18 days of incubation. We also observed the production of ammonia from the nitro groups of the explosive compounds in the culture media. This ammonia served as a nitrogen source for the bacterial growth, and the concentration of ammonia later dropped to <0.5 mg/L. The sulfate-reducing bacteria may be useful in the anaerobic treatment of explosives-contaminated soil.