Use of Stable Oxygen and Carbon Isotope Analyses for Monitoring the Pathways and Rates of Intrinsic and Enhanced <i>in Situ</i> Biodegradation

Abstract

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.