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Trichloroethylene (TCE) is a carcinogenic organic chemical impacting water resources worldwide. Its breakdown by reductive vs. oxidative degradation involves different types of chemical bonds. Hence, if distinct isotope effects are reflected in dual element (carbon and chlorine) isotope values, such trends could help distinguishing both processes in the environment. This work explored dual element isotope trends associated with TCE oxidation by two pure bacterial cultures: Pseudomonas putida F1 and Methylosinus trichosporium OB3b, where the latter expresses either soluble methane-monooxygenase (sMMO) or particulate methanemonooxygenase (pMMO). Carbon and chlorine isotope enrichment factors of TCE (ε 13 C =-11.5,-2.4 and-4.2‰; ε 37 Cl = 0.3,-1.3 and-2.4‰ respectively) differed strongly between the strains. The dual element isotope trend for strain F1 (ε 13 C/ε 37 Cl =-38) reflected, as expected, primary carbon and negligible chlorine isotope effects, whereas unexpectedly large chlorine isotope effects became apparent in the trend obtained with strain OB3b (ε 13 C/ε 37 Cl = +1.7 for sMMO and pMMO). Therefore, although dual element isotope analysis partly reflects predicted differences in oxidative vs. reductive (ε 13 C/ε 37 Cl = 3.4 to 5.7) degradation, the unexpected OB3b fractionation data may challenge field interpretation.

BACKGROUND: The fuel additive, methyl tert-butyl ether (MTBE), is one of the most frequently occurring groundwater pollutants worldwide. The present study focused on assessing microbial degradation of MTBE in groundwater below a gasoline station situated above the Israeli Coastal Aquifer. Groundwater was monitored in six monitoring wells over a time period of 2.5 years. RESULTS: The site water geochemistry indicated that anaerobic conditions prevailed in two of the most contaminated wells. During a year MTBE concentrations declined from 81 to 4.2 mg L −1 , concurrently, with a significant change in 13 C from −31.4 to −11.8 ‰ in a source area monitoring well. MTBE biodegradation during microcosm experiments under sulfate reducing and methanogenic conditions resulted in carbon isotopic enrichment factors ( ) of −17.2 ± 1.1 ‰. CONCLUSION: Significant decrease in MTBE concentrations, along with pronounced changes in carbon isotope composition, indicates in situ degradation of MTBE occurring at the site. This evidence was supported by a microcosm experiment with indigenous microbial cultures. The estimated rate of biodegradation is 0.7 year −1 , which is about 70% of the total natural attenuation (NA) processes.Assessing the natural attenuation of MTBE at the field site Assuming that MTBE biodegradation within the groundwater follows first-order kinetics, a rate constant for the process can be calculated based on shifts in 13 C (Equation (2)). Alternatively, a rate constant can be assessed by measuring the concentration J Chem Technol Biotechnol 2016; 91: 1638-1645

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