Compounds Specific Isotope Analysis to Recognize and Evaluate Natural Attenuation of Methane in Groundwater

Biodegradation can be an important mechanism for the natural attenuation of methane in groundwater. Biodegradation of methane can be carried out by three classes of microorganisms: bacteria that use oxygen as the electron acceptor, bacteria that use nitrate as the electron acceptor, and archaea that form a syntrophic association with sulfate-reducing bacteria. At sites that have been impacted with methane from unconventional gas development, sulfate is likely to be the important electron acceptor. All three classes of microorganisms fractionate the stable isotopes of carbon and hydrogen in methane as the methane is metabolized. It should be possible to recognize biodegradation of methane from the associated fractionation of carbon or hydrogen isotopes. Under sulfate-reducing conditions, degradation of as little as 50% of the methane originally present in the groundwater can shift the value of δ¹³C by 10‰. For years, geochemists in the oil and gas industry have used stable isotopes to identify the source of methane in a reservoir. The same approach and the same techniques should have application to recognize and evaluate the natural attenuation of methane that has escaped the reservoir and entered shallow groundwater.