A New Screening Method for Methane in Soil Gas Using Existing Groundwater Monitoring Wells

Methane in soil gas may have undesirable consequences. The soil gas may be able to form a flammable mixture with air and present an explosion hazard. Aerobic biodegradation of the methane in soil gas may consume oxygen that would otherwise be available for biodegradation of gasoline hydrocarbons. The consumption of oxygen by methane may increase the chance of completing a vapor intrusion pathway for benzene. A protocol was developed to sample soil gas from conventional groundwater monitoring wells that had some portion of their screen in the vadose zone. This protocol was applied at 12 gasoline stations in Oklahoma. The soil gas as collected from the monitoring wells was not flammable, due to low concentrations of oxygen. However, soil gas at five of the 12 sites could form flammable mixtures in air. To allow a simple comparison of the possible effect of methane on vapor intrusion of benzene, characteristics of the sites were matched to computer simulations published by others. At three of 11 sites, the methane in the soil gas might cause unacceptable concentrations of benzene in a hypothetical receptor. Ethanol is readily fermented to methane. The increased use of ethanol in gasoline raises concerns about methane at gasoline spill sites. At five sites with high concentrations of methane, the concentration of C in methane was used to determine the source of the methane. At four sites, the majority of the methane was produced from anaerobic biodegradation of petroleum, and not from ethanol or another biofuel. At three sites, the maximum possible contribution of methane from ethanol or another biofuel was 5.1% of total methane. At a fourth site, the maximum contribution of ethanol or another biofuel to methane was 31% of the total methane. At the fifth site, the methane came from a leak of natural gas.