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Monitoring Production of Methane and Carbon Dioxide and Consumption of Oxygen at Spills of Gasoline at UST Release Sites
Citation:
JEWELL, K. P., J. P. SKENDER, AND J. T. WILSON. Monitoring Production of Methane and Carbon Dioxide and Consumption of Oxygen at Spills of Gasoline at UST Release Sites. Presented at API Hydrocarbon Summit, Waterloo, ON, CANADA, July 13 - 15, 2011.
Impact/Purpose:
Presentation (via webinar) for the API Hydrocarbon Summit (July 13-15, 2011)
Description:
Methane is rarely measured at fuel spill sites, and most commonly the measurements are made on samples of ground water. Many ground water monitoring wells are intentionally screened across the water table. This was done to allow them to sample free product. However, if there is sufficient screen interval above the water table, a conventional monitoring well can be used to sample soil gas. We developed a sampling approach that allows us to pump soil gas from the riser of monitoring wells, and then use field equipment to monitor the concentrations of methane, oxygen and carbon dioxide in the soil gas in real time. The long term goal is to develop a robust and affordable sampling protocol that can be used by contractors or consultants to evaluate methane accumulation in soil gas at the same time that they sample water in wells for contamination. We visited twelve gasoline service stations in Oklahoma that have had recent confirmed releases from the UST. We sampled soil gas from monitoring wells in the UST tank pit, or in the aquifer immediately adjacent to the tank pit. When methane was present, the concentration of oxygen was low. As a result, the soil gas was not explosive. However, when the concentration of methane in gas is 14% or higher, it can mix with air to form an explosive mixture. At five of the thirteen sites, the soil gas from at least one monitoring well had methane concentrations above 14%. 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. 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.