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Influence of Methane at UST Spill Sites on Biodegradation and Transport of Benzene in Soil Gas
Citation:
JEWELL, K., J. T. WILSON, AND J. P. SKENDER. Influence of Methane at UST Spill Sites on Biodegradation and Transport of Benzene in Soil Gas. Presented at International Symposium on Bioremediation and Sustainable Environmental Technologies, Reno, NV, June 27 - 30, 2011.
Impact/Purpose:
Presentation for the International Symposium on Bioremediation and Sustainable Environmental Technologies - (Reno, NV; June 27 -30, 2011)
Description:
Background/Objectives: Methane can be produced at gasoline spill sites from the biodegradation of biofuels, such as ethanol, or from the fermentation of petroleum constituents such as the aromatic hydrocarbons. If the methane finds its way into soil gas, the aerobic biodegradation of methane in soil gas may consume oxygen that would otherwise be used for biodegradation of gasoline hydrocarbons such as benzene. If biodegradation of gasoline hydrocarbons is inhibited, this will increase the chance of completing a vapor intrusion pathway for the gasoline hydrocarbons. Approach/Activities: At many gasoline service stations, ground water monitoring wells are screened across the water table. This allows the wells to also sample free product, and to accommodate modest changes in the elevation of the water table. Because there is screen above the water table, these wells can also be used to sample the soil gas immediately above the water table. We developed a protocol that uses conventional ground water monitoring wells to sample soil gas at gasoline spill sites. We applied this protocol to twelve gasoline stations in Oklahoma. To evaluate the influence of high concentrations of methane and low concentrations of oxygen on biodegradation of benzene, we compared the concentrations of methane and oxygen in the soil gas to the concentration of benzene in soil gas. Many risk evaluations for vapor intrusion assume that benzene in soil gas originated from the ground water immediately below the soil gas. Instead of actually measuring the concentration of benzene in soil gas, the Henry’s Law Constant for benzene is used to estimate the concentration in soil gas from the concentration of benzene in water produced from the monitoring well. To determine whether the benzene in soil gas originated from ground water, or came to the well through lateral transport as a vapor, we compared the concentrations of benzene in ground water from the well to the concentrations of benzene in soil gas. Results/Lessons Learned: Across the twelve sites, the maximum concentrations of methane in soil gas varied from 0.004% by volume to 63% by volume. Eight of the twelve sites had methane concentrations above 1%. When concentrations of methane in soil gas were above 1%, the concentrations of oxygen were below 2%. The oxygen demand for biodegradation of methane reduced the supply of oxygen needed for aerobic biodegradation of benzene. As a result, the highest concentrations of benzene (10 to 400 ppm v/v) were also associated with methane concentrations above 1%. The U.S. EPA screening level for concentrations of benzene in deep soil gas is near 1 ppm by volume. When the concentrations of methane exceeded 1%, the concentrations of benzene exceeded the screening level by ten fold to forty fold. With one exception, when the concentration of methane was less than 1%, concentrations of benzene were undetected or were near the screening level. We compared concentrations of benzene in soil gas to concentrations of benzene in ground at six sites. At four sites the concentrations of benzene in ground water underestimated the concentrations of benzene in soil gas. At these four sites, it was more likely that the source of benzene in soil gas was benzene that originated some distance away from the monitoring well. The U.S. Environmental Protection Agency through its Office of Research and Development funded and managed the research described here through in-house efforts. It has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency, or of the Petroleum Storage Tank Division of the Oklahoma Corporation Commission, and no official endorsement should be inferred. All research projects making conclusions or recommendations based on environmental data and funded by the U.S. Environmental Protection Agency are required to participate in the Agency Quality Assurance Program. This project was conducted under an approved Quality Assurance Project Plan. The procedures specified in this plan were used without exception. Information on the plan and documentation of the quality assurance activities and results are available from John Wilson. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. NOTE: Download of presentation slides is not available.