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Using Simple Field Instruments to Monitor for Biological Production of Methane at Gasoline Spill Sites
Citation:
JEWELL, K. P., J. T. WILSON, AND K. Barron. Using Simple Field Instruments to Monitor for Biological Production of Methane at Gasoline Spill Sites. Presented at 21st Annual National Tanks Conference and Expo, Sacramento, CA, March 20 - April 01, 2009.
Impact/Purpose:
Purpose was to evaluate a simple and inexpensive meter that is marketed to screen for methane vapors.
Description:
When gasoline containing ethanol is spilled to ground water, natural anaerobic biodegradation of the ethanol can produce copious quantities of methane gas, which bubbles out of the ground water and enters the unsaturated zone. Depending on local circumstances, the concentration of methane may exceed the lower explosive limit. In addition, the methane can exclude oxygen, or can provide a substrate for the biological consumption of oxygen in the soil gas, and as a result, the methane can diminish the role of aerobic biodegradation in limiting the intrusion of benzene vapors into structures. State Agencies and the consultants that serve the tank owners need a simple and affordable technology to screen sites for the production of methane. We evaluated a simple and inexpensive meter that is marketed to screen for methane vapors in coal mines. We used the meter to determine the concentrations of methane in the soil gas of tank fill at gasoline service stations in Oklahoma. Sites are selected where the tank fill is monitored with conventional two inch PVC monitoring wells that are screened above the water table. The well plug is replaced with a sampling cap, and soil gas is pumped at 10 liters per minute from the well for twenty minutes. The pump effluent is delivered to the meter, which records the concentrations of oxygen and the concentrations of methane as determined by an infra red cell. To evaluate the accuracy of the field meter, gas samples were collected and returned to the laboratory for analysis by gas chromatography. The lower explosive limit of methane in air is 5%. A concentration of 5% methane was taken as an arbitrary indication for concentrations of concern. The field meter indicated that concentrations of methane met or exceeded 5% at seven of nine sites. Based on GC analysis, the concentrations of methane exceed 5% at only three of the nine sites. An analysis of the composition of stable isotopes indicated that the methane at two of the three sites was produced by anaerobic biodegradation of the petroleum constituents of the fuel. At the other site, the methane came from a leak of natural gas.
