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Description
A laboratory-scale soil vapor extraction test was conducted to evaluate the mechanisms that affect removal of free-phase hydrocarbon from the subsurface. The test was performed inside a steel-reinforced acrylic tank, 185 cm long by 185 cm high by 25 cm wide, which was filled with #60 industrial grade silica sand. A water table was initially established with a piezometric head of 54 cm above the base of the tank and a gradient of 0.01 to simulate a field groundwater system. Once this system was allowed to equilibrate, a gas tracer test, using sulfur hexafluoride, was performed to measure airflow velocities as a function of elevation above the air/water interface. Results of this procedure showed a rapid decrease in airflow velocities as the visible capillary fringe was approached, with a more gradual decrease in velocities below the capillary fringe. Twenty liters (5.3 gallons) of gasoline (free-phase hydrocarbon) was added through the side of the tank over the course of six weeks following the tracer experiment. Upon equilibration, the oil/air interface at the inlet side was 68.5 cm and the piezometric surface was 54.1 cm above the base of the tank, respectively. Vapor extraction began at a rate of approximately 25.5 liters/min. (0.9 scfm), which resulted in an initially high mass removal rate of 3600 g/day (25%/day) of gasoline range organics. This mass recovery rate decreased within the first 1. 5 hours of extraction. During the course of five weeks of vapor extraction, cumulative mass recovery rates of total GRO (gasoline range organics) and eleven individual gasoline components, including BTEX and MTBE (methyl-tert-butyl-ether), continued to increase in a generally linear trend. Recovery rates at the end of five weeks were 50 g/day (0.35%/day) for total GRO and 2 g/day (0.4%/day) for benzene. MTBE showed an affinity to volatilize and be remediated through vapor extraction. MTBE comprised 10.5 percent of the gasoline used for this experiment. Since vapor extraction began, approximately 40 percent of that initial mass had been removed and its mass removal rate continued to be approximately 13.5 g/day (0.88%/day) at the end of the experiment. The results of this experiment suggest that an important aspect of the success of vapor extraction for remediation of free-product hydrocarbon is the upward movement of hydrocarbon into an enhanced oil capillary fringe produced by the application of a vacuum. Application of the vacuum resulted in a decrease of the free product layer from 40 cm to 17 cm, as measured in manometers located at the inlet side of the tank, despite the fact that less than 16% of the mass of hydrocarbon had been removed from the tank. In addition, an analysis of mass removal rates versus elevation above the oil/air interface showed that virtually 100 percent of the mass removed is derived from only one of the eleven ports~ the port located immediately above the top of the oil capillary fringe. Measurements of concentrations of gasoline at the port just 7.6 cm above the oil capillary fringe showed negligible mass removal. Measurements of air fluxes through each of the ports showed that approximately 95 percent of the airflow is coming from ports located 7.6 cm and higher above the oil capillary fringe. These data indicate that free-product was drawn upwards into the capillary zone by the vacuum, and that volatilization from this zone controls the bulk removal of the free-product. These data also indicate that vapor extraction systems designed to remove free-product layers should have a minimal-length screen section and that this screen should be placed as close to the oil/air interface as possible.
