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Passive vapor extraction involves wells that are screened in the unsaturated zone and open to the atmosphere. Gas will flow out of the subsurface through the open well during periods of low barometric pressure. Field and modeling studies have been completed to evaluate enhancements for a passive vapor extraction system at a site contaminated with carbon tetrachloride on the Hanford nuclear reservation near Richland, Washington. During a 38‐hour period of low barometric pressure, approximately 500 m3 of air were vented from the subsurface. Approximately 27 grams of carbon tetrachloride were removed from the subsurface during this same outflow event. On an annual basis, more than 15 kilograms of carbon tetrachloride have been removed from each of several passive extraction wells. Computer simulations based on the field data indicate that surface covers smaller than 30 m radius will result in relatively small enhancements of flow. However, with larger surface seals (i.e., up to 90 m radius), volumetric flow rates more than doubled. Simulations showed that check valves might increase the rate at which subsurface gases are extracted by a factor of nearly three. These estimates are sensitive to dispersion coefficients. If not properly designed, filters used to treat effluent gases from passive extraction systems can significantly reduce the effectiveness of these systems.

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