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The site is a small gasoline retail station which borders a residential area to the south. The surficial geology of the area is described as a uniform medium grained sand which overlies a dense clay at a depth of 65'below surface. Groundwater occurs under water table conditions at an average depth of 8' below the surface. The spill was discovered in the spring of 1998, when free phase NAPL began to seep into the basement of a residence 140' from the station. This event occurred during a historically high, seasonal rise (3') in the water table. Gasoline vapors in the home quickly approached explosive conditions prompting health and fire officials to evacuate the residents and cut electric service to the home. Emergency abatement actions were initiated immediately and consisted of sealing the foundation, forced-air ventilation of the basement and operation of a soil vapor extraction system around the perimeter of the home. Shortly thereafter, a pump and treat system was installed as an Interim Remedial Measure, to lower the water table locally and maintain hydraulic control of the area and prevent further migration of the NAPL and dissolved components.
A records search revealed that dissolved contaminants had been documented at the site as early as 1995, and that the site was under review by the regulatory agency for enforcement action.
An inspection of the UST system identified a minor leak in a dispenser valve as the source of the spill, suggesting that a slow, continuous release of fuel had been occurring for at least 3 years.
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