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i j Ms. Margaret Lagorio September 24, 1996 Page 2 ARCO notified the San Joaquin Local Health District and other interested agencies of the product loss. Production from the City of Lodi Municipal Well 12, located approximately 240 feet from the leak site, was stopped immediately. Municipal Well 2 was restarted in July 1993. BC was hired to conduct investigative work at the station. Previous Subsurface Investigations In April 1987, a site investigation was conducted at the station by'BC_ The objectives of the investigation were to determine the groundwater gradient, identify product in soil and/or groundwater beneath the site, evaluate the site geology, and obtain information necessary for evaluating potential remedial actions at the site. Three monitoring wells were drilled, W-1, W-2, and W-3, and groundwater and soil samples were collected at these sites. The soil at these locations did not appear to contain significant levels of hydrocarbons; however, benzene, toluene, and xylene (BTX) concentrations from one or more of the monitoring well groundwater samples exceeded Department of Toxic Substances Control action levels. Three additional monitoring wells, W-4, W-5, and W-6, were drilled off-site in July 1987 to determine the extent of the plume to the south. In July 1987, between one and two feet of free product was measured in Wells W-2 and W-3; the product was removed to unmeasurable levels. In August 1987, Well W-3 had accumulated almost 8 inches of product. Product was removed from wells on a biweekly basis until the remedial system was installed. Off-site Wells W-7 and W-8 were installed in November 1987 to better define the extent of the hydrocarbon plume west of the station. Wells W-2A and W-2B were installed in June 1988. Quarterly sampling of all on and off-site wells was begun in September 1988 to systematically track benzene, toluene, ethylbenzene, and xylenes (BTEX) and total petroleum hydrocarbons as gasoline (TPH-G) trends. The dominant factor in hydrocarbon occurrence in groundwater beneath the station appeared to be hydrocarbons within the vadose zone that intermittently continued to migrate downward and e e e groundwater. Therefore, two boreholes, B-1 and B-2, were drilled on site in July -199 , n order to determine the lateral and vertical extent of hydrocarbon contamination in the vadose zone, and in order to facilitate pilot testing for design of a soil remediation system. In April and May 1990, five additional monitoring wells were installed, W-IA, W-9, W-10, W-11, and W-12, to further define the plume and determine the zero line; groundwater extraction well E-1 was installed; eight vapor extraction wells were installed; and monitoring Use or disclosure of data continued on this sheet is subject to the restriction specified at the beginning of this document. 33821CORR ESPIC LO S URE.DOC