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�. N.01 ATC Associates Inc. M%TC 1117 Lone Palm Avenue, Suite 201 Modesto,California 95351 209-579-2221 Fax: 209-579-2225 ASSOCIATES INC . In July 1993, a subsurface investigation was conducted at the site to determine the extent of petroleum hydrocarbons present in soil and to determine if groundwater had been impacted. The investigation included drilling eight soil borings and collection of soil samples. Three of the borings were completed as shallow groundwater monitoring wells (MW-1, MW-2, and MW-3). Analyses of soil samples indicated the greatest concentration of petroleum hydrocarbons in soil occurred from approximately ten feet below ground surface (bgs) to the groundwater surface at a depth of approximately 2111 feet bgs and within a radius of 35 feet from the former UST pit. '3015 In August 1993, monitoring wells MW-1, MW-2, and MW-3 were monitored and sampled. Laboratory analytical results indicated that the samples collected from MW-2 and MW-3 contained detectable levels of petroleum hydrocarbons in the groundwater. The sample collected from MW-1 was reported to contain no measurable concentrations of petroleum hydrocarbons. A quarterly groundwater monitoring and sampling program was initiated in March 1994. In August 1995, Smith Technology conducted a subsurface site investigation to further define the extent of contamination in groundwater at the site. Smith Technology supervised the installation of monitoring wells MW-4, MW-5, and MW-6. The three new wells were developed and sampled along with original monitoring wells MW-1, MW-2, and MW-3. Analytical results of groundwater samples collected from the six monitoring wells indicated the presence of elevated concentrations of petroleum hydrocarbons in the vicinity of MW-3 and MW-6 in the northeastern comer of the site. — In November 1998, ATC conducted a subsurface site investigation to evaluate the vertical and horizontal extent of hydrocarbon-impacted soil. ATC supervised the advancement of three soil 0 borin s B1 B2, and B3) from approximately 30 to 50 feet bgs. Analytical results of soil sam r ertical extent of soil contamination had been defined, but the horizontal extent , Impacted soil was not fully identified in the northeastern corner of the site. Site activities are 31 y summarized in ATC's Summary Report of Additional Subsurface Investigation, dated January 27, 1999. From August 20 to 22, 2002, ATC conducted pump testing activities to evaluate aquifer properties and the radius of influence associated with groundwater extraction from MW-3. The groundwater extraction pump test yielded a pumping rate of approximately 5.55 gallons per minute and a radius of influence of approximately 85 feet. Refer to ATC's Feasibility Study Report, dated December 2, 2003, for additional information. On March 21, 2008, ATC conducted a soil vapor study to evaluate the potential for vapor intrusion hazards into nearby structures originating from subsurface soil and/or groundwater beneath the site. Analytical results of the soil vapor sampling activities indicate human health and the environment, S:\environmental\6205Vteports\additional subsurface investigation workplan.wpd 2