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ATC Associates Inc. (: V9%TC 1117 Lone Palm Avenue,Suite 201 y Modesto,California 95351 209-579-2221 fax:209-579-2225 ASSOC I ATES 1 N C. low MW1. Analytical data indicated that soil and groundwater samples contained detectable concentrations of petroleum hydrocarbons. Field activities and analytical results are detailed in ATC's Summary Report of Additional Subsurface Investigation, dated January 8, 1999. In December 1999, ATC personnel supervised the advancement of soil borings MW2, MW3, and MW4 each to a depth of approximately 65 feet bgs to evaluate groundwater for the 6' presence of petroleum hydrocarbons and to determine the groundwater gradient direction and magnitude. Each boring was completed as a monitoring well. Analytical data indicated that soil samples collected from boring MW2 contained detectable concentrations of petroleum 6,, hydrocarbons. Petroleum hydrocarbons were also detected in the groundwater sample collected from monitoring well MW2. A groundwater sample collected from MW3 contained very low concentrations of only a few of the petroleum analytes. Petroleum hydrocarbons were not detected in the groundwater sample obtained from MW4. Field activities and analytical results are summarized in ATC's Summary Report of Additional Subsurface Investigation, dated February 24, 2000. �.. In December 2001, soil boring VEW1 was advanced to approximately 35 feet bgs, and borings VEW2 and VEW3 were each advanced to an approximate depth of 45 feet bgs. Borings MW5, MW6, and MW7 were each advanced to approximately 65 feet bgs and were completed as 2- inch diameter groundwater monitoring wells. Soil borings VEW1, VEW2, and VEW3 were completed as 2-inch diameter vapor extraction wells. Boring CPT1 was advanced to approximately 100 feet bgs utilizing a truck-mounted Cone Penetrometer Testing (CPT) rig. >r In 2002, ATC performed a soil vapor extraction (SVE) pilot test using mobile extraction equipment that yielded a radius of influence (ROI) of approximately 30 feet at a vacuum of 11.5 inches of water. Approximately 2.64 pounds of total petroleum hydrocarbons were removed from the subsurface during the pilot test. Field activities and analytical results are presented in ATC's Remedial Investigation Feasibility Study dated November 19, 2002. From October 2004 to August 2006, ATC performed additional SVE system tests to provide information from which an efficient and effective permanent SVE system could be designed. In December 2006, ATC completed permitting and began installation of a SVE system with �., startup of the system occurring on January 17, 2007. The system continued operating until February 13, 2009, when volatile organic vapor concentrations became too low to justify sustained continuous operation. On July 14, 2009, after 151 days of non-operation, the SVE Ir system was restarted in order to conduct a rebound test. Concentrations of volatile organic compounds (VOCs) measured from the influent air stream were determined to not significantly increased beyond the concentrations measured prior to the system shut down in February 2009. The SVE system rebound test supported historical data that indicated influent vapor phase �.• hydrocarbon concentrations had decreased to levels that warranted terminating system operations. The test also suggested that cycling the system would not be a cost effective alternative. ry During operation of the SVE system between January 17, 2007 and February 13, 2009, the system operated for a total of 16,500 hours and successfully removed an estimated 5,436.76 pounds of total petroleum hydrocarbons as gasoline (TPHg), and an estimated 823.39 pounds 6d of benzene, toluene, ethyl benzene, and total xylenes (BTEX) constituents or a total of estimate of 6,260.15 pounds of vapor phase hydrocarbons. Using an average density of gasoline of 6.25 S:\Environmental\62577\Reports\201 I Soil Gas.doc 2 ATC Associates Inc. I..r V