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Remedial Action Plan June 14, 2017
<br /> Project#Z054000052 Page 2
<br /> 2.0 SITE HISTORY
<br /> 2.1 SITE DESCRIPTION
<br /> The site is a former retail gasoline station and convenience store located on the southwest corner of Pacific
<br /> Avenue and Rivara Road in Stockton, California. The gasoline station is not currently in operation.
<br /> Redevelopment of the site into a 7-Eleven convenience store was completed in June 2011.
<br /> A site plan based on recent and historical information is presented as Figure 2. It should be noted that some
<br /> discrepancies regarding un-surveyed boring locations on historical figures used to create the site plan remain
<br /> unresolved.
<br /> 2.2 PREVIOUS INVESTIGATIONS
<br /> From May 23 through June 6, 1991, SSB Environmental Consultants, Inc. (SSB) installed four groundwater
<br /> monitoring wells (MW-1 through MW-4). Two monitoring wells were located adjacent to the underground
<br /> storage tanks (USTs) and two monitoring wells were installed near the pump islands. The monitoring wells
<br /> were installed to depths of 76 to 80 feet (ft); groundwater was encountered at 58 ft below ground surface (bgs).
<br /> Laboratory analysis of a soil sample collected from well MW-2 at a depth of 20 ft indicated the highest
<br /> concentrations of total petroleum hydrocarbons (TPH), and benzene at 380 milligrams per kilogram (mg/kg)
<br /> and 33 mg/kg, respectively. Laboratory analysis of groundwater samples collected ranged from
<br /> 36.4 micrograms per liter (pg/L) to 5,270 pg/L benzene in wells MW-4 and MW-2, respectively (SSB, 1991).
<br /> On April 22, 1994, Thrifty installed three vadose zone wells (VWA, VWB, and VWC) to depths of 35 ft, 48 ft,
<br /> and 45 ft bgs, respectively. Thrifty also installed two ground water monitoring wells (MW-7 and MW-8) to 56 ft
<br /> bgs and 58 ft bgs, respectively. Groundwater was encountered at approximately 50 ft bgs. Laboratory
<br /> analysis of soil samples detected TPH concentrations up to 21,000 mg/kg, and benzene concentrations up to
<br /> 36 mg/kg. Laboratory analysis of groundwater samples detected 5,500 pg/L TPH and 830 pg/L benzene in
<br /> MW-7, and 130,000 pg/L TPH and 7,200 pg/L benzene in MW-8. Thrifty investigated the OPW vault area near
<br /> MW-8. The vault was approximately 10 inches in diameter and covered what appeared to be a sewer line
<br /> clean-out. A 24-inch diameter hole was excavated adjacent to the OPW vault and a cast iron pipe that
<br /> appeared to be a sewer line was encountered at approximately 3 feet bgs. The pipe was connected to the
<br /> OPW pipe opening, but was not conclusively identified as a sewer line (Thrifty, 1994).
<br /> In July 1995, Thrifty Oil Company installed three off-site groundwater monitoring wells (MW-5, MW-6, and MW-
<br /> 9) to approximately 60 feet bgs. Petroleum hydrocarbons were not detected in the soil samples collected from
<br /> MW-5, MW-6, and MW-9. Dissolved-phase hydrocarbons were detected in groundwater at maximum
<br /> concentrations of 1,000 pg/L TPH (MW-9), 3.4 pg/L benzene (MW-9), 34 pg/L ethylbenzene (MW-9), and
<br /> 2.0 pg/L xylenes (MW-9) (Thrifty, 1995).
<br /> In July 1995, JDR Environmental (JDR) properly destroyed three vadose wells (VWA, VWB, and VWC) and
<br /> three groundwater monitoring wells (MW-1, MW-2, and MW-8) as required by the SJCEHD prior to station
<br /> remodeling activities (JDR, 1995).
<br /> In July 1995, five single-walled, steel USTs were removed from the site and disposed of by Erickson of
<br /> Richmond, California. Approximately 1,500 cubic yards of soil were removed from the UST excavation and
<br /> transported to Forward Incorporated, of Stockton, California, for disposal. Ten soil samples (Tank Pit-1
<br /> through Tank Pit-9 and Tank-E-9') were collected from beneath the five USTs after excavation, and seven soil
<br /> samples (PL-1 through PL-7) were collected at a depth of 2 to 4 ft from beneath the product lines. TPH and
<br /> benzene were detected in soil samples collected from beneath the USTs at concentrations up to 12,000 mg/kg
<br /> TPH and 33 mg/kg benzene. Soil was excavated to approximately 15 feet bgs (Alisto, 1995).
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