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Air Sparging Pilot Test Related Activities Report and Remedial Action Work Plan Former Countryside Market, Stockton, California April 30, 2015 2.0 BACKGROUND AND PHYSICAL SETTING 2.1 Site Location and Description The Site is located in eastern San Joaquin County, approximately two miles east of the city limits of Stockton, and southeast of the intersection of Copperopolis Road and Tulsa Road. The Site vicinity is primarily rural residential and agricultural. Currently, the Site is vacant with the current property owner occupying a single-family residence on a separate parcel of land directly south of the Site. The Former Countryside Market acted as a gasoline service station and convenience stored that dispensed fuel via two 1,000-gallon USTs. It does not appear a permit to operate/own USTs was completed so the age of the tanks is unclear. However, USTs were removed under San Joaquin County Environmental Health Division (SJCEHD) permit in March 1993, which is presented in Section 2.4. 2.2 Physical Setting The City of Stockton is centrally located in San Joaquin County in the north end of the San Joaquin Valley. The Site lies approximately six miles east of Stockton's city center at an elevation of 52 feet above mean sea level (amsl). Results of this Site investigation indicate fine alluvial sediments to depths of 120 feet below ground surface (bgs) consisting of sandy clay and clayey sand. Groundwater was initially encountered at approximately 92.5 to 93 feet bgs during drilling activities. Depth to groundwater in monitoring wells ranged from 98.10 to 99.73 feet below top of casing (TOC) during the first quarter 2015 groundwater monitoring activities (SGI, 2015a). Water levels have been consistently dropping likely due to regional drought conditions. 2.3 Previous Environmental Investigations In March 1993, Stockton Service Equipment Company removed two 1,000-gallon leaded- gasoline USTs from the Site. Four soil samples (TK1N, TK1S, TK2N, and TK2S) were collected from beneath the USTs at depths of approximately 7.5 feet bgs. The samples were analyzed for total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and xylenes (BTEX compounds), ethylene dibromide (EDB), and total lead. TPHg was detected in all four samples at concentrations ranging from 770 to 6,500 milligrams per kilogram (mg/kg). The excavated soil was subsequently returned to the excavation, and no investigation was performed (UEC, 2001). Historical soil samples results are included in Appendix B. In October 1995, approximately 40 cubic yards of hydrocarbon-impacted soil was excavated and replaced with clean backfill. The former UST pit was excavated to 11 and 13 feet bgs. Soil sample CM-1-13 collected from the bottom of the excavation indicated TPHg, BTEX compounds at concentrations of 14,000 mg/kg and not detected (ND) at a laboratory reporting limit (RL) of 0.005 mg/kg, 37 mg/kg, 78 mg/kg, and 340 mg/kg, respectively. The excavation was backfilled with clean stockpiled soil and clean pea gravel (UEC, 1995). 2-1 The Source Group, Inc.