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0 0 GeoHydrologic Consultants, Inc. SSB Environmental Consultants, Inc. (SSB)conducted a well survey within a half-mile radius during their preliminary investigation. Their findings indicated a City of Stockton well (No. 11) is located at the corner of Inglewood and Leslie Avenues (approximately 2,100 feet southeast of the site). A number of private wells are located within 1,000 feet to the west of the site in a county residential area and one to the northwest in the Carden School. In addition, a number of wells are located in a small commercial center south of the site; however,they are no longer used,as the occupants currently utilize the City of Stockton water system (Thrifty, 1995). In May 1991, SSB encountered groundwater at a depth of approximately 58 feet bgs during installation of groundwater monitoring wells. The hydraulic gradient was measured to be 0.0017 feet per foot towards the east-southeast. In July 1995, groundwater was encountered at a depth of approximately 48 feet,and the hydraulic gradient was measured to be 0.0013 feet per foot towards the east-southeast(Thrifty,August 1995). Subsequent measurements have varied flow directions although the static groundwater level has gradually risen since this time(Thrifty,2006). Groundwater was encountered during previous drilling activities at approximately 33 to 58 feet bgs. During the November 27, 2007 groundwater monitoring measurements, depth to water in the Site wells was between 35.16 and 37.85 feet below top of casing (btc). The hydraulic flow under the Site is northeast at a gradient of approximately 0.009 feet per foot(ft/ft). 2.2 Previous Site Assessment Activities Previous site assessments were conducted in 1991, 1994, 1995, 1996,and 1999 at Thrifty Station 172. Groundwater monitoring and sampling has occurred at the site since June 6, 1991. From May 23 through June 6, 1991, SSB installed four(4)ground water monitoring wells (MW-1, MW-2, MW-3,and MW-4)at locations indicated on Figure 2. Two(2)monitoring wells were located adjacent to the USTs and two (2) monitoring wells were installed near the pump island.The monitoring wells were installed to depths of 76 to 80 feet,and groundwater was encountered at 58 feet bgs. Laboratory analysis of soil samples collected from the well MW-2 at a depth of 20 feet indicated the highest concentration of TPH and benzene at 380 milligrams per kilogram (mg/kg)and 33 mg/kg, respectively. Laboratory analysis of groundwater samples collected ranged from 36.4 micrograms per liter(pg/L)to 5,270 µg/L,benzene in wells MW-4 and MW-2, respectively. On April 22, 1994,Thrifty installed three(3)vadose zone wells(VWA, VWB, and V WC)to depths of 35 feet,48 feet,and 45 feet bgs. Thrifty also installed two(2) ground water monitoring wells MW-7 and MW-8 to 56 feet and 58 feet, respectively(Figure 2). Groundwater was encountered at approximately 50 feet bgs. Laboratory analysis of soil samples detected TPH concentrations from non-detectable (ND)to 21,000 mg/kg, and detected benzene concentrations from ND to 36 mg/kg. Laboratory analysis of groundwater samples detected 5,500 pg/L TPH and 830 µg/L benzene in MW-7,and 130,000 pg/L TPH and 7,200 pg/L benzene in MW-8. TOC 172 UST Removal Report Final Page 3