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S E C 0 R 7-Eleven Store #14117 Additional Site Assessment Report and Fate and Transport Model February 27, 2004 Page 2 (Table 2) The onsite borings reported benzene, TPHg, and MtBE at maximum concentrations of 125 milligrams per kilogram (mg/kg) (GP-17020'), 42,900 mg/kg (GP-15015'), and 234 mg/kg (GP-14@ 10'), respectively (Fluor Daniel GTI, 1998) On May 13 and 14, 1999, four groundwater monitoring wells (MW-1 through MW-4) were advanced to 51 feet below ground surface (bgs) Benzene, TPHg, and MtBE in soil were reported at maximum concentrations of 2 89 mg/kg (MW-3-15'), 429 mg/kg (MW-3-15'), and 0 119 mg/kg (MW-4-20'), respectively(IT Corporation, 1999) (Table 2) On October 30, 2001, three groundwater monitoring wells (MW-5, MW-6, and MW-7) were advanced to 20 feet bgs TPHg, BTEX, and MtBE in soil were not detected above laboratory reporting limits, except for 0 006 mg/kg total xylenes (MW-7020') Selected halocarbons also reported in the soil samples indicated possible dry cleaner plume contributions (IT Corporation, 1999) An active USA Gasoline (former Exxon) service station is located east of the site During quarterly monitoring and sampling at the site, groundwater is typically encountered at approximately seven feet bgs During the most recent monitoring and sampling event (second quarter 2003), depth to water ranged from approximately 6 6 to 8 4 feet below top of casing Historical groundwater flow direction has varied in all directions, with gradients between 0 0001 and 0 001 feet per foot SOIL BORING, SAMPLING, AND WELL INSTALLATION Permiffing/Site-Specific Health and Safety Plan/Utility Clearance Soil boring permits for MW-8 and SB-1 were obtained from the San Joaquin County Environmental Health Department (SJCEHD) prior to installation of the wells (Attachment A) SECOR prepared a site-specific Health and Safety Plan for well Installation and sampling activities at the site as required by the Occupational Health and Safety Administration (OSHA) Standard "Hazardous Waste Operations and Emergency Response" guidelines (29 CFR 1910 120) The document was reviewed and signed by all SECOR personnel and subcontractors performing work at the site Prior to conducting subsurface work at the site, USA was contacted to delineate subsurface piping near the site with surface markings In addition, a private utility locator service was contracted to clear each of the proposed drilling locations Soil Borings On August 14, 2003, SECOR supervised Cascade Drilling of Rancho Cordova, California, to drill and install one groundwater monitoring well and one soil boring at the locations shown on Figure 2 The borings were advanced to an approximate depth of 50 feet bgs using a truck-mounted drilling rig equipped with 8-inch diameter hollow-stem augers (Table 1) 14117 Vertical Assessment and Fate and Transport Model doc SECOR Intemational Incorporated