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' 2.0 SITE HISTORY 21 Background ' The site is located on the northeast corner of Harding Way and EI Dorado Street in Stockton, California (Figures 1 and 2) The site is a Former 76 Unocal service station that dispensed gasoline and diesel fuel beginning in 1982 Tanks were removed in 1999 and no fuel has been ' dispensed after that point ' 22 Previous Investigations On April 7, 1999, three 10,000-gallon capacity underground storage tanks (USTs), which contained gasoline and diesel fuel, were removed from the site Excavation dimensions were approximately 35 feet by 40 feet by 15 feet deep Initial remedial activities included source removal of obviously impacted sod to a depth of approximately 18 feet below ground surface (bgs) beneath the center of the western fuel tank Additional remedial activities included source removal of obviously impacted soil in the area beneath the former dispenser and product lines A concrete pad restricted the lateral extent of excavation During excavation of impacted sod, unidentified fiberglass piping was found Additional excavation and sampling was completed on April 8, 1999 to remove the unidentified piping Samples were collected from the bottom and sidewalls of the excavated area Maximum concentrations of total petroleum hydrocarbons as gasoline (TPHg) and total petroleum hydrocarbons as diesel (TPHd) were reported at 3,000 miligrams per kilogram (mg/kg) (PBB-2@13') and 1,400 mg/kg (PBB-1@13'), respectively Fallowing removal of the former station canopy, canopy footings and kiosk, additional over- excavation was completed in the dispenser area Over-excavation was terminated when ' obviously impacted soil was removed, the structural integrity of the sidewalk was endangered, or groundwater was encountered A total of approximately 1,707 tons of soil was transported off-site for disposal at the Forward Landfill in Manteca, California Historical maps of soil sample locations and areas of over-excavation are included in Appendix 2 and historical sod ' analytical data is included in Table 1 On December 9 and 10, 1999, four groundwater monitoring wells (MW-1 through MW-4) were ' installed to investigate the extent of petroleum hydrocarbons in soil and groundwater beneath the site Well MWA was completed at 50 feet bgs, and wells MW-2 through MW-4 were completed at 45 feet bgs Petroleum hydrocarbons were not detected in any soil samples from beneath the site Groundwater sampling from the new wells was initiated on January 5, 2000 ' Concentrations of total petroleum hydrocarbons as gasoline (TPHg), benzene, toluene, ethylbenzene, and xylenes (BTEX), and methyl tertiary butyl ether (MtBE) were generally not detected in the wells, with the exception of minor concentrations of benzene and/or MtBE (EPA ' Method 8260) detected in wells MW-3 and MW-4 Initial depth to water was approximately 28 to 29 feet bgs The direction of groundwater flow was established in the easterly direction, at an approximate gradient of 0 003 foot/foot Historical soil analytical data is included in Table 1, soil ' boring and well completion details are included in Table 2 On July 20 and 21, 2000, four Geoprobe borings (GP-1 through GP-4) were completed by IT to a depth of 60 feet bgs to investigate the vertical extent of petroleum hydrocarbons in sod and ' groundwater Petroleum hydrocarbon-impacted soil and groundwater were encountered at depths of approximately 40 to 60 feet bgs Maximum concentrations of TPHg were reported at 4,570 mg/kg (GP-2 at 40') and 3,650 mg/kg (GP-3 at 60') Maximum concentrations of benzene Updated SCM doe 2 August 13,2004 ' 77CP 64409 05 0005