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• • 25 Octobei 2002 AGE-NC Project No 02-0964 Page 2 of 8 The Modesto, Riverbank and Turlock Lake Formations and overlying Recent alluvium are the principal sources of domestic ground water in the 13,500-square mile San Joaquin Valley Ground Watei Basin(Basin 5-22) Based on review of the Lanes of Equal Depth to Groundwater Sprang 1999 map published by the San Joaquin County Flood Control District and Water Conservation District (FCD&WCD), the estimated depth to ground water at the site to be 30 feet bsg Review of the map Lines of Equal Elevation of Groundwater Sprang 1998 (FCD&WCD) depicts the regional gi ound water flow di ection towards the northwest and west Review of previous ground water monitoring at the site and historical depth to gi ound water maps(FCD&WCD)dating back to 1971 suggests that ground water depth near the site has fluctuated between 50 feet bsg and 60 feet bsg 2 2 UNDERGROUND STORAGE TANK REMOVAL An unnamed sen vice station operated at the site from the 1940's until the 1960's Reportedly foul USTs wei e filled with sand and abandoned in place in the late 1960's A Standaid Oil Service Station operated at the site from the 1960s until 1980 Subsequently, the • USTs associated with this fuel system were removed, the location of the formei USTs during this period is uncertain, but is believed to be at the northeast comes of the site Neither UST decomirussioning was verified with soil sample collection 23 PREVIOUS SITE ASSESSMENTS Ten soil borings have been advanced and three ground water monitoring wells have been installed at the site to assess the vertical and lateral extent of impacted soil and ground water The following is a sunnrnaly of milestone events at the site On 22 May 2000, a total of seven on-site soil borings were established by Hydra-Geo Consultants Inc with a Geopi obe sampling ng Boi Ings A, B and C were established on the northeast cornel of the site and borings D through G were established on the west and southern edge of the former service station location,to a depth of 45 feet below surface grade(bsg) Based on laboratory analysis • Total petroleum hydrocarbons as gasoline and diesel (TPH-g and TPH-d) were detected in soil samples collected during boring operations at concentrations as high as 300 milligrams per kilogram(mg/kg) and 2,400 mg/kg, respectively, in the sample from boring A at 30 feet bsg BTEX compounds were detected in the soil samples at maximum concentrations of 0 5 ring/kg(toluene), 1 5 mg/kg(ethylbenzene)and 30 mg/kg(xylenes)in the sample from boring . A at 40 feet bsg • TPH-g and TPH-d wei e detected in grab ground water samples collected during the soil boring operations at concentrations as high as 470,000 micrograms per liter (pg/1) and 230,000 µg/l, respectively, in the sample from boring A Dissolved BTEX compounds were .advanced GeoEnvmonmental,lite