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' 29 January 2001 AGE-NC Project No 00-0725 ' Page 2 of I 1 ' both marine and continental sources The thickness of the sedimentary fill ranges from thin veneers along the valley edges to more than 20,000 feet in the south central portion of the valley The sedimentary fofmations range in age from Jurassic to Recent with the older deposits being primarily ' marine in origin and the younger deposits being primarily continental Continental-derived sediments were primarily deposited in lacustrine, fluvial, and alluvial environments with sources being the mountain ranges surrounding the valley (Olmsted and Davis, 1961) The site is located on ' unconsolidated and semi-consolidated alluvium, lake, playa and terrace deposits of Quaternary age (California Division of Mines and Geology, 1977) ' The estimated depth to ground water at the property is approximately 10 feet below surface grade (bsg),based on the map titled Lines of Equal Depth to Groundwater Spring 1999 published by the San Joaquin County Flood Control District and Water Conservation District (FCD&WCD) Based ' upon a previous investigation performed by AGE, ground water depth at the site ranges between 10 and 12 feet bsg It is inferred from the map titled Lanes of Equal Elevation of Groundwater Sprang 1999, also published by the FCD&WCD, that ground water flows toward the northwest in the general area of the site, but this may be modified by changing recharge and discharge patterns Ground water is considered to be of beneficial use and is used for domestic, industrial and liecommercial purposes ' 2 3 UNDERGROUND STORAGE TANK REMOVAL A gasoline fueling system reportedly operated on the site for approximately thirty years from 1940 ' until 1970 The UST system is assumed to have been removed after 1970, however, no records of the UST removal are available with the Tracy Fire Department, Tracy Building Department or the PHS-EHD 24 PREVIOUS SUBSURFACE INVESTIGATION On 04 May 2000, five soil probe borings were advanced at the site under observation by a representative of the PHS-EHD The probe borings were advanced in the area of the former UST ' excavation, the southern and central dispenser islands and two locations which are in the assumed down-gradient direction All borings were advanced to depths of 20 feet bsg A `grab' ground water was collected and analyzed from each soil boring ' Total petroleum hydrocarbons quantified as gasoline (TPH-g) was detected in soil samples in the former dispensers area and at lower concentrations in the former UST excavation The vertical extent ' of hydrocarbon-impacted soil was approximately 15 feet bsg and above 20 feet bsg Table 1 • summarizes the analytical data of the soil sample analyzed ' Advanced GeoEnvironmental,Inc I