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Pombo Property Subsurface Investigation Report Page: 7 point, and at the end of the sampling day. De-ionized or organic-free water (also • provided by the laboratory) was then introduced into the bailer as a final stage of ' cleaning. This water was then poured into sample vials to be used as a bailer blank. Immediately prior to sampling, water levels and well depth measurements were ' taken in all wells at the site. Each well was evacuated using a stainless steel bladder pump, which does not introduce air into the water column. The pH, temperature, and electrical conductivity of the water were monitored as the well was evacuated. A field log of well monitoring information was maintained (see Appendix C - Well Monitoring Data Sheets). Instruments calibrated to known 1 standards were used to monitor these parameters. At least 5 well volumes were evacuated from each well, and more, if necessary, until the above parameters stabilized. The wells or other sampling points were sampled in order of the least ' to the most contaminated, as best known or estimated. The samples were carefully poured from a Teflon bailer into clean glass vials with Teflon-lined screw caps. Care was taken to ensure that no air space existed in the vials by ?' inverting to check for bubbles and re-sampling if necessary. The groundwater samples were analyzed or extracted within 14 days according to their EPA methods. The samples were analyzed for Total Petroleum Hydrocarbons (gasoline & diesel), BTX&E (Benzene, Toluene, Xylene, and Ethyl Benzene) and MTBE by EPA Method 8020. Analysis was conducted by ' GeoAnalytical Laboratories; a state certified analytical laboratory. ' 3.0 SITE CONDITIONS s 3.1 Site Geology & Groundwater Occurrence The Pombo Ranch is located on the southwestern edge of the San Joaquin Delta. The shallow unconsolidated sediments present at Pombo Ranch are composed fine-grained overbank and coarse grained channel deposits sourced from ' Cretaceous and Tertiary rocks exposed in the Coast Range Mountains to the west. Soil boring logs are presented in Appendix B. r' The sediments encountered in the area of the underground fuel storage tank in this investigation (Geoprobew borings 9, 10, 11, 12 and monitoring wells 1, 2, and 3, ' July 1999) are composed of tan silty plastic clays which grade into a tan very fine grained, well sorted, silty sand at 15 to 20 below grade. A thin shallow sandy unit (upper sand) is present within the upper silty clay in boring B 133/9, B10, B11, MW 1, and MW3. This sandy unit appears to be confin to semi-confined beneath the overlying silty clay in borings B3/9, B 10, and B 11. The shallow sandy unit is not present in borings to the east and south of the former underground storage tank. A coarse sand to pebble gravel was encountered at 5 feet in boring B 12. The well sorted sand present at the base of the borings (lowe .Z . sand) dives to the west with an apparent dip of approximately 25 degrees.