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. Soil Remediation Feasibility Study Work Plan - Unocal Station No 0187 Apnl 30, 1996 SITE CHARACTERISTICS The geology and hydrogeology is based upon previous investigations in which soil borings were drilled and wells were installed under the direction of GSL aMbsurface -Geology and H,ydrogeologv Soil encountered beneath the site consists predominantly of clay from 0 to approximately 40 feet bgs Interbedded sand (approximately four to 12 feet thick) were encountered in most borings. The clay is underlain by a sand unit from approximately 40 feet bgs to the total depth explored of 55 feet bgs. This sandy unit was not encountered in boring U-6 Approximately one-foot of clay was encountered beneath the sandy unit at the base of borings U-4, and approximately three feet of silt was encountered beneath the sandy unit in the base of boring U-7 Groundwater has been measured at depths ranging from approximately 38 5 to 48 feet bsg. First encountered groundwater appears to be unconfined and exists within a sand zone. The groundwater gradient has vaned from 0 01 to 0 02 and the flow direction has been to the north and northeast Chemical Distribution in Soil • The estimated distribution of residual petroleum hydrocarbons in the soil beneath the site are based on analytical data from on-site and off-site borings. Data from the UST removal conducted in 1992 indicates there are no residual petroleum hydrocarbons in the vicinity of the former gasoline or waste oil USTs south of the former station building, the former hydraulic lifts, or the former dispenser islands Vadose zone soil beneath and adjacent to the old UST pit in the southeast corner of the site has been impacted by petroleum hydrocarbons up to 20,000 ppm The estimated lateral extent of gasoline hydrocarbons in soil is shown on Figure 3. The vertical extent of gasoline hydrocarbons in soil beneath and adjacent to the old UST pit extends to approximately 38 to 48 feet bsg, depending on the depth to the water table. TPHg and benzene concentrations for soil samples collected from soil borings during previous investigations are shown on Figure 4 Tables of historical soil chemical analytical data, including data from the 1992 UST removal and a map showing the locations of these samples, are included in Appendix A The approximate volume of soil containing gasoline hydrocarbons is approximately 8,900 cubic yards The estimated mass of TPHg and benzene in soil is 56,819 and 236 pounds, respectively. tna0E87 SRW 4 4827 05.k