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PROBLEM ASSESSMENT REPORT 6131 Pacific Ave , Stockton, CA Aegis Project No. 90-020 Paye 5 3.0 RESULTS OF SUBSURFACE INVESTIGATION The following subsections present the results of the site assessment Phase of the investigation. Data collected during the pre-tank replacement site assessment and tank closure will be included in this section where pertinent. Data collected during the pre-tank assessment investigation will be discussed In detail in Section 4 . 0. A total of five soil borings (MW-1 through MW-3 , VE-1 and VE-2) were advanced at the locations shown on Figure 3. Soil borings MW-1 through MW-3 were advanced to 65. 0 feet below grade. Soil borings VE-1 and VE- 2 were advanced to 45.0 and 20. 0 feet below grade respectively. Logs of the soil borings are included in Appendix B. Soil borings MW-1 through MW-3 were completed as 4" diameter ground water monitoring wells. Soil boring VE-1 and VE-2 were completed as vapor extraction wells. Monitoring and vapor extraction well construction detail diagrams are included in Appendix C. A total of 15 soil samples from the borings were submitted to a state-certified-laboratory for analyses. Copies of soil sample analytical laboratory reports are included in Appendix D. Ground water samples were collected from the monitoring wells. Copies of ground water sample analytical laboratory reports are included in Appendix E. 3.1 Soil Conditions 1 Soil borings VE-1 and VE-2 were drilled through the original tank basin to depths of 20 and 45 feet below grade, respectively. The purpose of the borings was to ascertain the depth to which petroleum hydrocarbons in soil had migrated, to quantify the levels of petroleum hydrocarbons in the native soils beneath the original tank basin and to install test vapor extraction wells. Soil borings MW-1 through MW-3 were drilled to determine the extent of vertical and horizontal migration of petroleum hydrocarbons in soil and to determine the presence of petroleum hydrocarbons in ground 1 water. As shown on the boring logs, silts varying from clayey to sandy composition were the predominant soil type encountered (Unified Soil Classification System symbol ML) . Generally, silty clays occurred at shallow depths, and were underlain Dy Ynterlayered horizons of sandy silts. The interlayered sandy silts were underlain by clayey silts. ' Soil types ranged from fine grained silty clays to medium grained silty sand, to sand. A saturated horizon was encountered at approximately 53 feet below grade in the monitoring well soil borings.