ARCHIVED REPORTS_XR0008606 CASE 1

Home Browse Search

measured on December 10, 1991. The general direction of ground water gradient is to the north-northwest. There are a number of existing water wells in use within the City of Tracy. Three wells, including one domestic well, are located within a 1/2-mile radius of the site (see Appendix A), (Kapreahan 1988) 3.0 SUMMARY OF PREVIOUS ACTIVITIES Information pertaining to past activities, as summarized in this section, is based on reports prepared by Kaprealian Engineering, Inc. (KEI), and Alton Geosmence • August 1986: Four underground storage tanks (two 10,000-gallon; one 5,000-gallon, and one 250-gallon waste oil tank) were removed from the site on August 19, 1991 Ground water was encountered in the tank cavity at approximately 7 fbg KEI was retained by Mobil Oil Corporation to conduct a an assessment of soil and ground water conditions. One soil sample was collected at 6 8 fbg from the side wall of the waste oil tank cavity; an additional soil sample was collected beneath product delivery lines at 4 fbg; and a composite of three ground water samples were collected from the tank cavity and analyzed for total petroleum hydrocarbons (TPH), benzene, toluene, ethylbenzene, and total xylenes (BTEX), and halogenated volatile organic compounds (HVOCs) Analysis of the soil sample collected from the waste oil tank detected TPH at 61 parts per million (ppm) Analysis for TPH of the composite water sample detected 30,000 parts per billion (ppb). HVOCs were not detected in any of the samples (Kapreahan 1986a). Subsequently, KEI installed four 2-mch- diameter ground water monitoring wells (MW-1 through MW4) to total depths of 20 fbg (Kaprealian 1986b) • September 2, 1986: During initial ground water sampling, no free product was observed in any of the monitoring wells, however, high levels of TPH were detected in the samples collected from MW-3 at 320,000 ppb and MW-4 at 28,000 ppb (Kaprealian 1986b) • February 1987: Based on results of the preliminary investigative work, KEI installed three additional 2-inch-diameter monitoring wells (MW-6 through MW-8) and one 4-inch-diameter monitoring well (MW-5). Monitoring Well MW-5 was installed onsite, while the other three wells were installed offsite, as shown in Figure 2. The highest TPH concentration detected in soil samples collected during installation of these four wells was 24 ppm (MW-8 at 3 fbg) No free product or sheen was noted in any ground water samples collected from the new monitoring wells, however, analysis of the samples showed detectable TPH concentrations ranging from 6,300 ppb in MW-7 to 41,000 ppb in MW-5, while detectable concentrations of TPH . were not present in MW-1 (Kapreahan 1987). 2