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Harlin Knoll, Senior REHS, LOP/Site Mitigation Unit IV April 9, 2001 PHS/EHD Page 2 excavation. The 10-foot bgs sample from GP-1 indicated 2,900 mg/kg and the 10- foot sample from GP-9 indicated 140 mg/kg diesel range hydrocarbons. Each of these samples was collected from the pea gravel present in the bottom of the former excavation. Although the boring logs indicate strong to moderate odor at each of these sample points,the PID readings for each are zero. No BTEX compounds were detected in any of the soil samples and gasoline range hydrocarbons were only detected in the 10-foot soil sample from GP-9, at a concentration of 3.3 mg/kg. While these samples indicate the presence of residual hydrocarbons in the floor of the former excavation,the concentrations detected show a marked decrease from the concentrations detected during the tank removal. At the time of the tank removal, diesel range hydrocarbons ranged to 3,800 mg/kg, benzene to 600 ug/kg, and xylenes to 1,600 ug/kg. • Significant concentrations of diesel and gasoline range hydrocarbons persist within the groundwater in the former tank excavation and are detectable at low concentrations in groundwater outside the immediate area of the former excavation. Diesel range hydrocarbons in the groundwater range from 5.3 mg/l to 330 mg/1 and gasoline range hydrocarbons in the groundwater range from 0.15 to 0.87 mg/l. .. Concentrations of diesel range hydrocarbons decrease rapidly to levels below 1.0 mg/1 in the groundwater surrounding the former excavation. Gasoline range hydrocarbons are not detected in the groundwater outside the area described above. The limited migration of this material is likely due to the fine-grained nature, and related low permeability, of the soil at the site. BTEX is not detected in groundwater samples collected outside the bounds of the former tank excavation. The benzene concentrations that were detected within the former excavation indicate a marked decrease from those reported at the time of the tank removal. Current benzene concentrations range to 8.6 ug/l, whereas, at the time of the tank removal,benzene was reported at 80 ug/l. • MTBE concentrations also show a significant decrease from the 3,200 ug/l in the groundwater reported at the time of the tank removal to a maximum of 36 ug/l during the recent sampling. The MTBE concentrations that exceed the SWRCB's water quality goal for MTBE of 5 ug/l and CalEPA's Office of Environmental Health Hazard Assessment's Public Health Goal of 13 ug/l are present within the tank excavation and immediate vicinity and also do not appear to be migrating significantly. Based on the information contained in the attached report, and summarized above, it appears that the impacts related to the former underground storage tanks at the site are attenuating naturally. It is also clear that due to the fine-grained, low permeability, nature of the soil in the area, the impacts are relatively immobile. DA. -EHDPHS 040901.dm