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' Stockton Meat Plant - Case Closure Page 7 March, 1996 the residual hydrocarbons (PNAs, trace volatile organics) are less than those concentrations for a target cancer risk of 1 x 10-5 and a non-carcinogenic ' hazard index of 1 Thus, the risk assessments concludes the hydrocarbons at the site do not pose unacceptable health risks to workers and unacceptable impacts to groundwater quality EXTENT OF HYDROCARBONS IN SOIL AND GROUNDWATER ' Soil Figure 2-2, and Tables 1-1, 1-2, and 2-1 of ERM-West's site assessment (Attachment E) summarizes the extent the hydrocarbons in soil adjacent to the former tank site The data indicates that soil concentrations range from non-detectable to a high of 3,200 mg/kg From ground surface to 20 to 25 feet below ground surface (BGS), soil samples are reported with non-detectable concentrations of hydrocarbons (Table 2-1) A review of the investigation ' report performed in April, 1987 (Attachment B) also supports the premise that hydrocarbons in soil from ground surface to about 20 to 25 feet BGS are not present Of the 9 borings and eight soil samples taken during the work, only one sample was reported with hydrocarbons B-2 at about 21 feet (1,200 mg/kg) All other samples ranging from depths between 13 to 22 feet BGS were reported with less than 10 mg/kg as TPH The majority of reportable hydrocarbon soil samples are detected in the sandy soil strata that predominates the site at about 20 to 25 feet BGS Geological I cross-sections across the site (Figures 3, 4, and 5 - Attachment D, and Figures 4 and 5 - Attachment B), indicate the tank site and the surrounding area of the Safeway site are underlain by a thick, low permeable silty to clayey material This lithologic unit starts at ground surface and terminates 17 to 18 feet BGS Below this unit is a sandy zone of high permeability It is primarily in this zone that the higher soil hydrocarbons are reported by investigators IBased on the above soil sampling results and the identified lithologY, the question becomes if hydrocarbons are reported at depths of 20 to 25 feet in the Isandy zone, why in over 21 boring and HydroPunch samples have hydrocarbons not been reported in the upper silty, clayey strata? One reason is the diesel may be moving through the sandy zone from an off-site source This premise is also supported by the higher hydrocarbon concentrations detected in the Krazen borehole (HydroPunch) sample approximately 200 feet west of the tank site and lust north of Navy Drive (i a upgradient) Groundwater The last quarterly ground water monitoring report dated June 15, 1995, is iincluded in Attachment F Table 1 summarizes pertinent ground water data I