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1 ✓ 25 groundwater levels at the center of the depression rose approximately 30 feet during the interval and that the large depression centered on Stockton no longer exists This rising groundwater condition brackets the tune period in which the service station at the Site was ' remodeled and underground storage tanks were removed ' Figures 5-1 and 5-2 indicate that the maximum concentrations in both soul and groundwater samples collected while drilling and sampling NP-1 at the Site occurred at a depth ' of 60 feet bgs Soil samples taken at three depths (spaced on 10-foot intervals) both above and below the depth interval of unpact were nondetect for both TPHg and benzene The historic record of groundwater levels in the Site vicinity, the removal of the USTs ' during the 1967 station remodel, the 1988 station demolition and fuel dispensing equipment removal, and the vertical distnbution of constituent concentrations as indicated by the NP-1 ' data, indicate that the Site release occurred during a previous time when groundwater levels were lower and that groundwater has subsequently risen into the zone of soil impact The NP- 1 data also indicate that impacts from the removed surface source and residual concentrations in vadose zone soils are not the result of ongoing migration downward and impact to saturated ' zone soils The Site release has resulted in a dissolved petroleum hydrocarbon groundwater plume of limited extent Free-phase petroleum hydrocarbons and MTBE have not been detected at ' the Site ' 6.3 LATERAL EXTENT OF SITE PETROLEUM HYDROCARBON IMPACTS 6.3.1 Soillmpacts 1 - ' Previous investigations conducted at the Site indicate that hydrocarbon concentrations m the soil appear to be limited to the area around the former product islands No offsite soil ' impacts have been observed or documented The SJCPHS concluded in a letter received by UNOCAL on January 9, 1997 that the assessment of soil contamination at the Site was