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two 4- `! 4.0 CONCLUSIONS AND RECOMMENDATIONS This environmental site investigation included the drilling of three soil borings for soil and • groundwater grab sampling using a Geoprobe rig. Initial attempts to advance SB-6 failed due to drilling refusal into a buried concrete pad at a depth of 15 feet bgs. Soil borings SB-7 and SB-8 met with drilling refusal into concrete at a depth of 8.5 feet bgs and were terminated. Field and chemical data collected during this investigation indicate that diesel hydrocarbons have impacted vadose zone soil beneath the southern end of the former UST. Gasoline hydrocarbons were also detected at low concentrations in the vadose zone soil samples. Impacted soil extends from at least 20 feet to 30 feet bgs. The soil sample collected at 30 feet bgs contained the highest concentrations of TPH-as-diesel (10,000 ppm) and TPH-as-gasoline (32 ppm). Trace concentrations of toluene, ethylbenzene and xylenes were detected in the vadose zone soil samples. Benzene, MTBE and other oxygenate compounds were not detected. The soil sample collected from the capillary fringe at 35 feet bgs did not contain detectable concentrations of diesel or gasoline hydrocarbons. The groundwater grab sample collected at 36 feet bgs from SB-6 did not contain detectable concentrations of TPH-as-diesel but did contain 27 ppb of TPH-as-gasoline. BTEX, MTBE and volatile organic and oxygenate compounds were not detected in the soil sample collected from the capillary fringe at 35 feet bgs or in the groundwater grab sample. Based on the results of the soil and groundwater grab sampling and given the logistical constraints inherent at the site, Ramage Environmental recommends that the site be considered for regulatory closure as a "low-risk soils-only" case. Case closure is supported by the following: • No diesel or gasoline hydrocarbons in the capillary fringe soil; • No diesel and insignificant gasoline hydrocarbons in groundwater; • No benzene, MTBE or other oxygenate compound detected in soil or groundwater. Logistically, additional environmental corrective actions would be difficult and expensive to implement. Underlying the site is a buried concrete pad and a former building foundation of unknown dimension. These structures would be costly to penetrate and would impede remediation efforts. The site is located in a densely constructed area of downtown Stockton. Additional logistic constraints associated with this location include buried utility complexes, traffic and pedestrians. 11