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EXECUTIVE SUMMARY I Ramage Environmental drilled three soil borings (SB-6, SB-7 and SB-8) for soil and groundwater grab sampling using a Geoprobe rig. While drilling S13-6, a concrete pad was encountered on two attempts at a depth of 16 feet bgs before a third step-out location missed the slab and was fff completed to a depth of 40 feet bgs. Soil boring SB-7 was located next to an existing OPW street box. This boring was not completed due to drilling refusal at 8.5 feet bgs and no samples were collected for analysis. Soil boring SB-8 also met with drilling refusal into concrete at a depth of 8.5 feet bgs and was terminated at that depth. Results of the site 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, t 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. Ramage Environmental recommends that the site be considered for regulatory closure as a "low-risk soils-only" case. A case for site closure is supported by the absence of diesel or gasoline hydrocarbons in the capillary fringe soil, the absence of diesel and significant gasoline hydrocarbons in groundwater and the absence of benzene, MTBE or other oxygenate compounds in soil or groundwater. In addition, environmental corrective actions would be logistically 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 that includes buried utilities, traffic and pedestrians. Ramage Environmental recognizes that the lateral extent of impacted vadose zone soil is not L defined to the south and that diesel concentrations of 10,000 ppm are present within 6 feet of the current groundwater surface. As such, the recommendation to consider the site for closure is based on the following assumptions: • The lateral extent of residual diesel hydrocarbons in a southerly direction from the former LUST does not exceed 20 feet beyond SB-6. This allows for an assumed finite amount of diesel plume mass; Concentrations of petroleum hydrocarbons do not increase in the undefined part of the plume; • Groundwater elevations will not increase in the future such that contact between residual diesel hydrocarbons and non-impacted groundwater does not occur; Any future leaching of residual diesel hydrocarbons to groundwater via rainfall infiltration is not significant. 3