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1992 Third Quarter Report,Site Code 1178 City of Stockton,Police Facility October 14, 1992 Page 5 feet below grade. Groundwater samples were collected and analyzed for BTEX and TPHG by State-certified RESNA Environmental Laboratories. Prior to sample collection, the well was purged of approximately 41 gallons of water to collect a representative groundwater sample. The purged water was contained in a 55 gallon drum (DOT H), labelled, and stored on site. While purging, groundwater parameters (pH, temperature, and conductivity)were monitored and allowed to stabilize. All water samples were collected using a disposable bailer with a bottom- emptying device. The field purge log is included in Attachment 3. Laboratory results indicate dissolved hydrocarbon constituents are not present at or above detection limits in the monitoring well. Analytical results are summarized in Table 3. Laboratory data sheets and chain-of-custody documentation are included in Attachment 4. SUMMARYICONCLUSIONS Contaminant source control has been achieved at the site with the repair of the vapor recovery and product lines. All hydrocarbon impacted soils practically reachable have been removed and are currently stockpiled at the City Municipal Service Center. The result of field observations during the excavation and subsequent laboratory analysis of the 11 confirmation samples indicate limited hydrocarbon contamination has occurred as a result of leakage of unleaded gasoline to the subsurface. Due to excavation limitations (i.e., concrete encased electrical lines, police station building)impacted soils remain under the concrete encased electrical lines and under the police station structure. It is estimated that between 17 and 50 yd' of impacted soil remain in-place (Figure 3). It is estimated that between 13 and 40 gallons of gasoline remain in the soil. The remaining soil cannot be excavated without endangering the structural integrity of the existing building. Vertical migration of contaminants appears to be limited to the upper eight feet of soil due to the presence of a very stiff clay layer beginning at approximately 6.5 feet below grade. Groundwater samples collected from the site monitoring well (MW1) over a three year period indicate that groundwater has not been impacted by the line leak. Groundwater currently exists at a depth of approximately 46.5 feet below grade, and is separated from impacted soil by approximately 38 feet of very stiff silts and clays. The low permeability soils should limit the rate of vertical migration of the residual soil contamination. In addition, the surface above and surrounding the area is graded to prevent ponding and covered by impermeable surfaces (asphalt, concrete and the police station structure) QRTLYRPT%QR090292.FNL