SITE INFORMATION AND CORRESPONDENCE

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01-04-94 12:28PY FROM UTSUREG ON f TO 912094640138 P003/007 Mr. Derrick Adal • December 17 , 1993 Page 3 collected to determine shallow (e10 foot) , intermediate (10-25 feet) and deep (30-55 feet) chlorinated hydrocarbon distributions in all locations. The absence of chlorinated hydrocarbons in ground water in the area north of the tank needs to be confirmed by ground water sampling and analysis. At a minimum, GSU recommends installing three boreholes with depth-discrete ground water sampling for both solvents and total petroleum hydrocarbon (TPH) analysis. A monitoring well may be a requirement of the Regional Water. Quality Control Board (RWQCB) for tank closures; therefore, the RWQCB should be consulted. Local ground water elevation information would be useful. General Comment 2 : Hydrogeologic Setting The Report references that ground water elevations for the LCN site are 5 to 15 feet below sea level (55 to 65 BCS) . This information conflicts with Exhibit 6 which shows the elevation of ground water elevation for the site to be at sea level. Also, the resolution of contours presented in Exhibit 6 is not clear. This indicates that at a site scale, the hydrogeologic information lacks sufficient detail to be meaningful. Recommendation: Exhibit 6 should clearly show ground water elevation values at posted well locations for reference. Additionally, conflicting depth to ground water data should be clarified or corrected. General Comment 3: Attenuation of Chlorinated Hydrocarbons WZI interprets that vertical variations in solvent concentrations are attributable to attenuation. Therefore, there is only a slight potential for ground water contamination. GSU does not concur with this interpretation. With the exception or Sample T-1 (15 feet) , the chlorinated nydrocarbons appear to be in the same concentration range for boreholes T-4, T-6, T-1, and AB-3. Additionally, a shallow sample at T-1 exhibits a lower value than a deeper sample. The limited data do not appear to support the attenuation hypothesis. Boring samples greater than 25 feet BGS at the northwest edge of the tank and greater than 40 feet BGS for the rest of the tank area have not been collected. Therefore, the extent of contamination is not clearly known (see Comment- 1) . contaminants appear to have migrated at least 40 feet. GSU interprets that the data represent a potential for ground water contamination.