SITE INFORMATION AND CORRESPONDENCE

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0 NOV-07 97 09 : 55 FROM : CONDOR TURLOCK 209-668 -9248 70 : 46834330995 PRGE : 02 P CaneCar w Mr.eni Ron Rowe November G, 1997 Page - z - Analysis of the Monitor Well data indicates that contours of the water table elevations have the shape of a low-relief mound centered in the vicinity of MW- 1 and MW-2. Hydrocarbons were detected in MW- 1 , -21 -61 -7 and -8 as the result of sampling on 3/30/94. Since the end of March 1994 the elevation of the water table has increased by approximately 10 feet to elevations between -27 and -28 feet amsl in July 1997. Only the groundwater sample from MW-2 had a significant increase in the content of dissolved hydrocarbons (30 to 49 mg/1) and the groundwater samples from MW-4 (0. 003 mg/1) and MW-5 (11.0006 mg/1) compared to non-detect in 1994 following the 10 foot rise in the water table. The decrease in dissolved hydrocarbons in groundwater samples from the other monitor wells suggests that little has been mobilized down gradient by the change in water levels and that the plume is fairly stable except for relatively minor migration down gradient to the north. Groundwater quality in the area of the Soil Vapor Extraction (SVE) system is very poor. Benzene levels have been as high as 91 mg/1 (8/ 1/96) and are currently reported at 55 mg/1 ( 10/22/97) . MTBE has increased from 7.8 mg/l (8/1 /96) to 37 mg/1 (10/22197) and may be deteriorating groundwater quality with the increase in groundwater elevation. Due to the high levels of gasoline components and gasoline additives, combined with how little is known currently about MTBE migration and toxicity, it seems prudent to decrease the contaminant levels in groundwater as rapidly as possible. 'Demonstrate that the cleanup system is feasible, cost-effective and able to achieve the desired level of Buil and groundwater cleanup required by San Joaquin County Health Services, and Section 2725, Chapter /6, of the Underground .Sturage Tank Regulations (Article 11). " We believe that the installation of the soil vapor extraction system with catalytic oxidation ( Cat-Ox) of the hydrocarbon vapors removed is an appropriate interim remedial action in accordance with Sea 2722(b)(5 ) Chapter 16, 23 CCR. With implementation of interim remedial measures no further investigations of the distribution of hydrocarbons in soil is warranted until further monitoring data are acquired. Monitoring water quality of samples from the existing monitor wells should provide sufficient data for assessing the progress of the interim action. No further action is recommended unless northward migration of the hydrocarbon plume is indicated by the monitoring data. To meet the requirements of Section 2725, Chapter 16, of the Underground Storage Tank Regulations (Article 11) . We believe that following a period of operation of the Cat-Ox system of approximately one year, a risk-based corrective action study should be performed to examine risk levels posed to human health and establish achievable cleanup goals.