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ENSR International [4L�CE�V C lJ 10411 Old Placerville Road Suite 210 February' 17, 2004 Sacramento,CA 95827-2508 FEB 1 8 2004 (916)362-7100 FAX (916)362-8100 ENVIRlsiVNlt: 1 HEALTH www.ensr.com ' PERMIT/SERVICES Ms. Margaret Lagorio San Joaquin Public Health Services ' 304 East Weber Avenue, 3`o Floor Stockton, CA 95201-0388 ' RE: Quarterly Groundwater Monitoring and Ozone Pilot Test Status Report, Fourth Quarter 2003, Former Unocal Facility #E 0187 437 East Miner Avenue, Stockton, CA Dear Ms. Lagorio: ' ENSR Corporation (ENSR) has been authorized by Union Oil Company of California (Unocal) to prepare this report summarizing quarterly groundwater monitoring and ozone pilot test status at the site located at 437 East Miner Avenue, Stockton, California (Figure 1). This report presents ' the results of quarterly groundwater monitoring and ozone pilot test status through the Fourth Quarter 2003. Fourth Quarter monitoring was coordinated with Shaw Environmental, consultants for the former Vintage Car Wash located at 601 East Miner Avenue. The work was performed in accordance with the field methods and procedures included in Attachment A. The locations of former and current site features are illustrated on Figure 2. ' Groundwater Level Measurements Depth to groundwater measurements were recorded in groundwater monitoring wells U-1 through U-19 and U-22 through U-26 and piezometers NP-1 and NP-2 (Table 1). Depth to water ' measurements in wells U-20 and U-21 were not collected due to cars parked over the wells. Three groundwater-bearing zones (A, B, and C) exist below the subject site and surrounding area. Zone A is screened from 15 to 60 feet, Zone B is screened from 60 to 80 feet, and Zone C is ' screened from 108 to 120 feet. Groundwater measurements from wells and piezometers screened in water bearing zone A (U-1 through U-9, U-11, NP-1-22 and NP-2-22) beneath the Unocal site and from wells located at the former Vintage Car Wash, approximately 450 feet ' northeast of the Unocal site, were used to construct the zone A groundwater elevation contour map included as Figure 3. ENSR's field staff failed to collect depth to water measurements within a 24 hour period as required in EPA guidelines. Therefore, groundwater contour maps ' constructed rasing Fourth Quarter 2003 data may show anomalies in flow direction. In the future, field personnel will adhere to the field methods and procedures included in Attachment A. During the Fourth Quarter 2003 monitoring event, the groundwater flow direction for water bearing ' zone A was generally north-northwest with an average hydraulic gradient of approximately 0.019. Groundwater flow for water bearing zone A northeast of the Unocal site was generally northeast with an average hydraulic gradient of approximately 0.005 Groundwater measurements from wells screened in zone B (U-13, U-14, U-16, U-18, U-20, U-22, U-24, NP-1-65, NP-2-60) were used to construct the groundwater elevation contour map included J:\Projects\069401325-01871Final\Reports\4th Qtr 2003-QMR RSR.doc ' 35 Years of Excellence in Environmental Services