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I 1 Ell��t� ' ENSR International 10411 Old Placerville Road May 4, 2005 Suite 210 ' Sacramento, CA 95827-2508 (916)362-7100 FAX (916)362-8100 Ms. Margaret Lagorio www.ensr.com ' San Joaquin County Public Health Services L �WED 304 East Weber Avenue, 3rd Floor ' Stockton, CA 95201-0388 MAY 2 02.005 RE: Quarterly Groundwater Monitoring and Ozone MicrospargINMRONMENT HEALTH System Status Report, First Quarter 2005, PERMIT/SEIRVICES Former Unocal Station No. 0187 437 East Miner Avenue, Stockton, California 1 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 microsparge system 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 microsparge system status through the first quarter 2005. First 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 LOincluded 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 a' U-26, UV-1 through UV-3, and triple completion piezometer wells NP-1 and NP-2 on February 14, 2005 (Table 1). Three groundwater-bearing zones (zone A, zone B, and zone C) exist below the subject site and surrounding area. Zone A extends from approximately 15 to 60 feet below ground surface (bgs), zone B extends from approximately 60 to 80 feet bgs, and zone C extends from approximately 108 to 120 feet bgs. i Groundwater measurements from wells and piezometers screened in water bearing zone A ' (U-1 through U-9, U-11, UV-1 through UV-3, 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. During the first quarter 2005 monitoring event, the groundwater flow direction for water bearing zone A was generally southeast with a hydraulic gradient ranging from approximately ' 0.006 feet per foot (ft/ft) to 0.007 ft/ft. 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, and NP-2-60) and zone B wells at the former Vintage Car Wash site were used to J:1Pmjectsl0694010187-3251WorkinglReports101871 st Qtr 05-QMR RSR.doc ( 7 Celebrating 35 Years of Excellence in Environmental Services