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E11�yt® _ ENSR International T 10411 Old Placerville Road M r F I W E Suite 210 Sacramento,CA 95827-2508 December 22, 2004 DEC 2 8 2004 (916)362-7100 FAX (916)362-8100 ENVIRUNVItIV t HEALTH wwwensrcom PERMI I'/SERVICES Ms Margaret Lagario San Joaquin County Public Health Services Environmental Health Department 304 East Weber Avenue, 3rd Floor Stockton, CA 95202 RE Quarterly Groundwater Monitoring and Ozone Microsparge System Status Report, I Fourth Quarter 2004 Former Unocal Station No 2859 1665 Pacific Avenue, Stockton, California IDear Ms Lagono ENSR Corporation (ENSR) has been authorized by Union Oil Company of California (Unocal) to prepare this report summarizing quarterly groundwater monitoring and the status of the ozone microsparging system through the fourth quarter 2004, for the former Unocal site located at 1665 Pacific Avenue, Stockton, California (Figure 1) 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 monitoring wells MW-1 through MW-21, and In piezometer well NP-1 on October 4, 2004 (Table 1) Groundwater measurements from wells screened across water-bearing zone A (MW-1 through MW-11, MWA3, MWA5, MW-17, and MW-21)were used to construct the groundwater elevation contour map Included as Figure 3 The groundwater flow direction for water-bearing zone A was generally east-northeast with an average hydraulic gradient of approximately 0 003 feet per foot (ft/ft) with a second component of flow towards the southeast with an average hydraulic gradient of approximately 0 018 ft/ft ' Groundwater measurements recorded from wells screened across water-bearing zone B (MW-12, MW-14, MW-16, MW-18 through MW-20 and NP-1) were used to construct the groundwater elevation contour map included as Figure 4 The groundwater flow direction for water bearing zone B was divergent towards the north-northeast with an average hydraulic gradient of I approximately 0 005 ftlft and northwest with an average hydraulic gradient of approximately 0 009 ft/ft Copies of the groundwater sampling information sheets are included in Attachment B A summary of groundwater elevations measured to date is presented In Table 1 IGroundwater Sampling and Analytical Results Groundwater samples were collected October 5, 2004 and October 6, 2004, from wells MW-1 through MW-21 and NP-1 The groundwater samples were submitted to California Laboratory Services (CLS) in Rancho Cordova, California for analyses of benzene, toluene, ethylbenzene IkDcsacramentolprojectslProjectsk0694012859 357WinallReports12859 4th Qtr 04-QMR RSR doc Icelebrating 35 Years of Excellence in Environmental Services