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California Department of Transportation April 24, 2000 Stockton Maintenance Station, Stockton, California Project No 400296-01 2. INTRODUCTION AND SCOPE OF SERVICES This report summarizes the results of the First Quarter 2000 groundwater monitoring activities conducted on March 27 through March 29, 2000 at the Stockton Maintenance Station, located in Stockton, California (Figure 1) The work conducted by Ninyo & Moore was in accordance with Task Order No 10-987901-NP and Caltrans Contract No 43AO012 Our scope of services in- clude 1) attendance of a prework site visit, 2) preparation of a site safety plan, 3) measurement of the depth to static groundwater at selected wells, 4) purging of a minimum of three well vol- umes to labeled reconditioned 55-gallon drums, 5) collection and chemical analysis of groundwater samples, and 6) preparation of this report documenting our field activities and findings 3. STI'E BACKGROUND According to a report entitled, Final Report, Fourth Quarter 1998 Groundwater Monitoring Re lie port by Professional Service Industries (PSI), dated February 19, 1999, one 5,000-gallon diesel underground storage tank (UST) and one 10,000-gallon unleaded gasoline UST, two pump is- lands and associated product piping were removed from the site in 1989 Subsequent to the removal, soil samples were collected to determine the extent of soil contamination Analytical ' results of soil samples collected indicated maximum concentrations of total petroleum hydrocar- bons as gasoline (TPH-G) to be 2,000 milligrams per kilogram (mg/kg) and total petroleum hydrocarbons as diesel (TPH-D) to be 58 mg/kg ' Twelve groundwater monitoring wells and two vapor extraction wells have been installed at the site The approximate locations of the monitoring and vapor extraction wells are shown in Figure ' 2 Quarterly groundwater monitoring has been conducted at the site on wells MW-1, MW-2, and MW-3 since April 1992 Wells MW-4 through MW-8 and VW-1 have been monitored since May ' 1995 Wells MW-9 through MW-12 and VW-2 have been monitored since February 1996 His- torically, high concentrations of TPH-G, TPH-D, and benzene, toluene, ethylbenzene, and total Xylenes (BTEX) have been detected in the on-site monitoring wells • stocktonDRdoc