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California Department of Transportation April 24, 2000 Shop 10 Maintenance Station, Stockton, California Project No 400296-01 2. INTRODUCTION AND SCONE OF SERVICES This report summarizes the results of the First Quarter 2000 groundwater monitoring activities conducted March 27 and 28, 2000 at the Shop 10 Maintenance Station, located in Stockton, Cali- fornia (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 include 1) atten- dance 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 volumes 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. SITE BACKGROUND According to a report entitled, Revised Report, Third Quarter 1998 Groundwater Monitoring Re- fart by Professional Service Industries (PSI), dated March 19, 1999, four underground storage tanks (USTs) were removed from the site in August 1986 Subsequent to the removal, fourteen soil borings were drilled on-site and four were drilled off-site between 1987 and 1991 to deter- mine the extent of soil contamination Analytical results of soil samples collected indicated maximum concentrations of total petroleum hydrocarbons as gasoline (TPH-G) to be 3,800 mil- ligrams per kilogram (mg/kg) and total petroleum hydrocarbons as diesel (TPH-D) to be 4,500 mg/kg Between 1988 and 1991, five on-site and three off-site groundwater monitoring wells (MW-1 through MW-8) were installed These wells have been subsequently abandoned, and replaced by additional groundwater monitoring and/or vapor extraction wells (MW-9 through MW-26, VW- 1, VM-1, and VM-2) in 1990 In August 1998, wells VW-1 and MW-26 were abandoned The approximate locations of the remaining monitoring wells are shown in Figure 2 Quarterly groundwater monitoring has been conducted at the site in the wells since 1990 Historically, con- centrations of TPH-G, TPH-D, and benzene, toluene, ethylbenzene, and total xylenes (BTEX) have been detected in the on-site monitoring wells shop 10 DR doc 2