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California Department of Transportation August 17, 1999 ' Stockton Maintenance Station, Stockton, California Project No 103872-02 2 INTRODUCTION AND SCOPE OF SERVICES This report summarizes the results of the Second Quarter 1999 groundwater monitoring activities conducted on June 24, June 25 and June 28, I999 at the Stockton Maintenance Station located in ' Stockton, Cahfointa (Figure 1) The work conducted by Ninyo & Moote was in accordance with Task Order No 10-911175-NB and Caltians Contract No 43A0012 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 analysts of groundwater samples, and 6) preparation of this report documenting our field activities and ' findings 1 3 SITE BACKGROUND According to a report entitled, Draft Report Fourth Quarter 1998 Groundwater Monitoring Re- 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 Swam&c 2