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CONS ❑ LTA NTS I NC E NVI RON MENTAL ❑ G E O T E C H N I C A L ❑ M A T E R I A L S _ Project No 58475-06-24 ' June 24, 2002 Mr Saiyed S Ali California Department of Transportation Central Sierra Environmental Services Branch Post Office Box 2048 Stockton, California 95201 Subject STOCKTON MAINTENANCE STATION ' SAN JOAQUIN COUNTY, CALIFORNIA CONTRACT NO 43AO078 TASK ORDER NO 10-987901-MU GROUNDWATER MONITORING REPORT-FIRST QUARTER 2002 ' Dear Mr Ah ' In accordance with Caltrans Task Order No 10-987901-MU, dated February 22, 2002, Geocon Consultants, Inc has provided groundwater monitoring services at the Stockton Maintenance Station located at 1604 South B Street in San Joaquin County, California (the Site) The Site consists of a ' Caltrans maintenance facility utilized for the storage of vehicles and equipment used for the maintenance of California highways The scope of services provided by Geocon included depth to groundwater measurements, the sampling of fourteen monitoring wells, and the submittal of the water samples to a California-certified laboratory for analytical testing The site location is depicted on the Vicinity Map,Figure 1 ' BACKGROUND In December 1989, one 5,000-gallon diesel underground storage tank (UST) and one 10,000-gallon ' gasoline UST, two pump islands and associated product piping were removed from the Site Analytical results of soil samples collected from the UST excavation indicated total petroleum hydrocarbons as gasoline and diesel (TPHg and TPHd) concentrations up to 2,000 and 58 milligrams ' per kilogram (mg/kg), respectively In September 1993, one 115-gallon backup diesel generator UST was removed from the Site Soil discoloration and odors were not noted during the 1993 UST removal activities TPHd and benzene, toluene, ethylbenzene and total xylenes (BTEX) were not detected in the soil samples collected from the UST excavation and stockpile Permits and/or closure ' documentation for the 1993 UST removal were not made available by Caltrans or the San Joaquin County Environmental Health Department (SJCEHD) The former UST locations are presented on Figure 2 ' Twelve groundwater monitoring wells MW-1 through MW-12 and two vapor extraction wells (VW-1 � g ( g ) p and VW-2) were installed at the Site between 1992 and 1996 Quarterly groundwater monitoring has been conducted at the Site since April 1992 for wells MW-1, MW-2 and MW-3, since May 1995 for wells MW-4 through MW-8 and VW-1, and since February 1996 for wells MW-9 through MW-12 and VW-2 Maximum TPHg, TPHd and benzene concentrations of 54,000, 2,900 and 13,700 micrograms per liter (µg/l), respectively, have been reported for the groundwater samples Maximum methyl tert- . butyl ether (MTBE) concentrations of 277 and 99 pg/l have been detected in groundwater samples analyzed by EPA Test Methods 8020 and 8260, respectively Historical groundwater flow beneath the ' Site is directed to the east and southeast A summary of historical analytical groundwater data is tabulated in Appendix A 11375 Sunrise Park Drive, Suite 100 C] Rancho Cordova, California 95742 13 Telephone (916) 852 9118 11 Fax (916) 852 9132