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California Department of Transportation June 7 1999 P , Shop 10 Maintenance Station, Stockton, California Project No 103872-02 • ' 4.3 ]Laboratory Analysis The groundwater samples were submitted for analysis to AAL, a State of California-certified ' hazardous waste analytical laboratory The samples were analyzed for TPH-g and TPH-d, using method EPA 8015 modified, for BTEX, methyl tertiary-butyl ether (MTBE), tertiary- amyl-methyl ether (TAME), isopropyl ether (DIPE), tertiary butyl (TBA), ethyl-tertiary- butyl ether (ETBE), and volatile organic compounds (VOCs) using method EPA 8260b A summary of the past and present laboratory results for the groundwater samples is presented in Table 1 Copies of laboratory results and chain-of-custody documents are included in Ap- pendix B 5. SUMMARY AND C®NC'LUSI®NS Ninyo & Moore measured depth-to-groundwater and sampled fourteen wells at the site between March 29 and April 1, 1999 Groundwater elevation data indicates groundwater flows to the southeast beneath the site with a hydraulic gradient of 0 0026 feet per foot The depth to groundwater has risen in wells MW-12 by 4 04 ft , MW-14 by 3 38 ft , MW-19 by 3 48 ft , MW- 24 by 3 48 ft , MW-25 by 3 37 ft , VM-1 by 3 35 ft , and VM-2 by 3 29 ft , respectively since the last monitoring event in December 1998 Wells MW-9, MW-11, MW-13, MW-16, and MW-20 Icould not be compared because no data was available at those wells from the December 1998 sampling even According to laboratory data concentrations of constituents have increased or decreased in water samples from several on site wells since the last groundwater sampling round in December, 1998 Since December 1998 Well MW-12 had decreases in BTEX concentration levels from 2 0 ppb, to non-detect for benzene, toluene and ethylbenzene and from 13 ppb to non detect for total xy- lenes TPH-d was detected at 400 ppb in well MW-13, compared to the <5000 ppb found there Decem- ber 1998 Benzene decreased from 1,200 ppb to 950 ppb Toluene, ethylbenzene, and total ,hap 10 4 fgJU71�®��1