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' California Department of Transportation June 9, 1999
<br /> Stockton Maintenance Station, Stockton, California Project No 103872-02
<br /> 4.3 Laboratory Analysis
<br /> The groundwater samples were submitted for analysis to AAL, a State of California-certified
<br /> hazardous waste analytical laboratory The samples were analyzed for TPH-g and TPH-d,
<br /> using method EPA 8015 modified, for BTEX, methyl tertiary-butyl ether (MTBE), tertiary-
<br /> amyl-methyl ether (TAME), isopropyl ether (DIPE), tertiary butyl (TBA), ethyl-tertiary-
<br /> butyl ether (ETBE), and volatile organic compounds (VOCs) using method EPA 8260b A
<br /> summary of the past and present laboratory results for the groundwater samples is presented
<br /> in Table 1 Copies of laboratory results and chain-of-custody documents are included in Ap-
<br /> pendix B
<br />' S. SUMMARY AND CONCLUSIONS
<br />' Ninyo & Moore measured depth-to-groundwater and sampled fourteen wells at the site on March
<br /> 31, April 1 and 2, 1999 Groundwater elevation data indicates groundwater flows to the south
<br /> beneath the site with a hydraulic gradient of 0 009 feet per foot Comparatively, groundwater
<br /> levels taken during the last monitoring event in December 1998 indicate groundwater movement
<br /> beneath the site towards the east under a hydraulic gradient of 0 01 feet per foot The depth to
<br /> groundwater has risen in the on-site wells since the last monitoring event Water level increases
<br />' are as follows MW-1, 3 53 ft , MW-2, 3 46 ft , MW-3, 2 82 ft , MW-4, 3 41 ft , MW-5, 3 47 ft ,
<br /> WM-6, 3 38 ft , MW-7, 3 47 ft , MW-8, 3 59 ft , MW-9, 3 52 ft , MW-10, 3 4 ft , MW-11, 3 54
<br />' ft , MW-12, 361 ft , VW-1, 351fl , and VW-2, 338ft
<br />' According to laboratory data well MW-1 showed decreased benzene concentrations from 5 0 ppb
<br /> to non detect since December 1998 Toluene concentrations decreased from 4 0 ppb to non-
<br /> detect, ethylbenzene from 3 0 ppb to non-detect, and total xylenes decreased from 14 ppb non-
<br /> detect
<br />' Benzene concentrations increased in well MW-2 from 1 0 ppb to 9 0 ppb, toluene increased from
<br /> non-detect to 3 2 ppb, ethylbenzene increased from non-detect to 3 1 ppb, and total xylenes in-
<br /> creased from non-detect to 12 to ppb
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