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California Department of Transportation July 2, 2001 Stockton Maintenance Station, Stockton, California Project No 400296001 • 4.3. Laboratory Analysis The groundwater samples were submitted for analysis to AL, a State of California-certified hazardous waste analytical laboratory Samples were also analyzed for TPH-G and TPH-D, using EPA method 8015 (modified) Samples were analyzed 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 EPA method 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 Appendix B 5. SUMMARY AND CONCLUSIONS Ninyo & Moore measured depth-to-groundwater and sampled fourteen wells at the site on March 20-21, 2001 Groundwater elevation data indicates that the groundwater flows to the southeast • beneath the site with a hydraulic gradient of 0 003 feet per foot Comparatively, groundwater levels taken during the last monitoring event in December indicate the same flow direction and a hydraulic gradient of 0 002 ft/ft Groundwater elevation has increased in all of the on-site wells since the last monitoring event Groundwater elevations have increased in monitoring wells MW-1 by 177 ft , MW-2 by 162 ft , MW-3 by 1 75 ft , MW-4 by 177 ft , MW-5 by 176 ft, MW-6 by 176 ft , MW-7 by 1 85 ft , MW-8 by 170 ft , MW-9 by 176 ft , MW-10 by 171 ft , MW-11 by 1 54 ft , MW-12 by 1 83 ft , VW-1 by 1 70 ft and VW-2 by 1 76 ft Laboratory analytical data has indicated changes in many of the compounds analyzed in samples taken from various wells on site since the December 2000 monitoring event In well MW-1, Benzene concentration decreased from 6 4 micrograms per liter (µg/l) to non- detectable Levels Total xylene concentration decreased from 21 µg/L to non-detectable levels In well MW-2, TPH-G concentration increased from 180 µg/1 to 200 µg/1 Benzene concentra- tion increased from 29 µg11 to 49 µg/l Total xylene concentration increased from 25 µg/l to 30 • Pg /I 0296-01 srpckton draft IQ01 doc 4 q'?§R