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i Igroundwater elevation than in wells MW-13 and MW-13A There appears to be cyclical water . level fluctuations at the site due to weather and proximity of the site to the Stockton Diverting ICanal Historical groundwater elevations are summarized in Table 2 Groundwater at the site has been sampled and analyzed for gasoline range total petroleum hydrocarbons (TPH-G), benzene, toluene, ethylbenzene, and xylenes, and gasoline oxygenates Methyl t Butyl Ether(MTBE), t-Butanol (TBA), Di-isopropyl Ether (DIPE), Ethyl-t-Butyl Ether (ETBE), t-Amyl Methyl Ether (TAME), 1,2-Dichloroethane (1,2-DCA), and Ethylene I Dibromide (EDB) A review of the groundwater analytical data indicates that the dissolved- phase hydrocarbon constituent concentrations of benzene, toluene, ethylbenzene, xylenes, and TPH-G have generally decreased or stabilized in all monitoring wells except MW-13 The fluctuations in MW-13 may be the result of soil impacts in the former tank pit leaching to groundwater Two dissolved-phase oxygenate constituents (MTBE and 1,2-DCA) have been detected in the groundwater MTBE concentrations were detected in monitoring wells MW-3, 3A, 3D, 11, and 13 MTBE concentrations appear to be increasing in monitoring wells MW-3A, 3D, and 11 and fluctuating in monitoring wells MW-3 and MW-13 1,2-DCA concentrations have only been detected in monitoring well MW-11, located off-site This constituent has not been detected in wells located in the UST release area or immediately downgradient of the release area and may not be related to the release at the Canteen Facility site Concentrations of dissolved-phase hydrocarbons in the groundwater exceed the CAMCL for benzene (1 0 ug/1), toluene(150 ug/1), ethylbenzene (700 ug/1), xylenes (1,750 ug/1), MTBE (13 ug/1), and 1,2-DCA (0 5 ug/1) TPH-G concentrations ranged from<50 to 170,000 ug/1 The majonty of the groundwater impacts are centered on the MW-3 and MW-13 serves wells Concentrations of 1,2-DCA have only been detected in off-site monitoring well MW-11 and may not be related to the UST release Historical groundwater analytical results are summarized in I Table 3 Isoconcentration maps showing the horizontal extent of the dissolved-phase hydrocarbons are presented in Appendix B 2.3 Sensitive Receptors A sensitive receptor survey for the site was conducted in April 2002 (Appendix C) According to the California Department of Water Resources—Central District records, four domestic water wells are located within 2000 feet of the site An area reconnaissance survey conducted by Advanced GeoEnvironmental, Inc identified three additional domestic water wells and seven I monitor wells within 2000 feet of the site The California Water Service Company reported one active municipal water well approximately 3,500 feet west of the site The nearest surface water body (approximately 1,200 feet) is the Stockton Diverting Canal located north and west of the Isite A Receptor Map showing the water well locations is included in Appendix C The groundwater gradient is consistently towards the south-southwest based on past monitoring events Four domestic water wells are located downgradient from the site The closest downgradient water well is located approximately 200 feet south of the site According to the . Water Well Drillers Report, this well is screened from 185 to 225 feet below ground surface Q,EGUI„v, 0 u� 3