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t ENSR , rNP-1 was located in the saturated zone(40-to 60-foot depth interval) These soils are considered not to have been excavated from the site and therefore are considered a source of residual contamination Concentrations of TPHg and total BTEX in boring B-5 increased with depth from 32 2 mg/kg of TPH and 2 4 Mg/kg total BTEX in the 15 5-foot sample to 370 mg/kg of TPHg and 139 mg/kg of total BTEX in the 20 5- foot sample Similarly, concentrations of TPHg and total BTEX were detected within the 40-to 60-foot ' interval (saturated zone) in NP-1 Concentrations increased with depth from 13 mg/kg of TPHg and 3 4 mg/kg total BTEX in the 40-foot sample to 290 mg/kg of TPHg and 82 5 mg/kg of total BTEX in the 60-foot sample No impacted soil was detected in the 70-foot sample Sod sampling conducted during the latest monitoring well installations(wells MW-11 through MW-21 during 2001 and 2002) reported no detectable levels of TPHg, benzene, and MTBE 3.4 Contaminant distribution to groundwater Groundwater quality data has been collected from the site over 18 years between 1988 and 2006 Over this period, 22 monitoring wells have been installed at the site, including 12 wells on-site and 10 wells off-site Fifteen of these wells (MW-1 through MW-11, MW-13, MW-15, MW-17, and MW-21) monitor groundwater at the groundwater interface (i e , Zone A), and the other seven wells (MW-12, MW-14, MW-i 6, MMI 8, ' MW-19, MW-20 and NP-1) monitor groundwater at depth from approximately 85 to 90 feet bgs (i e , Zone B) Refer to Table 3-2 for details on the construction of the monitoring wells The groundwater quality data is summarized in Table 3-3, Groundwater Analytical Results of Petroleum Constituents and Table 3-4, Groundwater Analytical Results of Oxygenate Compounds (including 1,2-DCA) The lateral extent of groundwater impacts have been determined by the current monitoring well network The center of the groundwater plume is located on-site near the eastern property boundary as indicated by the data obtained from monitoring wells MW-2, MW-10 and MW-1 i The plume has migrated off-site, based on the data from wells MW-4, MW-5, and MW-17 located on the east side of Pacific Avenue and well couplet MW-7 and MW-19 located approximately 300 feet northeast (downgradient) of the site However, recent TPHg and BTEX data from these five wells indicates that the size of the plume has decreased The groundwater analytical data from the most recent sampling round (January 2006) are depicted in Figure 3-5 (Zone A) and Figure 3-6 (Zone B) Consistent with the nature of the gasoline release at this site, TPHg and benzene were the constituents detected primarily in Zone A(groundwater interface) The highest concentrations of these constituents have been historically reported in three on-site wells - MW-2, MW-10, and MW-11 These three wells are all located in close proximity to one another and within 50 feet of the source area (i e ,former eastern pump island) The January 2006 data indicated concentrations of TPHg in these three wells ranging from 2,100 ug/I (MW-2) to 55,000 ug/I (MW-10) and benzene concentrations ranging from 550 ug/l (MW-2)to 2,500 ug/I (MW-10) (ENSR, 2006) An isopleth map for benzene tconcentrations in Zone A are depicted in Figure 3-7 1 2-DCA has also been detected in the monitoring WL'IIs, dt both on-bite and off-site locations These concentrations are indicative of residual concentrations and not a source area Results from the January 2006 event reported the detection of 1,2-DCA in four of the 15 wells for which it was analyzed These wells are downgradient of the site, three of which are in Zone A (MW-4, MW-5, and MW-17) and one in Zone B (MW-19) The highest concentration of 1,2-DCA was reported in MW-17 (87 ug/i) with decreasing concentrations to approximately 1 ug/I in the other three wells Well MW-17 has historically yielded the highest concentration of 1,2-DCA 3-3 �pO 25 ro« i