SITE INFORMATION AND CORRESPONDENCE

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Canepa's Car Wash 2 Page 248 E.Park Street 06 February 2 e 2 Stockton, CA as on adjacent off-site locations. Due to refusal, only CPT3 and CPT7 were able to achieve the proposed target depth of 150 feet bsg, and a grab groundwater sample was collected from that depth only from CPT3. The other CPT borings reached total depths between 102 and 108 feet bsg. Analytical results of the deepest grab water sample at 157 feet bsg from CPT3 were 3.3 micrograms per liter (,ug/I) benzene, 3.2 pg/I di- isopropyl ether (DIPE), and 0.8 pg/I 1,2-dicholoroethane (1,2-DCA). If not a result of cross contamination from shallower groundwater, this data indicates that dissolved contaminants may extend to this depth, but the potential for cross contamination from shallower groundwater must be considered. Shallow grab groundwater samples, generally collected between 29 and 33 feet bsg were locally intensely impacted, demonstrating that groundwater has become impacted since rising over 20 feet since 1994. The proposed shallow groundwater monitoring wells to include this interval in their screened zones are well justified technically and is common practice for site characterization. With the exception of CPT7 located near the suspected contaminant release source area, the highest concentrations of total petroleum hydrocarbon as gasoline and benzene were consistently detected in grab groundwater samples collected at depths approximately 80 feet to 86 feet bsg, well below the screen intervals of the current monitoring wells on site. The interval between 72 feet bsg in CPT1 and between 75 and 82.5 feet bsg in CPT2 was a high permeability interval interpreted as sand on those logs; the 82 to 86-foot interval in CPT3 through CPT7 were interpreted as finer grained soil with generally moderate to high permeability, locally with low permeability (the relative permeability being inferred by the EHD from the pore pressure log). The difference between the logs may be due to differing CPT tool responses and differing software interpretation programs utilized by the two different contractors. The data from the general depth interval justifies installation of groundwater monitoring wells in the interval to characterize the dissolved plume therein, but the hydrogeologic relationships between these sample points must be worked out. Grab groundwater samples were also collected from a deeper interval, generally between 98 and 109 feet bsg during the 2004 CPT investigation. This interval generally coincides with sand units of variable thickness. The groundwater samples were all impacted by petroleum hydrocarbon contaminants at concentrations ranging from 450 to 25,000 pg/I TPHg and from 3.4 to 1200 pg/1 benzene. Groundwater monitoring wells screened in this interval are justified by this data. Deeper grab groundwater samples were obtained from CPT3 at 126 to 130 feet bsg and at 152 to 157 feet bsg. Both were impacted by petroleum hydrocarbons and by 1,2-DCA and DIPE at sharply declining concentrations. Due to the sparse data at these depths and the declining contaminant concentrations, the EHD does not believe a series of wells should be set in these intervals at this time; but would consider one set across the 126 to 130-foot interval down-gradient of the source area. If that well or other wells screened across the 98 to 109-foot depth intervals prove to be impacted, deeper characterization will be justified. The EHD believes the concept behind the recommendation to install continuous multi- chamber tube (CMT) wells completed in the various depth zones was justified by the findings of the 2004 investigation as noted above, however as discussed at the Work Plan Comment Letter 0209