SITE INFORMATION AND CORRESPONDENCE

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ExxonMobil Oil Corporation - 2 - 25 November 2008 75 E. Alpine Ave., Stockton detections of petroleum hydrocarbons. Grab groundwater samples were not collected from the CPT borings. The Workplan proposes the following screened intervals for the deep wells: DW-1, 55'-70' bgs (15' length); DW-2, 62'-75' bgs (13' length); DW-3, 55'-75' bgs(20' length); and DW-4, 52'-67' bgs (15',length). Proposed locations for deep wells are: DW-1 near shallow well MW-4 (west of the main tank pit)', DW-2 near CPT-2 (approximately 45' north of the main tank pit), DW-3 near CPT-3 (east of the main tank pit), and DW-4 (in the car wash parking lot) at 30' east of CPT-5 (located north of CPT-4 in the EI Dorado Street right of way) and approximately 10' north of proposed shallow well MW-8, which was discussed above. Deep wells will initially require 8 5/8" steel conductor casing set to 48' bgs depth inside a 15" boring, prior to installing 2" wells in the subsequently drilled 8" boring that will be completed through the conductor casing and then deeper to total depth of the wells. Conductor casing is routinely installed to limit the possibility of cross contamination from the shallow groundwater zone into the deeper groundwater zone. Comments: • The proposed deep wells, which are screened above the CPT logged sand unit and within the CPT logged silt unit, appear to be based entirely on the MIP concentration readings without consideration of the CPT lithological logs, which should also be considered as criteria for selecting well screen intervals. Laterally continuous sand or gravel units, due to their higher permeability than fine grained units, typically serve as preferential pathways for groundwater plumes when there are no lithological barriers (acluatards) or hydrological factors (upward flow gradients) to prevent plume migration, and subsequently may result in accelerated plume migration due to the sands/gravels higher permeability and lower attenuation factors (retardation, etc.)than silts/clays. Since the MIP concentration data appear to indicate that downward migration to the deeper zone is occurring, and monitoring data show high concentrations of petroleum hydrocarbons in shallow onsite wells near the main tank pit, a laterally continuous deep sand unit (when found) is typically proposed as the target zone for monitoring with the deep wells. While the Workplan does say that the locations for well screens are approximate and will be adjusted in the field, the text does not specifically target or mention the deep sand zone for well screen placement. In order to assure that all preferential pathways are adequately investigated, I recommend that: 1)the deep sand zone shall be confirmed based on lithology encountered during the drilling of the wells, 2) upon confirmation that the deep sand zone exists, a field evaluation be conducted to determine the thickness of the sand zone for selection of an appropriate well screen length, and 3) the deep wells be screened in the sand zone to help assess both vertical and lateral extend of the groundwater plume, in accordance with the best professional judgment of the licensed geologist or engineer in charge of the well installations, and with the field boring data used as the final criteria for well screen placement. Otherwise, additional justification for not targeting the sand unit needs to be presented in writing to this office by 12 December 2008. • All well screens must be either 5' or 10' in length, based on the thickness of the sand unit as encountered in the field. Well screens longer than 10 feet are not approved for the deep wells. • You are to contact Mr. Kelly Reilly and make one final attempt to gain access to the car wash property prior to installing the deep wells. If offsite access is again refused for the offsite shallow and deep wells, then the three deep onsite wells are to be installed and sampled (soil and groundwater) without further delay.