SITE INFORMATION AND CORRESPONDENCE

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ExxonMobil Oil Corporation - 3 u July cuvo 75 E. Alpine Ave., Stockton• • C) Note that although low levels of petroleum hydrocarbons in soil were detected in soil samples at total depth in borings MW2, MW3, and MWS, TPHg and benzene have been detected in groundwater from those monitoring wells, up to a maximum of 32,600 ug/L and 18,900 ug/L, respectively. MTBE has also been detected up to 1,200 ug/L in MW2. Therefore, at least two additional onsite deep wells (three total) are necessary to determine vertical extent of groundwater pollution and to provide groundwater flow direction in the deep zone. Those onsite deep well locations shall be indicated in the new figure, with the final locations to be determined by CPT/MIP data and approved by Regional Board staff, prior to installation in this phase of the investigation. D) One shallow and one deep well shall be indicated on the new figure near the offsite CPT/MIP locations across-EI Dorado Street and east of the site, with the understanding that the CPT/MIP screening data will be used to determine a final location for those wells. If the CPT/MIP data do not detect total petroleum hydrocarbons in groundwater to a total depth of 120 feet at those offsite locations, then only one shallow well will be installed in the City of Stockton right of way across EI Dorado Street, to the east of the site, in this phase of the investigation. 3. Regional Board staff concurs to the findings of the screening level human health risk assessment; however, Central Valley Regional Board staff do not use the groundwater ESLs for comparison in groundwater risk assessments or as cleanup levels for regulatory closure. Instead, in future reports use the Regional Board Water Quality Goals (WQGs), which may be downloaded from our website at http,//www.waterboards.ca.gov/centralvalley/water issues/water quality standards limits/water quality goalsfindex.shtml. 4. ExxonMobil may request reductions in groundwater analyses for specific wells if the data show no impacts from those constituents after four consecutive quarters of monitoring, provided the data method detection limits are sufficiently low for comparison to applicable WQGs. 5. While the MIP logging is being used as a real time field screening tool to detect total petroleum hydrocarbons, this office does not accept the MIP data as evidence for vertical delineation of individual petroleum hydrocarbon constituents or for quantification of petroleum hydrocarbon mass in soil and groundwater. You may proceed with the CPT/MIP investigation with the understanding that the MIP data may only be used for placement of soil borings and groundwater monitoring wells. However, all fieldwork (CPT/MIP and conventional soil borings/monitoring wells) must commence no later than 15 September 2008. 6. A report of the work is due 60 days after completion of fieldwork, however the report must be submitted no later than 5 December 2008. The report shall provide the following: • All soil and groundwater analytical data and CPT/MIP logs; • Iso-concentration (at a minimum TPHg and benzene) and groundwater flow direction contour maps for each zone; • Geologic cross sections indicating lithology, petroleum hydrocarbon concentrations in soil and groundwater, and locations of wells and borings on each cross section, and a map view of showing the lines of all cross sections; Conclusions based on the data and logs; and A recommendation for active remedial action(s) to cleanup petroleum hydrocarbons in soil and groundwater in a cost effective and reasonable period of time. A workplan for a bench scale test (chemical injection) or a pilot study shall also be submitted by 5 December 2008.