SITE INFORMATION AND CORRESPONDENCE

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Ms. Kerry Heckman - 2 - 4 December 2006 2211 N. Wilson Way, Stockton groundwater to 100 feet below ground surface (ft bgs) and 80 feet laterally to the west from the Source. Another component of the Plume involves the gasoline lead scavenger 1,2-Dichloroetbane (1,2-DCA) emanating from the same Source, which has traveled deeper and farther than the gasoline and diesel-range components in the Plume, to 150 ft bgs and over twice the distance laterally than TPHg, in excess of 160 feet from the source (Figures 6 through 8) under the Site building. Maximum 302006 Report groundwater concentrations included: Total Petroleum Hydrocarbons as Gasoline (TPHg, vapor well VW-14B), 1,900,000 micrograms per Liter or ug/L; TPH as Diesel (TPHd, localized around MW-16 and near Stoddard spill location west of the Site building), 160,000 ug/L; Benzene (B), 7,400 ug/L; Toluene (T), 4,200 ug/L; Ethylbenzene (E) 810 ug/L; Xylenes (X), 5,900 ug/L; and 1,2-DCA, 330 ug/L. The results of the water levels taken from the monitoring wells in September 2006 showed that groundwater generally appeared to flow across the Site towards the west and north-northwest, from the shallow (60 ft bgs) to the deep (150 ft bgs) zones, respectively, in the aquifer. The two monitoring well sample results in the deepest monitored zones (195 and 245 ft bgs) were non-detect for all petroleum constituents. The 3Q2006 Report states that 74.5 gallons of gasoline was removed by SVE from the soil in the Source area from June through September of 2006. The SVE has removed approximately 3,088 gallons of gasoline since startup in July 2005. The SVE soil remediation and natural attenuation appear to be responsible for a reduction in the vertical and lateral extent of the groundwater plume; however the higher concentrations of pollution are persisting in the shallow zones near the Source. Air sparging near the Source, in conjunction with SVE, will be tested in a pilot study as discussed above. Monitoring and Reporting Program No. R5-2005-0827 (MRP) states in part that: "The Dischargers shall not implement any changes to this MRP unless and until a revised MRP is issued by the Executive Officer or until Regional Board staff approve those changes in writing." The 3Q2006 Report recommends continued groundwater monitoring with modifications to the MRP, which would result in a cost savings of fifty percent for analyses currently not required to monitor the groundwater plume. Specifically, the 3Q2006 Report recommends the following analyses reduction- 1. eduction:1. Sample for only 1,2-DCA in wells where other petroleum constituents have not been detected after multiple sampling events; 2. Removal of TPH as Stoddard Solvent (TPH-ss) from the list of analyses for all wells (TPHd analyses to continue as stated in number 3 below). Note that this reduction is already authorized as Footnote 2 of the Table on Page 3 of the MRP, and 3. Removal of TPHd from the list of analyses in wells that have not reported TPHd after multiple sampling events. Data from 16 quarters of groundwater monitoring show that the proposed reductions would not affect the data quality or produce data gaps that might require an additional investigation prior to completion of remedial action. Regional Board staff agrees with the recommendation and approves the changes in the MRP effective immediately, to be followed by a new MRP with a revised table on page 3 for sample analyses at a later date.