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ENSRIAECOM <br /> ENSR <br /> 10461 Old Placerville Road,Suite 170,Sacramento,California,95827-2508 <br /> T916.362.7100 F916.362.8100 w. .ensr.aecom.com <br /> October 19, 2007 <br /> Mr. James L. Barton <br /> Central Valley Water Board <br /> 11020 Sun Center Drive, Suite 200 <br /> Rancho Cordova, CA 95670 <br /> Subject: Addendum to Work Plan for Replacement of Decommissioned Monitoring Well 15 <br /> Cleanup and Abatement Order No. 5-00-0707 <br /> 5400 and 5606 Pacific Avenue, Stockton, California <br /> Chevron Facility 306620 (former Unocal 5098) <br /> ENSR Project No: 01 231-154-Al C <br /> Dear Mr. Barton: <br /> ENSR Corporation (ENSR), on behalf of Chevron Environmental Management Company(EMC), is <br /> pleased to present this addendum to the August 30, 2007 Work Plan for Replacement of the <br /> Decommissioned Monitoring Well MW-15(Workplan)for both the former Montgomery Ward, LLC(Wards) <br /> service station located at 5400 Pacific Avenue and the former Unocal service station, located at 5606 <br /> Pacific Avenue, Stockton, California (Figure 1 —Site Location Map), referenced collectively as the Site. <br /> This addendum addresses comments provided in the Central Valley Water Board (CVWB)Work Plan <br /> Review letter dated October, 4, 2007(Attachment A). <br /> Comment No. 1 <br /> The Workplan does not include an explanation why the new well will be screened from 25 to 40 feet bgs, <br /> which is 29 feet shallower than the well it replaces. Please provide the rationale for completing the well at <br /> a shallower depth in a revised Workplan. Note that MW-15's depth to groundwater over the last three <br /> years varied from 32 to 37 feet bgs. An alternative screened interval might be 28 to 43 feet bgs, <br /> depending upon the actual site lithology. <br /> Response No. 1 <br /> The MW-15 replacement well is proposed as a well screened across the water table based on historic <br /> water levels and historic analytical results, as shown in Table 1 of ENSR's Quarterly Groundwater <br /> Monitoring and Remediation Status Report and Updated Site Conceptual Model for Third Quarter of 2007 <br /> (October 10, 2007). MW-15 was originally installed in July 1994, and the KEI boring log shows depth to <br /> first-encountered groundwater of approximately 62 feet below ground surface(bgs). Monitoring well <br /> MW-17, located south of the proposed MW-15R,was installed in November 1995, and the KEI boring logs <br /> shows depth to first-encountered groundwater of approximately 56 feet bgs. The CVWB comment <br /> identifies the MWA 5 water level range over the last three years of 32 to 37 feet bgs. The proposed screen <br /> interval of 25 to 40 feet bgs also considered the water depth of 29.98 feet bgs measured in May 2001. <br /> Historic groundwater analytical results for MWA 5 identify eight consecutive non-detect events(January <br /> 2007 to April 2005), and the last detection of dissolved petroleum hydrocarbons occurred in January 2005 <br /> with a benzene concentration of 1.8 micrograms per liter(pg/Q. Monitoring well MW-17, located south of <br /> 01231-154 <br />