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Stantec Consulting Corporation 3017 Kilgore Road Suite 100 Rancho Cordova CA 95670 Tel:(916)861-0400 Fax:(916)861-0430 Stantec September 10, 2008 Mr. Mike Infurna San Joaquin County Environmental Health Department 600 East Main Street Stockton, California 95202 RE: Addendum to Interim Remediation Action Plan Former ARCO Service Station No. 443 2478 E. Oak Street Stockton, California 95205 File No.: 77BP.00443.02.0136 Dear Mr. Infuma: This letter, prepared by Stantec Consulting Corporation (Stantec) on behalf of Atlantic Richfield Company (ARC), a BP Affiliated Company, presents an addendum to the Interim Remediation Action Plan dated July 10, 2008 for the site referenced above. The original work plan called for the installation of four groundwater monitoring wells and the initiation of monthly groundwater extraction via vacuum truck. Based upon the e-mails dated July 28, 2008 and August 8, 2008, the letter dated August 11, 2008, and a phone conversation on August 28, 2008, this addendum has been prepared to address SJC/EHD requests regarding the proposed activities. The SJC/EHD Comments and Stantec Response The SJC/EHD letter dated August 11, 2008 requests the following two items for this addendum and Stantec's response: 1. The construction methods, materials, and dimensions of the cased deep well. The MW-8 soil boring will be advanced to approximately 95 ft bgs using a drill-rig equipped with 15-inch diameter, continuous flight, hollow-stem augers. An eight and 5/8 inch inside diameter, flush-threaded, steel casing will be set within the annulus of the auger. The area between the outside of the steel casing and the wall of the boring will then be filled with cement and/or grout to surface grade using a tremie pipe and allowed to set. The inside of the steel casing will be covered with a plug. After the boring has set, a drill-rig equipped with eight-inch diameter, continuous flight, hollow-stem auger will be advanced down the inside of the steel casing of the boring to a depth of 120 ft. At the terminal depth of the boring, a well will be installed within the annulus of the auger. The well will be constructed using two-inch diameter, flush-threaded, schedule 40 poly vinyl chloride well casing and 0.020-inch factory-slotted well screen. The well will be screened from approximately 105-120 ft bgs; however, the final total depth and screen interval will be determined in the field based upon observed conditions. Stainless steel centralizers will be attached to the well casing at 15 foot intervals from 15 ft bgs to 90 ft bgs, and an additional centralizer at 100 ft bgs. A sand filter pack will be placed in the annular space surrounding the entire screened interval and will extend approximately two ft above the top of the screened interval. A two foot thick bentonite annular seal will be placed above the filter pack 443 IRAP Addendum.doc 1