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0 PARSONS 2121 N. California Blvd.Suite 500. Walnut Creek,California 94596. (925)941-3700. Fax(925)979-9781 May 22, 2006 Ref: 442107 California Regional Water Quality Control Board Central Valley Region 11020 Sun Center Drive#200 Rancho Cordova, California 95670-6114 Attention: Mr. Gerald J. Djuth Subject: Revised Work Plan for Offsite Investigation Downgradient of MW-15 Applied Aerospace Structures Corporation Stockton, California Dear Mr. Djuth: On behalf of Atlantic Richfield Company (a BP affiliated company), Parsons is submitting this work plan for investigation of groundwater downgradient of monitoring wells MW-5R and MW-15 in the area southeast of the Applied Aerospace Structures Corporation (AASC) facility in Stockton, California. The investigation is designed address concerns of the Regional Board Staff expressed the 17 February 2006 letter "Extraction Well Concurrence and Request for Downgradient Investigation, Applied Aerospace Structures Corporation, Stockton, San Joaquin County". The Regional Board Staff observed that MW-5R and MW-15 are outside the current capture area of the groundwater extraction system and required that the extent of offsite TCE contamination be more completely defined downgradient of MW-15. This Revised Work Plan incorporates changes to the plan requested by the Water Board in the 21 April 2006 letter to Atlantic Richfield Company "Downgradient Investigation, Applied Aerospace Structures Corporation, Stockton, San Joaquin County". The revisions include adjustment of locations and spacing of borings in the inner arc of 5 borings, additional CPT stratigraphic profiles, additional overnight chemical analyses, and clarification of sampling depth targets. During late February 2006, a new extraction well, GW-7, was completed near the south property line. It was constructed at the mouth of the breezeway between Buildings #1 and #2 approximately 35 feet north of monitoring well MW-5R. On March 1 an abbreviated step-drawdown pumping test was conducted in GW-7. Pumping was conducted at successive rates of 2, 3, and 4 gpm and discharge was maintained constant for about 1 hour at each pumping rate. The maximum drawdown in the pumping well was 1 foot and the response in MW-5R was rapid and small (several tenths of a foot). Due to this finding, a larger capacity variable speed extraction pump rated at 7 gpm was installed in GW-7. This will ensure a capture area large enough to intercept contaminants migrating southeastward from the building area toward MW-5R and MW- 15 in the groundwater. GW-7 was started up on April 12 and the average production rate has been approximately 6.5 gpm.