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1117 Lone Palm Avenue, Suite B C' VATC, Modesto, CA 95351 � (��} '� http://www.atcassociates.com l`J h, 209.579.2221 Fax 209.579.2225 ASSOCIATES INC . JINN 3 112305 June 27, 2005 EMAROW04T HCES 1 54.14885.0001 pE?,M1T Ms. Margaret Lagorio San Joaquin County Environmental Health Department 304 E. Weber Street, Third Floor Stockton, California 95202 Subject: Addendum to Workplan for Additional Subsurface Investigation for 6732 East Highway 88, Stockton, California Dear Ms. Lagorio: ATC Associates Inc. has prepared this letter to serve as an addendum to ATC's May 2, 2005 Additional Workplan for Subsurface Investigation prepared on behalf of Ms. Lolly Bozzano for the performance of a subsurface investigation at the above referenced site(Figure 1). This addendum has been prepared based on June 1, 2005 correspondence from Ms. Margaret Lagorio of San Joaquin County Environmental Health Department(SJCEHD). Construction of Monitor and Vapor Wells To determine the lateral and vertical extent of soil and groundwater contamination at the site, and to evaluate the feasibility of vapor extraction as a remedial alternative for cleanup at the site, a total of two groundwater monitoring wells and three vapor extraction wells will be installed at the project site. The vapor extraction wells will be constructed of 2-inch diameter, schedule 40 PVC casing and 0.010" slot size screen (Figure 2). The bottom of the screen interval will be fitted with an end plug and the top of the casing will be fitted with an expandable, water-tight, locking cap. A well screen length in the vapor extraction wells will extend from a depth of 15 to 45 feet bgs. Above the screen will be a sand layer from 13 to 45 feet bgs, a bentonite layer from 10 to 13 feet bgs, and grout filling from 0 to 10 feet bgs. A traffic-rated well box will be installed flush with the surface. The groundwater monitoring wells will be installed downgradient from the former tank pit and will be utilized to monitor the downgradient concentrations of petroleum hydrocarbons in a lower water bearing zone. The wells will be constructed of 2-inch diameter schedule 40 PVC and will be completed to a depth of approximately 140 feet bgs (Figure 3). The groundwater wells will have 0.020" slot size screen from approximately 130 to 140 feet bgs inside of a conductor casing that will extend to a depth of 120 feet bgs. Based upon additional review of the logs from borings CPT] and CPT2, ATC is recommending a shorter screen length. Above the screen will be a sand layer from 125 to 140 feet bgs, a bentonite layer from 122 to 125 feet bgs, and grout filling from 0 to 122 feet bgs. A traffic-rated well box will be installed flush with the surface. All work will be performed in accordance with the methodologies and practices described in ATC's original workplan dated May 2, 2005. 1 ENVIRONMENTAL, GEOTECHNICAL AND MATERIALS PROFESSIONALS