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GAVIN NEWSOM <br /> (;nVFHNnn <br /> YAN <br /> Water Boards ENVIRONMENTAL <br /> GRRCI <br /> NV3RONMENTAL PROTECTION <br /> Central Valley Regional Water Quality Control Board <br /> 19 December 2022 <br /> Mr. Atir Chak, P.E. Mr. and Mrs. Mandeep and Rudy Singh <br /> Contract Program Manager 2652 Cornflower Street <br /> Phillips 66 Company Stockton, CA 95212 <br /> 3900 Kilroy Airport Way, Suite 210 <br /> Long Beach, CA 90806 <br /> RESPONSE TO WORK PLAN, TOSCO/UNOCAL #05643 AKA: #4409, <br /> 1502 NORTH EL DORADO STREET, STOCKTON, SAN JOAQUIN COUNTY <br /> Dear Ms. Steward, Mr. Mandeep, and Mrs. Singh: <br /> Central Valley Regional Water Quality Control Board (Central Valley Water Board) staff <br /> reviewed the Offsite Investigation Work Plan (Work Plan) dated 15 November 2022, <br /> submitted on your behalf by GHD. The Tosco/Unocal #05643 AKA: #4409 leaking <br /> underground storage tank case is located at 1502 North El Dorado Street in Stockton <br /> (Site). <br /> In the Work Plan, GHD proposed to install three down-gradient groundwater monitoring <br /> wells to provide additional lateral and vertical delineation in the B zone, approximately <br /> 65 and 85 feet below ground surface (ft bgs), and C zone, approximately 100 and 120 ft <br /> bgs. Two co-located wells, MW-27 and MW-28, are proposed to the east-northeast of <br /> existing well MW-23, with one well screened in the B zone and the other well screened <br /> in the C zone. Well MW-29 is proposed between the plume boundary and Domestic <br /> Well 10 and will be screened in the C zone. GHD proposes to screen the B zone well <br /> from 65 to 80 ft bgs and to screen the C zone wells from 105 to 120 ft bgs. <br /> GHD proposed to reduce the sampling frequency of wells MW-1, MW-2, MW-18, <br /> MW-19, MW-20, MW-21, MW-22, and MW-24 to annually during the first quarter. The <br /> wells proposed for sampling frequency reduction are all screened in either the A or B' <br /> (B Prime) zones. <br /> Central Valley Water Board staff has the following comments: <br /> 1. Staff concurs with the scope of work presented in the Work Plan, provided the <br /> following comments are taken into consideration. Existing B zone wells are <br /> screened between 68 and 83 ft bgs. GHD proposed to screen the B zone well <br /> from 65 to 80 ft bgs. This proposed screen interval is a bit shallower than <br /> MARK BRADFORD, CHAIR I PATRICK PULUPA, EsQ., EXECUTIVE OFFICER <br /> 11020 Sun Center Drive#200, Rancho Cordova, CA 95670 1 www.waterboards.ca.gov/centralvalley <br />