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Jamar Service - 2 - 30 December 2019 <br /> 4074 E. Main St. <br /> Stockton, San Joaquin County <br /> 1. As ATC has not yet performed the confirmation assessment, Central Valley Water <br /> Board staff does not concur with case closure at this time. Central Valley Water Board <br /> staff will evaluate Site closure after the confirmation assessment data has been <br /> submitted and reviewed. <br /> 2. ATC used BioScreen regression analysis software to model the maximum future <br /> downgradient extent of MTBE in groundwater. The center of the Site MTBE plume <br /> appears to have migrated downgradient to the vicinity of monitoring well MW-6C in <br /> around 2005 (14 years ago). As such, ATC used MW-6C to represent the source <br /> area, and used a 14-year time frame to model what current conditions should look like <br /> downgradient of MW-6C, and compare them to actual current conditions in MW-10C, <br /> approximately 180 feet east of MW-6C. This "calibration" model appears to show <br /> reasonable congruence with actual current conditions. ATC then modeled the plume <br /> over a 100-year time frame to show the mobility of the MTBE plume. Central Valley <br /> Water Board staff used the BioScreen software to re-create and evaluate ATC's <br /> modeling using the same input parameters. Using the 14-year time frame for the <br /> model, it appears that the BioScreen model indicates that the maximum current extent <br /> of MTBE should be between 320 and 360 feet east of MW-6C. ATCs proposed boring <br /> is approximately 350 feet east of MW-6C (approximately 190 feet east of MW-10C) <br /> and Central Valley Water Board staff concurs with the proposed scope of work. <br /> 3. By 3 April 2020, please submit an Assessment Report detailing the completion of the <br /> proposed scope of work and the resultant findings. Please include discussion of the <br /> results as compared to the modeling presented in the Revised Closure Request, and <br /> whether closure is warranted. <br /> If you have any questions or concerns regarding comments in this letter, please call the <br /> undersigned at (916) 464-4615 or by email at alan.buehlerla)waterboards.ca gov. <br /> Alan M. Buehler,.P.G. Christopher Flower <br /> Engineering Geologist Senior Engineering Geologist <br /> �SSIONq j . <br /> S\ot4AC 0,,,, <br /> a ALAN M. BUEHLER <br /> No.8340 BQBa'"fir .�� <br /> (Pr9�oF CAOF4tIFg4 *P <br /> cc: Technical Review Staff, SWRCB, UST Cleanup Fund, Sacramento <br /> Michael Kith, San Joaquin County Environmental Health Department, Stockton <br /> Jeanne Homsey, ATC Group Services Inc., Modesto <br />