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ACTON • <br /> MICKELSON• <br /> ENVIRONMENTAL, INC. <br /> Consulting Scientists,Engineers, and'Geologists <br /> APR 1 6 2004 <br /> -,,vii-._mv1rN1 HEALTH <br /> PERMIT/SERVICES <br /> April 13, 2004 Via Electronic Mail <br /> Ms. Devra Lewis <br /> Private Sites Cleanup Unit <br /> Central Valley Regional Water Quality Control Board <br /> 11020 Sun Center Drive #200 <br /> Rancho Cordova, California 95670 16014.01 <br /> Subject: Former Georgia-Pacific Corporation Tracy Chemical Packaging Plant <br /> 75 West Valpico Road, Tracy, California <br /> Dear Ms. Lewis, <br /> This correspondence is in response to your request for clear guidelines on decreasing the <br /> sampling frequencies of monitoring wells MW-21 and MW-22 at the former Georgia-Pacific <br /> Corporation (G-P) Chemical Packaging Plant located at 75 West Valpico Road in Tracy, <br /> California. <br /> On behalf of G-P, Acton • Mickelson • Environmental, Inc. (AME) proposes to use a statistical <br /> approach to confirm decreasing concentration trends before sampling frequencies would be <br /> reduced. Specifically, following each sampling event, a Mann-Kendall trend analysis would be <br /> performed on ground water concentration data collected since initiation of air/ozone sparge <br /> operations. The trend analysis would calculate the certainty of a decreasing chloroform <br /> concentration trend for each well. Sampling would continue with the current frequency until the <br /> trend analyses determine, with at least 90% certainty, that decreasing concentration trends are <br /> occurring in each well. Once the 90% certainty of decreasing concentration trends was achieved, <br /> the sampling frequency for each well would change to semi-annually. Operation of the air/ozone <br /> sparge system would continue with monthly monitoring. <br /> The Mann-Kendall trend analysis test is a nonparametric test (i.e., the data do not need to <br /> conform to any particular distribution). Missing values are allowed and data reported less than <br /> laboratory reporting limits may be used. The Mann-Kendall test can be viewed as a <br /> nonparametric test for zero slope of the linear regression of time-ordered data versus time. <br /> C:\Documents and Settings\Barbam Mickelson%ocal SettingsUemporary Intemet Files\Content.LE5V38WY3AZ\04-082.doc <br /> 5175 Hillsdale Circle,Suite 100 (916)939-7550 <br /> EI Dorado Hills,California 95762 Fax(916)939-7570 <br />