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Harold and Dena Knowles Page 3 of 3 <br /> Former Roek Construction 23 February 2009 <br /> 102 S. Wilson Way <br /> Review of the SCM for your site and the SCM for the Arco site (Site Conceptual Model, <br /> dated 13 March 2007) has lead the EHD to believe the plume of contaminants impacting <br /> monitoring wells MW-4, MW-14, MW-16A and MW-16B may have originated from the Top <br /> Filling site and the MTBE impacting monitoring wells MW-3 and MW-7 may have <br /> originated from the Arco site. The EHD also believes the MTBE impacting <br /> monitoring wells MW-11, MW-12, MW-15A and MW-15B also originated from the Arco <br /> site, but the wells are also impacted by 1,2-DCA from the subject site. Pending additional <br /> work and data analysis, the EHD will not require you to sample MW-3, MW-4, MW-7, <br /> MW-14, MW-16A or MW-16B, but do continue to gauge the depth to water in these wells <br /> during your groundwater monitoring events. <br /> The Central Valley Regional Water Quality Control Board (CVRWQCB) has identified <br /> vapor intrusion from contaminated soil and/or groundwater into structures to be a potential <br /> health hazard, therefore you are directed to evaluate the potential for vapor intrusion into <br /> the structures on your site or in the immediately surrounding neighborhood, if appropriate, <br /> generated by the contaminants in the subsurface released from your former UST system. <br /> Submit a work plan to the EHD by 15 June 2009 for that evaluation. Technical guidance <br /> documents for such an evaluation are available from the California Department of Toxic <br /> Substances Control (DTSC) and from the Los Angeles Regional Water Quality Control <br /> Board (LARWQCB). <br /> Questions or comments may be directed to Nuel Henderson at (209) 468-3436. <br /> Sincerely, <br /> Nuel C. Henderson, Jr., PG Margare Lagorio, REHS <br /> Engineering Geologist Program Coordinator <br /> C: James L.L. Barton - CVRWQCB 11020 Sun Center Dr. #200, Rancho Cordova, <br /> CA 95670 <br /> Gregory Stahl — GZA, 1714 Main Street, Escalon, CA 95320 <br /> Mohammid Asghar— 601 Wade Ave., Modesto, CA 95351 <br /> Tasnim Akhtar— 1514 E. Market St., Stockton, CA 95205 <br /> Comment and Directive Letter 0209 <br />