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Excerpts from directive letter of 03 March 2005: <br /> "In order for the EHD to concur with a site closure request, you must demonstrate <br /> plume stability and degradation, and make a reasonable estimate of the time <br /> required for the impacted groundwater to return to background conditions. As <br /> stated in previous EHD letters, at a minimum, this requires reasonable estimates <br /> of contaminant mass in soil and groundwater, an understanding and <br /> demonstration of the natural attenuation processes affecting the contaminants, <br /> and an understanding of the hydrogeology of the site." <br /> "The work plan should also include such additional assessment measures as <br /> your consultant(s) deems necessary to formulate reasonable updated <br /> contaminant mass estimates." <br /> At this time there are additional criteria required by the CVRWQCB for closure <br /> consideration: <br /> • Comparison of site data with ESLs and CHHSLs of the San Francisco Bay <br /> Regional Water Quality Control Board with a justification for contaminant <br /> levels that exceed the ESLs and CHHSLs. <br /> • An evaluation of the potential for vapor intrusion into occupied buildings in <br /> the area, especially the adjacent residential structu <br /> M' <br /> EHD recommends the following actions: ' <br /> • Conduct a soil gas survey of the site and immediate surroundings to <br /> evaluate the potential for vapor intrusion into nearby structures and to <br /> potentially locate unrecognized impacted media. <br /> • At a minimum, collect grab groundwater samples from sand unit(s) <br /> approximately 10 to 20 feet bsg southeast of the source areas. <br /> • Prepare a technical evaluation of the monitoring results during the GWE <br /> shutdown period. Compare contaminant/MNA concentration trends <br /> through time and across space. Project, if possible, the time-frame for the <br /> site to return to background conditions. <br />