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15406 Tracy Blvd,Tracy <br /> March, 2006 <br /> Page 2 <br /> 4 <br />` Although the overall MTBE concentration is in decline,possibly due to natural attenuation,dilution, <br /> and mass migration, the dissolved plume appears to be moving toward DW-1. In order to move the <br /> site toward final closure,a site hydrogeological model should be evaluated and a fate and transport <br /> model tested to determine if destruction and replacement of DW-1 is necessary,and if not,when the <br /> contamination plume would be attenuated to background conditions if no active remediation is <br /> recommended. Please submit a problem assessment report that includes 1) an updated site <br /> conceptual model,2) calculation of the mass flux of the MTBE plume, 3) a risk assessment for all <br /> potential receptors within a 2000-foot radius of the site (please note specifically"if and when" <br /> if DW-1 would be impacted),and if not,when the contamination would be degraded to background <br /> level,and 4) an evaluation of the potential for indoor soil vapor intrusion to the nearby buildings. <br /> i Please submit this report to EHD by May 15,2005. <br /> If y you have an questions, lease contact Jeffrey Won at 209 468-0335. <br /> Y Yq ,l' J Y' g ( ) <br /> tDonna Heran,RENS,Director <br /> Environmental Health-Department <br /> el <br /> f <br /> Jeffrey Wong,Senior REHS Nuel C.Henderson,Jr.,PG <br /> LOP/Site Mitigation Unit N LOP/Site Mitigation Unit N <br /> i <br /> i c: RWQCB,Central Valley Region—James L.L:Barton,PG <br /> k c: Advanced GeoEnvitonmental,Inc.— Eyrene L. Fisher <br /> t '- <br />� r <br /> i <br />{ <br /> I <br /> i <br /> k <br /> I <br /> f <br /> f - <br />