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URS ' Ms Margaret Lagorio Page 3 San Joaquin County Environmental Health Department ARCO 760 February 15, 2006 Mass estimates for baseline groundwater contamination are based on historic data collected in 2002 Present day mass estimates are based on current groundwater monitoring results (Tables 1 and 2) Figures showing the plume dimensions used for the historic and present-day mass estimates, tabulated calculations, and a list of conditions and assumptions are present in Attachment F Historical groundwater data suggest initial soil masses for TPHg, MtBE, and 1,2-DCA were �} approximately 1 58 pounds, 0 003 pounds, and 0 189 pounds,respectively Present-day groundwater data suggest soil masses for GRO, MtBE, and 1,2-DCA are approximately 172 pounds, 0 006 pounds, and 0 082 pounds, respectively SENSITIVE RECEPTORS The sensitive receptor survey conducted at the site identified three wells in the vicinity of the site within a 1/z-mile radius Only one water supply well location was identified approximately 1/a rile north of the site The map with the radii and wells identified is included in Attachment G City of Lodi municipal i well#1R, as reported by the SJCEHD, is located approximately 812 feet east of the site r • CONCLUSIONS AND RECOMMENDATIONS The following information fulfills the RWQCB Central Valley Region's outstanding requirements for site closure The RWQCB checklist for No Further Action at UST sites is included as Attachment H o All USTs and appurtenances have been removed from the site The extent of soil and groundwater contamination has been ascertained from the investigations conducted at the site � a Total mass of 1,2 DCA in groundwater has been reduced from 0 19 pounds in 2002 to 0 08 pounds in 2005 Existing MtBE concentrations in site wells are below the maximum contaminant levels, and benzene is not detected at the site at or above the detection limit Based on the review of historical soil and groundwater investigations, URS requests that ARCO Station �} No 760 be considered "low risk" and recommends closure and No Further Action A checklist of data required by the RWQCB Central Valley Region for no further action at UST sites is presented as Attachment H This document is based on data, site conditions, and other information that are generally applicable as of the date of the document The conclusions and recommendations herein are therefore applicable only to that time frame K 1WprocesW5 BP ARC01007601Closure Rpt1760 Closure Report doc