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CONESTOGA-ROVERS <br /> &ASSOCIATES <br /> April 9,2009 Reference No.241511 <br /> -4- <br /> offsite (downgradient) groundwater samples collected during our November 2007 site <br /> investigation(CRA,Site Investigation Report,January 16,2008). <br /> • Quarterly groundwater monitoring performed at the site indicates that the 1,2-DCA j <br /> plume is stable and that 1,2-DCA concentrations in groundwater beneath the site are <br /> declining. <br /> • There are no sensitive receptors onsite. Two water supply wells were identified within <br /> 1/2-mile of the site;one 650 feet north (upgradie t) of the site,and the other is located <br /> approximately 1,800 feet east-southeast of the.site. Neither of these wells is threatened <br /> by residual 1,2-DCA beneath this site. <br /> • According to Table E-1 of the San Francisco Ba Regional Water Quality Control Board <br /> document Screening for Environmental Concerns at Sites with Contaminated Soil and <br /> Groundwater-- November 2007 (revised May 008),the groundwater screening levels <br /> for evaluation of potential vapor intrusion con erns for 1,2-DCA is 200 µg/L for <br /> residential land use and 690µg-/L for commer ial land use. Current 1,2-DCA levels in <br /> groundwater beneath the site range from<0.50 to 6.2µg/L,well below these screening <br /> levels. Based on this there is no risk of vapor ' trusion to indoor air from the residual <br /> 1,2-DCA at this site. Further,the screening levels for benzene,toluene, ethylbenzene, <br /> and xylenes are not exceeded by current conce trations of these compounds in <br /> groundwater beneath the site and therefore dc not pose a risk to vapor intrusion. <br /> Based on the above we recommend no further action At this site and we request that the San <br /> Joaquin County Environmental Health Department evaluate this site for case closure. <br /> If you have any questions regarding this document pl 2ase contact Joe Neely (707-933-2361). <br /> Yours truly, <br /> CONESTOGA-ROVERS&ASSOCIATES <br /> aA <br /> W. <br /> rN0� <br /> JN/fb/1 Encl.-5t <br /> �lrC OF c�►���o�' <br /> World ide Engineering,Environmental,Construction,and IT Services <br />