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• Page 5 of 5 <br /> mass to the extent practicable;this leads the EHD to the opinion that implementation of the second 30-day <br /> DPE pilot test may be adequate to address the vadose zone sorbed contaminant mass. <br /> Dissolved contaminant concentrations in some wells increased following the initial 30-day DPE pilot test, but <br /> this may be due to the groundwater extraction (GWE) portion of the DPE drawing more intensely impacted <br /> groundwater to the extraction points. GWE is generally not the preferred method for addressing sorbed <br /> contaminant mass in the saturated zone, but as MTBE is the contaminant with excessive dissolved <br /> concentrations and MTBE has little sorption to soil and is one of the most mobile contaminants, the GWE <br /> portion of the DPE may prove to be effective at reducing the concentrations of this contaminant to acceptable <br /> levels in the source area. <br /> The remediation options reviewed by the EHD, continuing with the second 30-day pilot test or installing a <br /> permanent DPE system, and the associated costs, lead the EHD to the opinion that the second pilot test is <br /> the preferred option, but this should be discussed in detail with your consultant who has worked on the site in <br /> greater detail than the EHD. You mentioned a bioremediation option which the EHD had not been appraised <br /> of at the time of the discussion and therefore cannot comment on it. <br /> The goal of an active remediation system on the site would be to reduce contaminant masses so that the site <br /> will fit the closure profile of sites according to Appendix A of the Tri-Regional Guidelines or the LTCP. The <br /> site has not been reviewed in great detail by the EHD for the discussion earlier today, but based on the <br /> EHD's limited review and knowledge of the site as described above, the EHD is of the opinion that the <br /> second 30-day DPE pilot test should be implemented and evaluated; however, the EHD will carefully <br /> consider your consultant's recommendations and reasoning for how to bring the site to closure. <br /> Sincerely, <br /> Nuel Henderson, PG and Vicki McCartney, Senior REHS <br /> San Joaquin County <br /> Environmental Health Department <br /> 1868 East Hazelton Avenue <br /> Stockton, California 95205 <br /> Phone: (209)468-9852 <br /> Email: vmccartnevasicehd.com <br /> i. <br /> 2/14/2013 <br />