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Mr.John&Ms.Maxine Fen�lo 23 April 2009 <br /> 2662 N.Wilson Way Page 2 of 3 <br /> Stockton,California <br /> I <br /> i <br /> The EHD does not agree that the release is not continuing to impact groundwater. The <br /> EHD plotted up the total petroleum hydrocarbons quantified as gasoline (TPHg), benzene <br /> and 1,2-dichloroethane (1,2-DCA) concentrations of the period of monitoring from well <br /> MW-1 and obtained the following graph: <br /> Harry's Auto MWA <br /> 1000000 <br /> 100000 <br /> 10000 <br /> a <br /> CL <br /> CX <br /> m <br /> 0 —MW-1 TPHg <br /> IN-- Benzene <br /> m <br /> —a—MW-1 1,2-DCA <br /> v <br /> c <br /> 0 <br /> U <br /> 100 <br /> 10 <br /> 1 <br /> Monitoring Dates <br /> The graph shows that over the eight-plus years of groundwater monitoring of MWA, <br /> TPHg and benzene concentrations may have declined marginally, and 1,2-DCA <br /> concentrations appear to have actually increased slightly. This implies to the EHD that <br /> the flux rate of contaminants from soil to groundwater is in equilibrium with the <br /> mechanisms of natural attenuation, such as absorption, adsorption, dispersion, diffusion, <br /> and biological breakdown. The forces of natural attenuation cannot be expected to return <br /> the site to background conditions in a reasonable period of time; therefore active <br /> remediation is still required. <br /> The limited data available from laboratory analysis of soil samples show that soil is <br /> intensely impacted from 15 feet to 55 feet below surface grade, and may extend deeper. <br /> With groundwater recently at approximately 56 feet bsg, the documented impacted soil is <br /> in the vadose zone and potentially amenable to SVE. You have already installed two SVE <br /> wells, one air sparging well and three observation wells. The site is prepared for the SVE <br /> pilot test. Successful remediation of the impacted soil in the vadose zone should reduce <br /> the contaminant flux rate to groundwater and enable natural attenuation to start reducing <br /> the dissolved contaminant concentrations. Whether the effect on groundwater will be <br /> Work Suspension Request Response 0409 <br />