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George W. Lockwood,PE Page 3 of 4 <br /> Re: Russell Chapin 2009 Petition 07 May 2009 <br /> 1766 Monte Diablo Avenue,Stockton,California <br /> TPHg and Benzene in MW-4 Chapin Bros. <br /> 1000000 <br /> 4/30/1997 SVE on <br /> 100000 <br /> 8/9/2007 GWE on <br /> a <br /> 10000 <br /> CL <br /> CL <br /> c <br /> o t MW4 TPHg <br /> W 1000 1/26/2002 W o MW-4 Benzene <br /> c <br /> /1999 SV <br /> c <br /> 0 <br /> v 100 <br /> 5/10/2005 GW off <br /> 10- <br /> 1 <br /> Ah ob A� e� e9 Ao A° Ati o'� A`O <br /> Monitoring Events <br /> The dates of monitoring events immediately prior to initiating or terminating operation of <br /> a remediation system are labeled on the graphs with enlarged symbols and the <br /> remediation action. <br /> The graphs show significant rebound after each remedial action was terminated, and an <br /> intense response to initiation of each remedial action. On average, dissolved <br /> concentrations of TPHg and benzene in the two wells are an order of magnitude less <br /> during the more recent non-remediation period, which would be a 90% reduction of <br /> dissolved mass, but this has not been shown to correlate to a 90% reduction of sorbed <br /> mass. Factors that probably influence the flux rate from sorbed to dissolved mass <br /> include groundwater elevation, which has varied from approximately 21.5 feet below <br /> surface grade (bsg) in 1994 to 10 feet bsg in 1998, and is currently about 16 feet bsg. <br /> The rising groundwater drowned much impacted soil and removed it from the influence <br /> of the soil vapor extraction system, so that mass has been influenced only by natural <br /> attenuation and soil washing related to the groundwater extraction system, a rate for <br /> neither mechanism has been established. An important point illustrated by the graphs is <br /> that contaminant concentrations rise to high levels when the site is not under active <br /> remediation. This concerns the EHD as the immediate area includes residential and <br /> public school use and groundwater has historically been as shallow as 10 feet bsg. A <br /> vapor intrusion survey has not been conducted. <br /> A final note, Mr. Chapin often includes the statement in his appeals and petitions he <br /> putatively attributes to the EHD that "Cost is a non-issue"; where this came from is not <br /> known, no current staff member of the EHD recalls ever saying or hearing it said; the <br /> EHD strives to be sure that approved actions are cost effective. The EHD has stated that <br /> 2009 Appeal Response 0509 <br />