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ENVY <br /> A M E R I C A <br /> Mr. Duane White <br /> California Department of Toxic Substances Control <br /> September 24, 2004 <br /> Page 3 of 6 <br /> G. More than 12 years of groundwater monitoring data have been collected from <br /> the Site to date. Hydrogeologic conditions at the Site are well understood. <br /> H. The COC concentrations in groundwater are generally stable and/or declining. <br /> I. Natural attenuation (including biological, physical, and chemical processes) of <br /> CDCs in groundwater at the Site is occurring and will continue to reduce the <br /> mass and mobility of the residual COCs remaining in the subsurface. Individual <br /> COCs in groundwater will attenuate at different rates depending on various <br /> factors, including the groundwater zone, proximity to the former source areas, <br /> and remediation activities completed. <br /> J. For example, benzene concentrations in MW-9/9R have been declining <br /> consistently since 1992 (see the following graph). The benzene concentrations <br /> in 2004 are three orders of magnitude less than those in 1992. Based on the <br /> existing data, the benzene concentrations in this well are estimated to attenuate <br /> at a rate of approximately 0.14 percent per day and are anticipated to meet RAO <br /> in 2008. <br /> Benzene Concentrations in the Groundwater Samples Collected from MW-919R <br /> Former MGP Site,Stockton, California <br /> 10000 <br /> j <br /> 1000 - <br /> Z <br /> n <br /> E 100 <br /> a <br /> QExponential Trendline <br /> 10y= 5E+23eonol.a:._._ <br /> R2=0,9121 <br /> O <br /> E <br /> c0 -RAO=0.5 p9/L —.__ ____-___.._.._ ..___. <br /> 0.1 <br /> w m w w w w w m w w o 0 0 <br /> t t t c t t t t c c t t t t <br /> Date <br /> CADocuments and settings\miavieAl-oval Settings\Temporary Internet Files\OLK86\DTSC response Itr Sep2004 revised.doc <br />