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DAN KIRK <br /> PAGE 2 <br /> To date, vapor extraction technology has been implemented at the site with a significant volume of <br /> hydrocarbons being removed and destroyed. However, given the existing design, this system has <br /> likely reached its full potential at this time. There are several design changes available that may <br /> improve the efficiency of the current system. These include the evaluation of the influence of the <br /> current vapor wells at the site and the potential enhancement of hydrocarbon removal efficiency by the <br /> installation of additional vapor extraction wells, possibly with more effective screen intervals. Test <br /> wells can be installed initially to determine if additional and/or differently designed wells would be <br /> productive and cost effective. Data obtained from the well heads should be provided to support the <br /> efficiency or inefficiency of the vapor extraction wells. In addition, an air sparge test well should be <br /> installed and tested to determine if this technology would be a cost effective approach to increase <br /> hydrocarbon removal from the subsurface. Also, the utilization of passive air inlet wells (to enhance <br /> subsurface air flow towards operating wells) should be considered. The option proposed by Fugro of <br /> the addition of dissolved oxygen to the groundwater can also be compared with these other <br /> alternatives for efficiency and cost effectiveness. <br /> The type of hydrocarbon contain ment/destruction equipment used at the site should be determined by <br /> the mass of hydrocarbons needing containment/destruction. The type of technology implemented at <br /> the site must be based on the evaluation of feasibility studies of several different applications of <br /> technology. Also, any change in technology at this site will require an additional public participation <br /> period in accordance with state law. <br /> Therefore, PHS/EHD cannot concur with this request for a change in technology without the benefit of <br /> feasibility studies of this and other technologies as discussed above. However, PHS/EHD appreciates <br /> your aggressive approach to the site remediation. The additional efforts described above should not <br /> excessively add to the overall cost of the project or to the duration to site certification if implemented <br /> soon. These efforts should be considered one of the concluding phases to the ultimate goal of site <br /> certification. <br /> If you have any questions or if you would like to discuss this letter in more detail, please contact Linda <br /> Turkatte, Senior REHS, at (209) 468-3441. <br /> Donna Heran, REHS, Director <br /> Environmental Health Division <br /> Linda A. Turkatte, Senior REHS Diane M. Hinson, REHS <br /> Site Mitigation Unit Supervisor <br /> LT <br /> c: CVRWQCB, Beth Thayer <br /> c: Fugro West Inc, David Poulsen <br />