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' S <br /> gy <br /> Jerry Moore <br /> 5491 F Street I <br /> Page 2 of 3 <br /> " I <br /> southeast. At this time it appears that the lateral extent of impacted ground water i <br /> has been determined in all but the down-gradient direction,. toward the northeast. <br /> The vertical extent of impacted ground water has not been delineated. <br /> F <br /> Notingthe nearby down-gradient sensitive receptor, AEI discounted natural <br /> Y , . <br /> inadequate to <br /> reasonably address impacted round water. Having <br /> attenuation as inade y P 9 <br /> a q <br /> achieved a poor radius of influence during the aquifer pumping test, AD <br /> determined that impacted ground water on the site would be best addressed by <br /> ozone sparging. In the work plan, AEI proposes to install ten ozone sparging <br /> wells with sparging points set at 25 feet below surface grade (bsg). AEI provided <br /> no technical justification or basis for the locations or. dept�s of the ozone <br /> sparging points. <br /> EHD concurs with the reasoning to justify the use of ozone on impacted ground <br /> water, but needs additional data to justify the number and depths of the proposed <br /> i <br /> sparge points, and the layout of the sparge wells. With an adequate justification } <br /> for the ozone sparging system design criteria, EHD will "approve the WP as an <br /> interim remedial measure. A final remediation work plan cannot be approved until + <br /> the plumes of impacted soil and ground water have been fully delineated and <br /> �- characterized. Please submit a work plan addendum with the requested ozone <br /> sparge system information by 16 February 2004. j <br /> Please submit a work plan by 15 March .2004 to complete. the site <br /> characterization, addressing the vertical extent and the down-gradient extent of. <br /> impacted ground water. Also, during the next scheduled ground water monitoring <br /> event, please analyze the ground water samples for 1 ,2-dichloroethane (1,2- <br /> DCA) and ethylene dibromide (EDB) in addition to the normal suite of analytes. <br /> The requested work plan and future assessment, remediation or closure 1 <br /> proposals would benefit greatly if presented in the context of a site conceptual <br /> model. Simply stated, the purposes for such a model are to demonstrate where i <br /> the contaminants came from, where they are at the present time, how they move <br /> through the subsurface, how they will respond to changes in the ground water <br /> flow characteristics or to potential remediation efforts, what the contaminants' <br /> ultimate environmental fate will be, and to help evaluate the risk posed by the <br /> contaminants to ground water supplies and sensitive receptors. To be sure that <br /> the SCM will adequately address the assessment, remediation and final closure <br /> requirements for this site, please include the following in the SCM: <br /> Local and regional plan view maps with sources,' boring and monitoring <br /> • well locations, lines of cross section, extent of contaminants in each i <br /> media, direction and rate of ground water flow, and receptor locations, <br /> including water supply wells within 2,000 feet of the site. <br /> 1 <br /> t <br /> r <br />