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Jerry Moore 5491 F Street Page 2 of 3 southeast. At this time it appears that the lateral extent of impacted ground water has been determined in all but the down-gradient direction, toward the northeast. The vertical extent of impacted ground water has not been delineated. Noting the nearby down-gradient sensitive receptor, AEI discounted natural attenuation as inadequate to reasonably address impacted ground water. Having achieved a poor radius of influence during the aquifer pumping test, AEI determined that impacted ground water on the site would be best addressed by ozone sparging. In the work plan, AEI proposes to install ten ozone sparging wells with sparging points set at 25 feet below surface grade (bsg). AEI provided no technical justification or basis for the locations or depths of the ozone sparging points. - -- - - EHD concurs with the reasoning to justify the use of ozone on impacted ground water, but needs additional data to justify the number and depths of the proposed sparge points, and the layout of the sparge wells. With an adequate justification for the ozone sparging system design criteria, EHD will approve the WP as an interim remedial measure. A final remediation work plan cannot be approved until the plumes of impacted soil and ground water have been fully delineated and characterized. Please submit a work plan addendum with the requested ozone sparge system information by 15 December 2003. Please submit a work plan by 15 January 2004 to complete the site characterization, addressing the vertical extent and the down-gradient extent of impacted ground water. Also, during the next scheduled ground water monitoring event, please analyze the ground water samples for 1,2-dichloroethane (1,2- DCA) and ethylene dibromide (EDB) in addition to the normal suite of analytes. The requested work plan and future assessment, remediation or closure proposals would benefit greatly if presented in the context of a site conceptual model. Simply stated, the purposes for such a model are to demonstrate where the contaminants came from, where they are at the present time, how they move through the subsurface, how they will respond to changes in the ground water flow characteristics or to potential remediation efforts, what the contaminants' ultimate environmental fate will be, and to help evaluate the risk posed by the contaminants to ground water supplies and sensitive receptors. To be sure that the SCM will adequately address the assessment, remediation and final closure requirements for this site, please include the following in the SCM: • Local and regional plan view maps with sources, boring and monitoring well locations, lines of cross section, extent of contaminants in each media, direction and rate of ground water flow, and receptor locations, including water supply wells within 2,000 feet of the site.