SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Harold and Dena Knowles 102 S.Wilson Way Page 3 of 4 Without considering the methyl tert-butyl ether (MTBE) issue, total petroleum hydrocarbons as gasoline (TPH-g) locally has concentrations as high as 260,000 µg/I, benzene at 18,000 µg/I, and 1 ,2-DCA at 940 µg/I. As the odor and taste threshold for TPH-g is 5 µg/I and the maximum contaminant levels (MCLs) for benzene and 1,2-DCA are 1 µg/I and 0.5 µg/I, respectively, EHD believes that interim remediation of impacted ground water should be initiated pending acceptance of a final remediation plan once the impacted media have been fully delineated. GZA has presented a model of the MTBE plume encroaching on your site from the 101 S. Wilson Way and/or the 130 S. Wilson Way sites and has proposed a meeting to develop an effective and equitable approach to cleanup of the plumes. EHD is willing to facilitate such a meeting and will recommend it to the various parties and will also recommend that each responsible party develop a model that encompasses lithological, analytical and hydrological data from all three sites to compare concepts at the meeting. GZA also has requested a reduction of monitoring frequency, but EHD believes that with the MTBE plume apparently in a dynamic state now, EHD cannot approve such a reduction. At this time you are directed to: • Test the SVE system as indicated above, to determine the hydrocarbon mass extraction rate for the overall system and for individual wells; • Compare the estimated extracted mass to the estimated original hydrocarbon mass; Model the long-term trend of the hydrocarbon mass extraction rate and determine what the mass removal limit is likely to be; provide the assumptions and calculations to EHD; Prepare a site conceptual model (SCM) that demonstrates contaminant origins, migration pathways, and geological controls on contaminant distribution, a more detailed SCM description is included below; Prepare a work plan in the context of the SCM to complete the site assessment, paying particular attention to the vertical extent of impacted media; and Include analysis for 1,2-DCA in the analytical regimen for the site. EHD has listed below the components generally used in an SCM. Simply stated,' the purposes for an SCM 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. The components of an effective SCM commonly include: