SITE INFORMATION AND CORRESPONDENCE FILE 2

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Rationale for Continued Excavation Background Wright performed an investigation regarding contaminant extent and the search for potential additional contaminant sources in 2000 and 2001 prior to starting corrective action approved in the October 10, 2003 work plan. The primary historic source of petroleum fuel contaminants was the former on-site underground storage tanks near to and slightly south of Well MW-3. To date no data has been discovered that supported the possibility of additional sources. Additional soil and groundwater sampling was performed in June 2000 and June 2001 to address the extent of capillary fringe and dissolved groundwater contamination. This included borings located for determining the depth of contaminant penetration to depths of between about 25 and 30 feet. The boring locations presented in reports were located in the suspect areas mainly around MW-3 and MW-9. Chemical analysis data showed that some capillary fringe data indicated contaminant presence, and field soil vapor screening data did indicate some vadose-zone and capillary fringe contaminants in the 7 to 10 foot depth range (for example; push boring data for logs PB-15, PB-16, PB-17, PB- 18). Historic groundwater rise and fall in periodic groundwater monitoring affect capillary fluctuations of contaminant smear up an additional foot in the vadose zone, in our view. On the basis of these borings and the chemical analytical data collected from them, the interpreted area of potential contaminated soil estimate was projected at 300-Tons (188/cy) for the October 10, 2003 approved work plan. This cleanup was to primarily address on source removal with some limited groundwater extraction to attempt to "flush" mobile dissolved contaminants for removal. Some vadose contaminants were indicated by soil vapor data and by one soil sample. The extent of excavation is interpreted from the data, but the data itself is limited to the boring locations. Conclusions and Recommendations Current excavation has revealed additional discolored soil that contains at least some contaminant in the soil above and at the capillary fringe interval between MW-9 and MW-3. This area was not extensively sampled; however it was denoted as potentially contaminated in previous reports. Previous experience with soil excavations of similar types of contaminant problems indicates that the actual contaminant extent can be more widespread than interpreted from boring and sample analysis data. Wright calculated a minimum volume 300-Tons, (188/cy), on the basis of the quantitative chemical analysis data from the borings. While some additional contaminated soil was inferred from soil screening vapor data, soil vapor data is qualitative. The estimate of subsurface soil extent is variable and relies primarily upon boring location and the quantitative chemical analysis data collected. The estimated extent is interpreted using assumptions of extent and cannot be guaranteed given the actual limited data available. Page 2