SITE INFORMATION AND CORRESPONDENCE_1988-1991

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Remedial Investigation Report -7- 8 March 1991 DDRW, Sharpe increase seemingly as a result of rainfall infiltrating into the aquifer, suggesting that the sources of contamination may still be active. RESPONSE 14. Finding is noted. It is unclear whether the past increase in TCE concentration was observed only in the A zone or in all of the zones. It is also unclear whether the term "past" specifically refers to the contaminant concentration prior to implementation of the South Balloon Ground Water Treatment System in 1987 or the entire historical record. Based on the text and graphics in Chapter 6 of the Report, it appears that the contaminant concentrations in the North Balloon Area appear to be decreasing, not increasing. FINDING 15. VOC concentrations are declining in almost all South Balloon Area Wells. Short- term increases in VOC concentrations in South Balloon source area wells, previously observed and ascribed to continued release of VOCs from contaminated soils, have been completely overshadowed by the reduction in concentrations due to the operation of the South Balloon Area Groundwater Treatment System. RESPONSE 15. I concur with this finding. FINDING 16. VOC contaminant concentrations in the downgradient part of Plume 4 and 5 are higher in the deeper aquifer zones in striking contrast to the other areas where the A-zone aquifer is almost always the most highly contaminated. This pattern is most probably due to the presence of strong downward gradients associated with agricultural pumpage offpost to the west. RESPONSE 16. I concur with this finding. It is also possible that this is an older plume so that there has been more time since the waste disposal occurred for the contaminants to migrate downward through the aquifer. FINDING 17. Seven areas of VOC-contaminated soil have been identified, containing an aggregate of more than 26,000 yd' of soil contaminated above 100 µg/g. RESPONSE 17. Finding is noted. This number may be an accurate estimate of the VOC contaminated soils above 100 µg/l . However, the volume of contaminated soils above background would be much greater. The background concentration of VOCs is considered to be non-detect because these constituents are not naturally occurring. In addition, it is unknown why 100 µg/l is significant. An environmental fate analysis, such as the Designated Methodology, must be performed to determine the concentrations of residual waste constituents which pose a threat to water quality.