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9. Pa. 10-2. finding #10. "It does not appear that volatile contaminants, consisting primarily of TCE, originating on SHAD have migrated offsite to a significant degree. It is currently estimated that contaminated groundwater with TCE concentrations in excess of 150 ug %L are currently confined entirely to SHAD. The main South Balloon Area VOC plume does not extend offpost." TCE levels of 5 ppb are considered significant by U.S. EPA, and 1,2-DCE may be significant at even lower levels [proposed maximum contaminant level (PMOL) - 0.07 ppb.]. 10. Page 10-2. finding #12. "Although TCE is the dominant VOC at SHAD, one well cluster (413) exhibits higher concentrations of another analyte (TCLEE). This apparently represents a separate source of VOC contamination.' Future soil investigations should attempt to locate the source(s) of PCE (TCLEE). 11. Pg. 10-3. finding #16. "VOC concentrations are declining in almost all South Balloon Area wells. Short-term increases in VOC concentrations in South Balloon Area 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 operations of the South Balloon Area treatment system." Decreases might be attributable in part to water table lowering. Alternative explanations for a short term increase might be that a previously unidentified plume in the vicinity of an extraction well was intercepted, or that contaminants are not homogeneously distributed within the aquifer. If the latter is true, contaminants in the most permeable zones might be extracted easily at first, leaving contaminants in the less permeable zones to be extracted at a much slower rate. SHAD could test this by turning off the extraction system for a period of several days and then noting whether or not effluent concentrations increase when the system is started up again. 12. Pg. 10-3. finding #17. "VOC contaminant concentrations in the Runway Area are higher in the deeper aquifer zones, a striking contrast to the other areas where the N.