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Antonia Vorster -4- 25 August 1989 <br /> No downgradient D-zone wells (toward locations 418/445) exist in the <br /> area of contamination to ensure either no contamination or upward <br /> vertical downgradients. Well 401 D has also been non-detect, but is <br /> also approximately 700 feet upgradient of any contaminated areas. <br /> Hunter/ESE' s contention in their March 1989 Response to Comments that <br /> this shows the D-zone in the South Balloon is uncontaminated is <br /> irresponsible. <br /> b. Plume 2 - the A-zone plume is presently "defined" by wells 441 , 422, <br /> and A-2 (possibly even A-4 which is potentially drawing both the South <br /> Balloon and Plume 2 contamination toward it) . Well 441 is at the <br /> upgradient edge of the apparent contamination source identified by soil <br /> gas results. Wells to define the downgradient and north/northwest <br /> extent, and determine the internal plume levels downgradient of the <br /> source are needed. Proposed well 4A may do the latter if properly <br /> located which cannot be adequately determined from the submitted work <br /> plan (see comment 1) . The comments for the A-zone similarly apply to <br /> the B-zone, except definition is also needed to the south/southwest. <br /> In the C-zone, downgradient confirmation of the lack of contamination <br /> is needed since no downgradient wells exist. Proposed well 14C appears <br /> to be intended for this use but, as best can be made out from the <br /> submitted maps, seems considerably north of the expected plume <br /> centerline. No wells exist in the D-zone at present; future need of <br /> D-zone wells may be made contingent upon the C-zone results. <br /> C. Plume 3 - proposed wells such as the 12A-C cluster may be the <br /> appropriate step in this area but location 12 on the March 1989 <br /> submittal does not appear to be between the plume and downgradient <br /> agricultural wells ( location 12 is not even shown on the Work Plan <br /> maps) . The past and present use, and possible influence, of AGW-1 <br /> should also be assessed to determine if definition to the south is <br /> necessary. <br /> d. Plume 4 - based on soil gas results, this area contains five potential <br /> contaminant sources at E61 , I70, AK85, near well 442, and uparadient <br /> of well 443; the two sites near wells 442 and 443 being confirmed <br /> sources. Since these potential/actual sources are contiguous and/or <br /> upgradient from one another, any resultant contaminant plumes probably <br /> overlap. The degree of definition in this area, and the following <br /> comments are very dependent on what mitigation measures will be proposed <br /> (read: extent of ground water extraction zone) . <br /> The first two sources (E61 and I40) need wells to determine if <br /> contamination has resulted; proposed wells 4A-B presumably were for this <br /> purpose, but now it is unclear whether 4A-B are to gain information <br /> about this area or Plume 2. The A-zone contamination at both 442 and <br /> 443 needs lateral definition. It appears that 6A-B may partially <br /> accomplish this downgradient of 442, but wells 5A and 13A-B (again, as <br /> best as can be determined from the maps) appear to be located off-line <br /> to the north of the plume. At the potential source at AK-85, wells are <br />