SITE INFORMATION AND CORRESPONDENCE_1988-1991

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Extraction Well Memorandum -2- 17 December 1991 DDRW, Sharpe Site Page 2-10. The Plan indicates that the extraction well system was designed on the premise that the nearby agricultural supply wells will be closed. Because the extraction system is designed based on this assumption, Sharpe may ultimately need to install additional extraction wells to assure complete plume capture in the event that the agricultural supply well pumpage does not cease in the near future. The Work Plan clearly confirms that the agricultural wells exert hydraulic control over the western portion of the VOC plumes. An extraction system to counteract this problem would need to pump large volumes of groundwater and would require large increases in the capacity of the treatment plant design, thereby causing significant increases in the cost of the remediation. However, this option should be developed, including a cost estimate, in order to evaluate all options and to have a basis for comparison of the costs associated with cessation of the agricultural pumpage. Page 2-11 . The Work Plan states that extraction well NA-7 was installed too deep (in the B Zone) and is not functioning as intended. Therefore, Sharpe plans to install another extraction well , NA-8, in the A Zone. However, installation of NA-8 may not be entirely necessary. The analytical data from the nearby monitor well cluster 521 indicates that there are not significant differences in VOC concentrations between the A and B Zones. The Second Quarter 1991 data show the concentration of trichloroethylene (TCE) in MW-521A to be 11.7 micrograms per liter (µg/l ) and to be 5.8 ug/l in MW-5218. If the B Zone has a high enough transmissivity, Sharpe could consider pumping NA-7 at a higher rate which would affect plume capture in the A Zone because the zones are hydraulically connected. This may be more cost effective option. Page 2-11. Sharpe intends to install an extraction well , NA-9, near MW-456A where the concentration in the Second Quarter 1991 was reported to be 145.3 µg/l . An extraction well , NB-5, is also planned for installation in the B Zone near monitor well cluster 456. The TCE concentration in MW-4568 in the Second Quarter 1991 was 391.7 µg/l . (The TCE concentration in MW-456C was 3.9 µg/l . ) It is unclear why two extraction wells are needed in the A and B Zones near monitor wells cluster 456, when the installation of one extraction well with a longer screen length (than ten feet) may suffice. Sharpe needs to consider installation of one well with a long screen length near MW-456 with the screen length determined by the lithology. Page 2-13. The Plan states that an extraction well , NB-4, is planned for installation near monitor well cluster 438. The TCE concentration in samples from MW- 438A, B and C in the Second Quarter 1991 were 15.8, 12.1 and 18.8 µg/l , respectively. Because the contamination appears to be evenly distributed throughout the three zones at this location and based on the configuration of the plume as depicted in the figures in the Work Plan, it may be more effective to move NB-4 to the southeast to be more centrally located in the plume. The screen length of NB-4 should be determined based on the lithology, with the objective to extract from the A, B and C Zones as confirmed by nearby surrounding monitor well clusters.