SITE INFORMATION AND CORRESPONDENCE_1988-1991

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Ground Water FS Report -8- 16 September 1991 DDRW, Sharpe Site and selenium contaminated ground water (Table 3.2-5) . This discrepancy needs to be clarified. 21. Evaporation ponds with connector wells were retained as a viable disposal alternative for both the remediation of VOC and arsenic-selenium contaminated ground waters (Tables 3.2-2 and 3.2-5) . Part of the preliminary design of this alternative is to line the pond and to have the gravity feed connector wells around the outside perimeter of the pond, at various depths with direct discharge to the aquifer. It is unclear why the evaporation pond will be lined when part of the preliminary design also includes connector wells which will "infiltrate" treated ground water into the aquifer. It is also unclear why the pond was not designed with the connector wells in the base of the pond. If part of the design of this alternative is to use infiltration in conjunction with evaporation, then the pond should not be lined. The pond should be unlined to promote infiltration from this large body of standing water and to thus, increase the amount of water which can be discharged by using this alternative. The pond should be redesigned to perform as an evaporation/percolation pond. 22. The Draft FS Report did not clearly indicate whether the evaporation pond with connector wells would only be used for the disposal of treated ground water. If the evaporation pond is to be used for the disposal of storm water, then more stringent monitoring requirements should be required because the quality of the storm water run-off at the site is currently unknown. In addition, it was my understanding that the oxidation pond at the eastern end of the site was to be converted to a storm water retention basin. Sharpe needs to clarify if the evaporation pond and the storm water pond are to remain as two separate waste management units. 23. The proposed location of the evaporation pond with connector wells is between D and F Streets and 1st and 2nd Avenues on the Sharpe Site (Section 5.11.2) . This location is upgradient from Plume 7&8 (North Balloon Area) . The Draft FS Report states that this location for the pond and wells should not significantly alter the direction of ground water flow. However, this location of the pond and wells may alter the ground water gradient. The preliminary modeling of the evaporation pond and connector wells indicates that there will be mounding of the ground water table due to infiltration from the connector wells of the disposed treated ground water. This mounding effect may cause the gradients to increase. If the gradient increases, the velocity of ground water flow will also increase, which in turn will increase the rate of the plume migration. Although the Draft FS Report acknowledges that the gradient will steepen (page 5-58) , the North Balloon extraction system has not been demonstrated to have complete plume capture. Without complete plume capture, potential negative changes on the contaminant migration cannot be assured. It is premature to state that the increase in the hydraulic gradients would not significantly change the capture zones. Therefore, prior to the installation of the pond and wells, Sharpe needs to redesign the North Balloon extraction system so that the potential increase in the rate of plume movement will be accounted for in the final remedial design.