SITE INFORMATION AND CORRESPONDENCE_1992

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RD/RA Work Plan Memorandum -3- 30 March 1992 DDRW, Sharpe measures, including confirmation of pumping, would be taken if pumping by the agricultural supply wells occurs. Pumping tests may need to be delayed if the agricultural supply wells are pumping. However, we understand that Sharpe is pursuing the discontinuation of the use of the agricultural supply wells. Observation Well Distances. The Work Plan was unclear why 35, 60 and 130 feet were specified for distances of the observation wells from the pumping and injection wells. These distances are specific rather than equal incremental distances and should have been clarified in text. Flow Rates. The Work Plan indicated that flow rates (Q) would be held constant for the injection tests. However, the Work Plan did not specify whether Q would be held constant during the pumping tests. The Q should also be held constant for the pumping tests. Well Design. The Work Plan provided some information on the preliminary design for the injection wells and indicated that final well design would be determined in the field. It is unclear what criteria, such as geophysical logs and/or sieve analysis of sediment samples, will be used to determine final well design. Well efficiency should be of primary concern in the design of the injection wells, as well as for extraction wells. We have previously indicated (by letter dated 17 December 1991) that well efficiency will affect the remedial action and should be considered in well design, as well as other major issues such as screen length and location. Plume Delineation. The installation of monitor wells 524CD and 526CO were proposed for definition of Plume 4&5. These monitor wells will define the vertical extent of contamination in the CD Zone (approximately 180 to 200 feet below ground surface) as they are to be installed in the area of highest probable contamination. However, these wells will not define the lateral extent of the plume. Because more hydraulic information is needed for the CD Zone during the 504 pumping tests, MW-524CD is an appropriate location. However, this well is not needed to define the vertical extent of contamination because MW-504D currently serves this purpose in this area of the plume. The proposed location for MW-526CD should be changed to the area of the 509 well cluster. The deepest well in this cluster is in the C Zone which had a concentration of 81 micrograms per liter (µg/1 ) during the Third Quarter 1991 . Installation of the CD monitor well at 509 will be of more use for the long term monitoring of the vertical extent of contamination than the 526 location and may provide drawdown information during the pumping tests at 504. Replacement Well . The Work Plan indicates that monitor well 475A will be installed as a replacement to MW-407A for the purposes of the pumping tests. The Work Plan did not indicate whether or not MW-407A would remain open or whether this well would be abandoned. Observation Wells for Pumping Tests. Below are listed concerns regarding the proposals and suggestions to improve data collection during the pumping tests. 1 . It is unclear why monitoring of the C Zone is not proposed for the 453A and B pumping tests. The C Zone is to be monitored during pumping tests of other wells in the A and B Zones. The rationale for installation of observation wells should be consistent for all of the pumping tests.