SITE INFORMATION AND CORRESPONDENCE

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WASTE DISCHARGE REQUIREMENTS -2- DEFENSE DISTRIBUTION REGION WEST TRACY SAN JOAQUIN COUNTY monitoring will be required during the first three months of full scale operations to ensure that the waste characterization is complete. In addition, periodic (quarterly) waste characterization monitoring of the waste, during the operational phase, will be required. 7. The determination of the extent of the ground water contamination, both on-site and off-site, is continuing. Seventy-four monitoring wells have been installed, an additional 12 monitoring wells are currently under construction. Six contaminant source areas have been identified. These source areas are believed to be associated with past solvent storage, handling, and use practices, rather than ongoing sources (i .e. , underground tanks and leaking pipelines). Ground water contamination is predominantly within the upper 100 feet, although there are indications that TCE and PCE are moving vertically downward (to 150 feet) as the plume migrates to the northern off-Depot areas. 8. The objective of the interim ground water cleanup system is to control the migration of a portion of the most heavily TCE/PCE polluted ground water along the northwest boundary of DDRW, and to provide hydrological and water quality data to evaluate the system' s effectiveness and its potential application to other areas on the Depot as part of the site' s final remedial action. 9. A detailed description of the interim ground water cleanup system is contained in a report entitled "Work Plan - Interim Ground Water Withdrawal , Treatment, and Disposal System for DDRW" , dated 20 November 1989, and within Section C (Statement of Work) of the Defense Logistics Agency contract with Ensotech, Inc. , dated 26 September 1989. 10. The main components of the Dischargers' interim ground water cleanup system include the following: a. The extraction of approximately 120 gpm of contaminated ground water from six extraction wells. The extraction wells will be screened within the uppermost sand and gravel aquifer along the northwest boundary of DDRW. b. The treatment of the extracted contaminated ground water by air stripping. The air stripping tower's hydraulic design capacity is 500 gpm. The system has been designed to achieve a 99.8 % removal efficiency. A vapor control unit, utilizing granulated activated carbon, will be used to remove trace organics from the tower's exhaust air. c. The disposal of the treated ground water (approximately 120 gpm) to three shallow aquifer injection wells and/or to the existing storm drain holding (percolation and evaporation) pond during the summer months (May through October).