SITE INFORMATION AND CORRESPONDENCE

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WASTE DISCHARGE REQUIREMENTS -2- DEFENSE DEPOT TRACY SAN JOAQUIN COUNTY contaminants within the groundwater. Additional waste characterization monitoring will be required during the startup phase of the interim ground water cleanup system 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 groundwater contamination, both onsite and offsite, 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 groundwater cleanup system is to control the migration of a portion of the most heavily TCE/PCE polluted groundwater along the northwest boundary of DDT, 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 groundwater cleanup system is contained in a report entitled "Work Plan - Interim Groundwater Withdrawal , Treatment, and Disposal System for DDT" , 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 groundwater cleanup system include the following: (a) The extraction of approximately 120 gpm of contaminated groundwater from six extraction wells. The extraction wells will be screened within the uppermost sand and gravel aquifer along the northwest boundary of DDT. (b) The treatment of the extracted contaminated groundwater 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 groundwater (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).