SITE INFORMATION AND CORRESPONDENCE

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SWMU 4 (continued) lowOU 1 (continued) Evaluation and Selection of Preferred Alternative groundwater would be primarily discharged via injection The original ROD established three remedial action into shallow aquifers using the injection wells and infiltration:I objectives for SWMU 4: galleries(discharge structures consisting of below-ground Prevent the release from sediment of contaminants of ' G; gravel-filled trenches)located on the main depot property. concern(COCs)that could cause surface water concei r' trations to exceed federal Ambient Water Quality Criteria It has not been possible to implement fully the remedy as it for the protection of aquatic life; was intended in the ROD because the ROD significantly • Prevent ecological receptors from being exposed to COCs overestimated the capacity of the subsurface(soil and water above aquatic standards for surface water;and beneath the ground's surface)to accept the treated ground- • Prevent ecological receptors from being exposed to COCl water. Although additional infiltration galleries and in sediment. numerous injection wells have been installed at the site, it has not been possible to discharge sufficient water to The original ROD identified the following three remedi enable the extraction wells to operate properly to contain alternatives for meeting the objectives listed above: the plumes of groundwater contaminants. For this reason, •Alternative 1 —No Action; other options are necessary to supplement the subsurface •Alternative 2—Upstream Source Control;and discharge. •Alternative 3—Limited Excavation; Overflow Weir; Sediment Trap;and Stormwater Monitoring The proposed modification to the original ROD remedy is to add overland flow(surface discharge)as a supplemental The original ROD selected Alternative 3 as the alternative discharge system. that best met the remedial action objectives. Subsurface discharge is still preferred and will be used to The ROD Amendment proposes modifying Alternative 3 the extent possible. Use of both subsurface discharge and to remove the need for the limited excavation of sediments. overland flow will improve plume capture by allowing for The modified alternative is called Alternative 3a. Alternative full operation of the groundwater"pump and treat"system. 3a was evaluated against the NCP criteria listed in Figure 3, and Alternative 3a met all of these criteria. The evaluation DDJC-Tracy is preparing an application for the renewal found that although the original Alternative 3 may provide and revision of Waste Discharge Requirements(WDRs). some minimal improvement in long-term effectiveness, The WDRs must be revised before DDJC-Tracy can Alternative 3a has a significantly greater short-term discharge treated groundwater to the overland flow system. effectiveness and a much lower cost. For these reasons, Also,DDJC-Tracy is preparing an application for a Alternative 3a is preferred for implementation. National Pollutant Discharge Elimination System(NPDES) permit to allow discharge of treated groundwater to the West Side Irrigation Ditch,which is maintained by the West Side Irrigation District. 01.111 The addition of a surface water discharge option would provide more operational flexibility,in case there is a The groundwater at DDJC-Tracy(known as OU 1)has been decrease in the capacity of the infiltration galleries or the affected by various contaminants. The presence of these overland flow system. To help maintain the maximum contaminants in groundwater is believed to be the result of beneficial use of treated groundwater,surface water past depot operations. The contaminated groundwater is discharge will be used only as a"last resort"option. present in a plume that is migrating off site from the depot toward the northeast. This plume of contamination is Evaluation and Selection of Preferred Alternative identified primarily by concentrations of certain volatile The original ROD established three remedial action organic compounds(VOCS);the VOCs of greatest objectives for OU 1: concern are the cleaning solvents trichloroethylene(TCE) • Remediate the areas containing the highest level and tetrachloroethylene(PCE). of contaminants; • Minimize transport of the COCs; and The original ROD selected a"pump and treat"cleanup • Remediate COCs to the levels agreed to in the ROD. remedy for OU 1 groundwater. This remedy consisted of groundwater extraction wells,two groundwater treatment In order to better achieve these remedial objectives,the plants that use an air stripping technique to remove VOCs, ROD Amendment proposes modifying the selected remedy wellhead carbon treatment to remove dieldrin(a pesticide), (extraction,treatment,and discharge to the subsurface)to and reinjection of the treated groundwater into injection include discharge via overland flow and potential discharge galleries. The original ROD specified that the treated to the West Side Irrigation District, continued on page 8 7