10-10-1988 - Agenda Packet City Council - regular meeting

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5.0 RECOMMENDATIONS 5.1 General Delta recommends that a ground water recovery program be initiated due to the contaminant concentrations identified at this site. Based on site hydrogeologic conditions, we believe that a recovery trench system would yield the best contaminant volumes. We propose to install a recovery trench pump out system at the WSI site as shown in Figure 4. The effectiveness of the trench system will be evaluated. Modifications to the system and/or the need for additional recovery systems will be assessed. 5,2 Conceptual Design The east end of the recovery trench will contain a large diameter sump which will be equipped with a single total fluids submersible pump. The pump will route water from the trench sump to a diffused aeration system for treatment of volatile organic compounds. The treated discharge water will then flow into the sanitary sewer system. The conceptual design of the recovery system and the anticipated TCE mass balance of the system are shown in Figure 5. 5.3 Recovery Trench Design The recovery trench will extend from near monitoring well MW-2 toward the east to MW-4 (Figure 4). The total length of the trench will be approximately 250 feet and it will have a depth of approximately 18 feet. A cross -sectional sketch of the trench is provided in Figure 6. The width of the trench will be dependent on the slope stability encountered during the excavation. Drain tile will be placed near the bottom of the trench to enhance drainage toward the sump. The trench will be aackfilled with coarse washed gravel to a depth of approximately five feet below the existing ground surface (Figure 6). A plastic liner will be placed over the washed gravel to inhibit surface water infiltration and to prevent fine grained materials from entering the drainage bed. The top five feet will be backfilled with clean soil removed during the trench installation. Soil removed during the trench installation exhibiting contaminant concentrations above five parts per million (ppm) total organic vapors, as detected by the hNu photoionization meter, will be stockpiled and covered on -site. The recovery sump will be located at the eastern end of the recovery trench and will extend to a depth of approximately 18 feet below the ground surface. The top of the sump will be sealed with a lockable well cover. The sump will consist of a 12 inch casing slotted with 1/8 inch wide by three to six inch long slots cut at 45 degree angles. A cross section of the proposed sump is provided in Figure 7.