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'. -' r... 00 N N .~ GENERAL DESCRIPTION OF PROPOSED GROUND-WATER FLOW REDUCTION FEATURES o The slurry trench/wall would form a series of basins along the south side of the Wash, as shown in Drawing No. 1297-300-171. The detention basins would diminish subsurface flow, and help retard the tributary brackish surface water as it flows parallel to the sewage effluent flow on the north side of the Wash. High flows of fresh storm runoff filling any detention basin would spill across to the north side of the Wash. Thus, the detention basins would impound ground water and help attentuate fluctuations in surface tributary inflow. The evapotranspiration capacity of the existing vegetated areas within the proposed detention basins is expected to exceed the annual average tributary inflow. However, the flushing action of intermittent storm runoff is expected to continue to maintain near surface salinity levels tolerable to the vegetation. In effect, minimal changes in existing vegetated or inundated land area are expected within the detention basin system. In some areas more salt tolerant plants may replace existing cattails. The system would be conducive to management (if desired) for a diversity of vegetative communities. Existing wetlands on the north side of the Wash would be unprotected by the detention basin system and subject to drainage by natural erosive processes. The sewage treatment plant effluent is expected to be less dense at equivalent temperatures due to a reduced salt pickup. At the same temperature, a less salty effluent inflow current in Las Vegas Bay (Bay) may ri se closer to the water surface. The dens ity current may then stimulate greater algae growth. Although increased algae growth may support a more productive food chain to the benefit of the fishery, it is generally considered an adverse aesthetic impact to body contact recreation. To compensate for loss of salt induced density, temperature of the density current may be lowered. Prior to mixing with Colorado River water in the Bay, the Wash water may be cooled by increased exposure to ambient air and/or Bay water temperatures. Estimates of Wash discharge and salt load are shown on Table 1. The estimate for salt load reduction for future years depends on the adverse salinity impact of erosion control that may be implemented by other agencies. Evidence to date indicates a reduction in salt pickup with continued headcutting of the erosion channel. Since the impacts of possible erosion control attempts are uncertain, a total load reduction is shown in Table 1 for both erosion control and no erosion control conditions.