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M OJ C'f') N C' o SALINE GROUND-WATER EXTRACTION Following initial efforts of control 1 iTICJ LitLlefil'ld Springs dlld abandonment of that approach as discussed in the previous chapter, attention turned to the feasibility of extracting saline ground water from under the riverbed and adjacent flood plain. Duri ng peri ods of low flow between May and October the major source of water to the Lower Virgin River is the Littlefield Springs. The average flow at the USGS gage at Littlefield is approximately 65 ft3/s, 2,500 mg/L TDS. Note that the decrease in TDS at Littlefield is due to calcium carbonate precipitation and low TDS Beaver Dam Wash inflow. During the low flow period the only surface flow of water out of the basin into Lake Mead occurs during flood events. During this time, the river infiltrates into the streambed below Riverside, Nevada. Part of the infiltrated water is consumed by vegetation along the banks of the dry river; the rest enters Lake Mead as underflow. It was postulated that all the salt associated with the river at the Littlefield gage flowed downstream and entered Lake Mead with the underflow in the river. If this underflow could be intercepted, the 80,000 tons of salt in this underflow could be removed. Calculations indicated that based on an estimated 5 ft3/s of underflow the water would contain 32,000 mg/L of dissolved salts. Ground-Water Barrier Concept The estimated presence of shallow ground water with high salinity suggested the following plan. An underground water barrier could be constructed in the alluvium under the flood plain across the river channel. This would block the downstream migration of the saline ground water and permit its extraction from the alluvium by shallow wells. The barrier would be flush with the surface of the riverbed and flood plain so it would not interfere with high flows in the river. This is particularly important in the spring when high flows 14