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to.) Q ~ en dike around the points of eruption to collect the discharges and then convey the water by pipeline to an evaporation pond some 3 miles away. Two alternatives will be presented for construction of the retaining dike. The less expensive of the two would use a concrete wall for impounding the water .at the geyser site as compared to a more costly but more esthetically pleasing soil-cement dike in the second plan. LAS VEGAS WASH UNIT, NEVADA ,. Las Vegas Wash serves as a surface drain for all domestic, municipal, and industrial wastewaters from Las Vegas Valley. The average annual discharge from the Wash to Lake Mead is about 38,000 acre-feet which carries approximately 208,000 tons of dissolved solids. Because of the rapidly increasing population of the valley, the annual discharges of water and salt are expected to increase in the next few decades. One alternative would collect ground-water flows at a natural "barrier" with a grouted curtain wall and a series of perforated pipes. The collected discharges could be pumped to a nearby reverse osmosis desalting plant. Manmade surface flows in the Wash would be diverted and conveyed around the ground-water collection site. The product water from the desalting plant would be joined with the surface flows for invalley use or release to Lake Mead. The brine residue would be placed in a lined evaporation pond near the project site or pumped to Dry Lake northeast of Las Vegas for evaporation. Other alternatives such as export to Dry Lake for total evaporation are still under investigation. It is estimated that the Las Vegas Wash Unit would remove 138,000 tons of salt per year from the Colorado River System resulting in salinity reductions of about 13 mg/I at Imperial Dam. A special report showing possible solutions to the problem was completed in January 1974. Feasibility investigations on the Las Vegas Wash Unit are scheduled through FY 1976. LaVER KIN SPRINGS UNIT, UTAH The LaVerkin Springs are located in a l,800-foot-long reach of the Timpoweap Canyon of the Virgin River in southwestenl Utah. The springs discharge about 8,300 acre-feet of water and 109,000 tons of salt each year. A feasibility study has been prepared that shows 103,000 tons of th is salt could be removed annually I which would reduce total dissolved solids concentrations at Imperial Dam by about 9 mg/1. A feasibility report on the LaVerkin Springs Unit is undergoing an inhouse review at the present time. One alternative calls for the construction of an upper diversion dam upstream from the springs to divert the normal riverflows around the area of the springs. The lower control dam would be located just below the springs to form a pool from which the springs' flows would be pumped to the LaVerkin Desalting Plant, The average inflow to the reverse osmosis desalting plant would be 11.5 cfs through a 21.inch concrete pressure pipe. It will be necessary to pretreat the feedwater in the form of calcium reduction, cooling, and filtration. A desalting alternative might consist of three stages of vessels containing semipermeable membranes. Since the recovery of some water in each stage would increase the brine concentration passed to the succeeding stage, increasingly higher pressures would be required to effectively recover additional water. A product water of 8.3 cfs would be recovered at a salinity concentration of 500 mg/1. A brine discharge of 2.8 cfs would be at a concentration of 33,000 mg/1. The product water would be returned to the Virgin River through a 1,BOO-foot-long pipeline. A 12-inch pipeline almost 19,000 feet long would be used to pump the brine to a 440-acre evaporation pond formed by diking a natural depression about 4 miles north of Hurricane, Utah. The dikes would be rolled earthfill embankments lined with 10-mil polyvinyl chloride. The entire pond would be lined with 10-mil polyvinyl-chloride sheeting covered with 12 inches of earth, PALO VERDE IRRIGATION DISTRICT UNIT CALIFORNIA There are about 91,400 acres irrigated in the locally developed district along the Colorado River in California. The irrigation of the district lands results in a salt pickup of about 148,000 tons of salt pe' year. The irrigation management services and water systems improvement programs should result in a reduction of about 23,000 tons in dissolved solids per year. The corresponding reduction in salinity concentration at I mperial Dam is about 3 mg/1. Also a private power company is investigating the potential of using return flows from the Palo Verde drain for nuclear power plant cooling. The diversion of such saline flows could result in significant salinity reductions at Imperial Dam. 9