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e ...... l-& o Co') . Ie COLORADO RIVER SALINITY CONTROL PROGRAM (continued) Before water development in the valley, Las Vegas Wash has been a generally barren and sandy channel with contained discharge only during brief periods of major storm runoff. The growth of the communities in the valley contributed increasing amounts of wastewater discharge to the wash until the flow became perennial. Return flows to the wash are from secondary treated sewage plant effluent, industrial cooling water, and urban irrigation and agricultural drainage. The water from some of these sources contains nutrients, salts J and other solutes. However, all of the wastewater leaches additional salts from the soil as it flows into and through the Las Vegas Wash. Past investigations associated with plan development have been described in previous progress reports. Construction according to ~ draft definite plan was begun in 1977, but it was delayed in 1978 to allow time to reevaluate the changing ground water conditions in the Lower Las Vegas Valley. Status of Investigations The principle investigation objectives for the reevaluation were to identify the potential for cost effective salinity control and to formulate a practical and cost effective plan. Data collection and analysis have been sufficiently completed to indicate possible cost effective plans, but the formulation of such plans is con- tingent upon the waste-water management practices of valley water users. Therefore, development of a definite plan for the Las Vegas Unit has been delayed to compliment a schedule for Las Vegas Wash nutrient control planning conducted by a group of various other entities. During the time required to develop a comprehensive water and solute management plan with other entities, the monitoring of changes in the saline ground water would continue. The ground-water monitoring program would be augmented by the addition of a recharge control verification program. A 3 mile (2 km), 24-inch (61 em) pipeline would be installed to prevent once-through cooling water from infiltrating through salt laden soils and carrying up to 20,000 tons (18 000 t) per year of salt into Las Vegas Wash and thence to the Colorado River. By transferring the surface discharge conveyed in unlined ditches into the pipeline, the ground water flow in the adjacent area is expected to diminish. This reduction would reduce required evaporation and pond capacity for any future diversion of remaining saline ground water. The amount of reduced salt loading result ing from the recharge control would be verified by the ground-water monitoring program. This verifica- tion may be used to indicate the potential salinity control in other areas that may be affected by reducing ground water recharge. A study of the amount of salt stored in soils above the water table began in May 1980 and should be completed by September 1981. Accumulated salt in this zone could have a significant future impact on the Colorado River which may not be evident in dissolved salt flow analysis. 70