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<br />. <br /> <br />. <br /> <br />PLAN FORMULATION <br /> <br />A number of salinity control alternatives for the Palo Verde subarea <br /> <br />are being considered in the plan formulation process. Basically, the <br /> <br />alternative studies consist of analyzing the effect that various action <br /> <br />plans would have on reducing salt contribution from the area. <br /> <br />Because the Palo Verde subarea was shown to be causing most of the <br />District's salt discharge, the preliminary project evaluation discussed <br />in this report has been confined to that subarea. <br /> <br />The Palo Verde subarea lies in the southwest corner of the District, <br /> <br />adjacent to the mesa. It is approximately 11-1/2 miles long and <br /> <br />3-1/2 miles wide at its widest point, and contains a gross area of <br /> <br />approximately 12,550 acres. The Palo Verde Outfall Drain forms half <br /> <br />of its eastern boundary, separating much of the subarea from adjacent <br />subareas, About 10,300 acres are irrigated in the area. <br /> <br />In 1974, the onfarm irrigation efficiency was an estimated 42 percent <br /> <br /> <br />in the Palo Verde subarea, and the overall efficiency, including <br /> <br />lateral seepage, was 39 percent. <br /> <br />ln0"~.O <br />~~vl <br /> <br />The salt emanating from the Palo Verde subarea derives from the <br /> <br />saturated alluvium underlying the subarea, and the unsaturated soils <br /> <br />above the water table. The salt in the ground water is apparently <br /> <br />being flushed out by irrigation water percolating through the aquifer <br /> <br />to the nearest drain and canal seepage. Flushing occurs to a depth of <br /> <br />about 450 feet, the depth of the top of the less permeable Bouse formation, <br /> <br /> <br />although most of the flushing undoubtedly occurs at shallower depths, <br /> <br />13 <br />