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<br />It was estimated that in 1974 the onfarm irrigation efficiency <br />was 42 percent in the Palo Verde Subarea, and the overall efficiency, <br />including lateral seepage, was 39 percent. <br /> <br />Geohydrology. The ground water under the Palo Verde Subarea is <br />contained in alluvium of unknown thickness. Typical lithology is <br />indicated in Figure 1 which shows data obtained from a Geological <br />Survey well in the western portion of the Subarea. The alluvium of <br />interest in this study is that above the tighter Bouse formation, the <br />top of which lies at a depth of about 450 feet. <br />The water tab 1 e 1 i es at an average depth of 7 feet beneath the <br />Palo Verde Subarea. The quantity of water contained in the ground- <br />water aquifer is not known. Neutron probe measurements by the <br />Geo 1 ogi ca 1 Survey to depths of 16 feet i ndi cated an average water <br />content of 44 percent by volume below the water table [4], but it is <br />probable that the porosity of the deeper sediments is less than that. <br />Based on a review of published values for the types of sediments <br />present, a value of 33 percent was selected for this study. With this <br />porosity the volume of ground water beneath the 12,550-acre Palo Verde <br />Subarea was estimated to be 1,840,000 acre-feet between depths of 7 <br />feet and 450 feet. <br /> <br />Sources of Salt. The salt emanating from the Palo Verde Subarea <br />evidently comes from the saturated alluvium underlying the Subarea, <br />and the unsaturated soil above the water table. <br />The Geological Survey reports that no beds containing appreciable <br />quant it i es of sol ub 1 e mi nera 1 salts have been found in the all uvi a 1 <br />deposits, and concluded that the dissolved minerals now in the ground <br />water must have originated from the Colorado River or from local <br />recharge. [4] Consequently, the salt in the saturated all uvi urn is <br />believed to be in solute form. <br /> <br />nn",;r;"!:'S <br />\.'-....:.'---'tw <br /> <br />15 <br />