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<br />Sa 1 tin Ground Water. The Palo Verde Subarea 1 i es in the <br />part of the District where the greatest concentrations of dissolved <br />solids are found in the ground water, as illustrated in Figure 2. The <br />salt in the ground water is apparently being flushed out by irrigation <br />drainage percolating through the aquifer to the nearest drain. The <br />depth to which flushing occurs is assumed to be 450 feet, the depth of <br />the top of the 1 ess permeab 1 e Bouse formation, although most of the <br />flushing undoubtedly occurs at shallower depths. <br />The mi nera 1 constituents of vari ous waters i ndi cate that <br />flushing takes place. For example, Colorado River water contains <br />approximately 28 percent Na+Cl (mg/l basis), while well water samples <br />having a TDS over 3,000 mg/l range from 65 percent to 75 percent <br />Na+Cl. Drainage flows from the Palo Verde Subarea are high in Na+Cl, <br />ranging from 40 percent to 60 percent. By contrast, drainage from the <br />northeastern part of the District contains between 30 and 40 percent <br />Na+Cl. Some ion exchange of calcium for sodium may be taking place, <br />but it is not expected to have a significant influence on the compo- <br />sition of the return flow compared to flushing. It is very unlikely <br />that precipitation of some constituents is occurring at present water <br />use rates. <br />The salt content of the ground water was estimated by first <br />estimating the salinity of various parts of each ground-water zone, <br />and then applying that salinity to the estimated volume of water. The <br />resulting amount of salt, 11,400,000 tons, was considered to be <br />present in 1962 when most of the data was obtained. The breakdown by <br />groundwater zone is: shallow zone--260,000 tons; principal gravel <br />zone--1,440,000 tons; and deep zone--9,700,000 tons. The total <br />quantity was reduced to 9,750,000 tons to bring it to a current (1974) <br />level, equivalent to an average ground-water salinity of 3,890 mg/l. <br />With Colorado River water at a salinity of approximately 750 <br />mg/l, and wi th an overa 11 water use effi ci ency of 39 percent, the <br />irrigation leaching fraction and canal seepage have an average salin- <br />i ty of about 1,230 mg/L. There is a net salt di scharge from the <br />Subarea because the ground water in the aquifer has a sa 1 i ni ty in <br />excess of that concentration. <br /> <br />16 <br /> <br />002850 <br />