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<br />Mechanics of Salt Discharge. In a flushing situation involving <br />sa 1 i ne aquifers, the sa 1 i nity of the di scharge decreases gradually <br />with time until it attains that of the water causing the flushing--in <br />this case deep percolation from irrigation. The gradual decrease in <br />salinity is a geometric recession, mathematically speaking, as indi- <br />cated by various theoretical and laboratory tank model studies. [10] <br />In the Palo Verde Subarea the horizontal and vertical permeabili- <br />ties of the aquifer are unknown. However, by using the 1974 salt <br />discharge and estimated salt content, and by treating the decline as a <br />recession, it was possible to develop mathematical expressions to <br />predict the future salt discharge and the amount of remaining salt by <br />year. The resulting equations, plotted on Figure 5, reflect the <br />specific permeabilities of the aquifer and the present irrigation <br />efficiencies. <br />The plot of excess salt content (lower plot) is a geometric <br />recession initialized at 7 million tons in year zero (the present). <br />That amount is the sum of the estimated excess salt contained in the <br />ground water and the relatively small amount in the saturated soil. <br />The equation for the plot is y = 7 (. 97984l where xis the future <br />year. The initial slope of the curve is (-) 144,000 tons per year. <br />The geQmetric recession equation plots as a straight line on <br />semi logarithmic paper. <br />The plot of discharge per year (upper plot) was derived by find- <br />ing the equation for the slope of the lower curve and making a very <br />small adjustment to initialize it at 144,000 tons per year. The <br />resulting equation is y = 144 x 103 (.97984)x. <br />The plots show that the excess salt will be flushed out gradually <br />with time, and that the rate of salt discharge decreases gradually. <br />For example, in 50 years the discharge per year is expected to be only <br />slightly more than one-third the present rate. Such change is reflec- <br />ted in observed changes in the composition of the dissolved minerals <br />in drainage from the District. Samples taken in 1975 showed a lower <br />percentage of calcium and chloride than samples from 1967-1971 [2], <br /> <br />(' ~ r-, t',''''' '11) <br />U .~ 0::" ~ t> ~.. <br /> <br />19 <br />