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<br />indicating a decreasing proportion of sal ine ground water. Figure 3 <br />shows a slight downward trend in net salt discharge of the entire <br />District during the period 1971-1975, years apparently not affected by <br />storms. <br />The analysis of salinity control measures presented in subsequent <br />parts of the report required an assumption regarding the proportion of <br />ground water contained in the drainage from the Palo Verde Subarea. A <br />comparison of the salinities of deep percolation, ground water, and <br />drain flow indicated an apparent ground-water content of 57 percent in <br />the drai nage di scharge from the Subarea. Thi s proportion was used. <br /> <br />Study of Calcium Carbonate Precipitation <br />The Lower Co 1 orado Ri ver is supersaturated wi th respect to ca 1- <br />cium carbonate, and drainage from the Palo Verde Irrigation District <br />adds to that condition. Theoretically calcium carbonate should pre- <br />cipitate from the water in the drains and the river until the satura- <br />tion point is reached. If this actually happens, the river has a <br />natural way of shedding some of the its dissolved calcium and bicar- <br />bonate, which would tend to duplicate the effects of salinity control <br />measures relative to that salt. Consequently the matter was studied <br />by the U.S. Salinity Laboratory, Riverside, California. [8] <br />Extensive sampling of the Colorado River, Palo Verde Irrigation <br />District drains, All-American Canal, and East Highline Canal was <br />conducted in March and September 1977. Samples were taken as far <br />upstream as Lake Havasu, but the analysis concentrated on the reach <br />below the Outfall Drain because the patterns of diversions and return <br />flows are more complex farther upstream. <br />The ri ver was found to be supersaturated wi th ca 1 ci urn carbonate <br />from Lake Havasu south. The degree of supersaturation increased below <br />the Outfall Drain, and was greater when the proportion of drainage <br />water to river water increased. No decrease was found in the degree <br />of supersaturation from the Outfall Drain to the end of the East <br />Highline Canal, a distance of over 100 miles. Water in various drains <br /> <br />20 <br /> <br />OC286t' <br />