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<br />While salt loading adds a large variety of dissolved matter to the water, <br />on ly 10 el ements make up about 99 percent of the di sso 1 ved constituents. <br />These are: hydrogen, sodium, magnesium, potassium, calcium, silicon, <br />chlorine, oxygen, carbon, and sulfur. The elements occur in solution as <br />various ions, molecules, or radicals. The major part of the dissolved <br />constituents in Colorado River water is made up of the cations: calcium, <br />magnesium, and sodium; and the anions: sulfate, chloride, and bicarbonate. <br />These, plus minor amounts of other dissolved constituents, are commonly <br />referred to as salinity, salt, dissolved solids, or dissolved mineral <br />content. <br /> <br />Studies of the operations of the hydrosalinity cycle suggest that about <br />half the salts in the waters at Hoover Dam is derived from natural sources <br />and the other half from manmade sources, as shown in figure 3. The order <br />of magnitude from greatest to least is: natural sources, irrigation, <br />reservoir evaporation, out-of-basin export, and M&I (municipal and indus- <br />trial) sources. <br /> <br /> <br />47% Natural Sources <br /> <br />37% Irrigation <br /> <br />12% Reservoir Evaporation <br /> <br />3% exports <br /> <br />1%M&1 <br /> <br />Figure 3. - Salinity sources. <br /> <br />Physical and Economic Impacts <br /> <br />The high salt load of 9 million tons annually adversely affects more than <br />12 million people and about 1 million acres of irrigated farmland in the <br />Lower Colorado River Basin in the United States. In this area, the <br />total damages attributable to salinity in the Colorado River system as of <br />January 1982 were about $113 million per year. By the year 2010, without <br />control measures, these damages would amount to about $267 million per <br />year. These economi c impacts are based on Bureau of Reel amation studi es <br />which showed that annual direct and indirect losses amount to, in January <br />1982 dollars, about $513,300 per mg/L increase in salinity at Imperial Dam <br /> <br />4 <br />