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Show has estimated that in the 1935-48 period more than 9,000,000 tons of calcium carbonate and more than 1,000,000 tons of silica constituting about 7 percent of the dissolved solids in the waters entering Lake Mead were precipitated out during the 14-year period.* He found that a much larger quantity of salts, chiefly halite and gypsum have gone into solution within the reservoir basin in that period, the effect being a net increase in total dissolved solids discharging from the reservoir over those entering it of about 17 percent. These two phenomena counteract one another, making it impossible to fully evaluate the effects of either on the quality of water. Transportation of water outside the natural basin of the Colorado River by means of transmountain diversions results in the physical removal of the dissolved solids carried by that water. Transmountain diversions as such have been attacked by some sources because they remove some of the "better waters" of the basin. The fact remains, however, that trans-basin diversions remove both s.alts and water while in-basin use remove only the water allowing the residual s.alts to be carried back to the streams to worsen the quality of the downstream supply. In summation, consumptive use of water in the Upper Basin, as contemplated by and as provided for in the Colorado River Compact of 1922 necessarily affects the quality of the remaining waters. Essentially, it depletes the water supply but leaves the dissolved solids behind. Certain factors, including the deposition of less soluble salts from solution and the physical transportation of salts out of the basin by transmountain diversion, tend to mitigate this condition. Base exchange occurring as a result of the irrigation process will tend to change somewhat the percentages of the several dissolved constituents. Dissolved Constituents and Their Significance Natural waters vary greatly in the concentration and composition of dissolved constituents and correspondingly in their suitability for irrigation or other beneficial use. Further, the requirements for a good irrigation water may be inimical with those needed for other purposes. Domestic and industrial uses, for example, require a "soft" water whereas a desirable water for irrigation uses should be "hard." Waters of the Colorado River system will largely be used for irrigation purposes but there will also be a substantial demand for domestic and industrial supplies. When used for irrigation, some of the constituents are beneficial to plants, some in moderate concentration appear to have little effect on plants or soils, while others impair plant growth or are harmful to soils. In solution, a large proportion of the inorganic salts are ionized. The metallic elements called cations take a positive electrical charge while the nonmetallic elements or acid radicles called anions take a negative charge. The major cations-calcium, magnesium, sodium * "Lake Mead Comprehensive Survey of 1948-49," Volume II -28- |
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Original book: [State of Arizona, complainant v. State of California, Palo Verde Irrigation District, Coachella Valley County Water District, Metropolitan Water District of Southern California, City of Los Angeles, California, City of San Diego, California, and County of San Diego, California, defendants, United States of America, State of Nevada, State of New Mexico, State of Utah, interveners] : |