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W -..l ~ o water downstream. Researchers at Utah State University theorize that while these reservoirs are filling they are retaining a portion of the salts that would otherwise be flowing down to Imperial Dam and, consequently, lower salinity concentration levels are occurring at the measuring stations. This retention is believed to be a temporary phenomenon, and the projected levels through the year 2010 (without water quality improvement units) are expected to follow the overall rising trend, as shown in the graph. The high salt load of 10 million tons annually in the Lower Colorado River Basin adversely affects more than 10 million people and 1 million acres of irrigated farmland in the United States. The losses associated with municipal and industrial use occur primarily from increased water treatment costs, accelerated pipe corrosion and appliance wear, increased soap and detergent needs, and decreased water palatability. The Public Health Service recommends drinking water contain no more than 500 mg/L of TDS. For irrigators, the higher concentrations cause decreased crop yields, altered crop patterns, increased leaching and drainage requirements, and increased management costs. Agricultural losses (either through lower yields or increased production/management costs) begin when salinity levels of applied irrigation water reach 700 to 850 mg/L, depending upon soil conditions and type 'of crop grown. A summary of the effects is presented below: mg/L Effect Below 500 Above 500 700 to 850 Good drinking water Municipal and industrial losses begin Agricultural losses begin Salinity increases result from two processes: salt loading and salt concentration. Salt loading increases the amount of salt for a given amount of water, and salt concentration decreases the amount of water for a given amount of salt. Salt loading is the addition to the river system of mineral salts from natural and manmade sources. Salt concentration is the rise in salinity through streamflow depletions which concentrate the salt burden in the river system into a lesser volume of water. Generally, the application of irrigation water results in increased salt concentrations because of both loading and consumptive use. The salt load fluctuates annually with the overall Basin water supply. Estimates indicate that about half of the present salinity concentration in the Colorado River at Hoover Dam near Las Vegas is attributed to natural sources. The remaining half is man-induced as indicated in the chart. IV-2