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<br />MAY 1982 <br /> <br />Both natural runoff and irrigation contribute to the problem, either by <br /> <br />salt concentration or by salt loading. Salt concentration is caused by <br /> <br />removal of water from the river system through consumptive use by irrigated <br /> <br />crops and other vegetation, and by evaporation; mineral constituents are <br /> <br />thereby concentrated in the water that remains. Salt loading occurs as <br /> <br />ground water dissolves subsurface minerals while flowing through the salt <br /> <br />laden soils and shale layers. Although both salt concentration and salt <br /> <br />loading are at work, salt loading is the major cause of the salinity <br /> <br />increase. <br /> <br />Salt loading from irrigated cropland is related to subsurface return <br /> <br />flows. The irrigation water applied is generally of good quality and most <br /> <br />fields are irrigated in excess of normal crop needs. Deep percolation of <br /> <br />excess irrigation water causes substantial return flows through layers of <br /> <br />Mancos shale that contain large quantities of undissolved salt (solids). <br /> <br />The concentration of dissolved salts (solids) transferred to the percolating <br /> <br />water seriously degrade the quality of water delivered to the Colorado ~iver. <br /> <br />Of the 115,000 tons of salt that McElmo Creek delivers annually to the <br /> <br />Colorado River about 46,000 tons come from onfarm irrigation systems and <br /> <br />practices, and about 8,000 tons from small off-farm laterals (See Table <br /> <br />IV-I). With voluntary participation by seventy ~rcent of the farmers, the <br /> <br />salt load from onfarm sources can be reduced by as much as 38,000 tons (70 <br /> <br />percent) depending on which alternative plan is implemented. <br /> <br />IV-4 <br /> <br />001869 <br />