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<br />w <br />....... <br />w <br />o <br /> <br />had been leached by 76 mm of simulated rainfall that was applied during <br />one hour. Nevertheless, on the alluvial valley floors of the Colorado <br />Plateau, water use by phreatophytes concentrates salts in the subsurface <br />(U.S. Bureau of Land Management, 1984, Table 7), and SC concentrations <br />of 5,800 to 11,000 mg/l were measured in shallow groundwater beneath <br />salt cedar (Tamari pentandra)(Woessner et aI, 1984). In fact, in the <br />lower Virgin River about one-third of the water is used, and one-third <br />of the salts are stored by phreatophytes. <br />By far, the most common method of soluble mineral accumulation is <br />evaporation of soil water. Evaporation from the soil surface continues <br />as water is replenished by upward capillary movement (Laronne, 1977). <br />In fact, the "critical" groundwater level, the depth at which <br />evaporation rate decreases only slightly wi th further increase of water <br />table depth, is about 150-200 cm for fine sandy loams, but it is as much <br />as 1500 cm for coarse sandy soils. Salt concentration gradients (and <br />thermal gradients causing diffusion in soil water) are known to increase <br />the upward flow of soil water, although buildup of salt crust may reduce <br />evaporation (Qayyum, 1961). <br />Anthropogenic activity leading to accumulation of soluble minerals <br />is very common in irrigated semiarid lands. Man-induced salinization <br />is caused by raising the water table by over-irrigation and improper <br />irrigation scheduling, poor drainage, and canal seepage. Other <br />activities leading to salinization are the use of saline water for <br />irrigation, under irrigation, and poorly levelled land which prevents <br />leaching (U.S.D.A., 1954, Ayers and Westcot, 1976, Evans, Walker and <br />Skogerboe, 1981). Nevertheless, it is the high soluble mineral content <br />of the Mancos Shale and similar formations that is the main source of <br />Colorado River salts. <br /> <br />Mineralogy of Soluble Minerals <br /> <br />~ <br /> <br />Samples of Mancos Shale, shales of the Lower Mesa Verde Group, and <br />associated surficial alluvium from the Grand Valley in Colorado and in <br />the vicinity of the confluence of the Price and Green Rivers, Utah have <br />been analyzed by Whittig (written comm., 1979) using X-ray diffraction <br />techniques in order to determine the mineralized composition of the <br />soluble materials. Calcite, dolomite, gypsum, and other more soluble <br /> <br />1 <br /> <br />18 <br />