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<br />("I") <br />r-. <br />'\1 <br />c) <br /> <br />-, <br /> <br />CHAPTER III <br /> <br />PROBLEMS AND NEEDS <br /> <br />In recognition of these facts, Congress passed the Colorado River <br /> <br />Basin Salinity Control Act (Public Law 93-320) on June 24, 1974. Title <br /> <br />I of the Act authorizes the construction of facilites to enable the <br /> <br />United States to comply with its obligations under the agreement with <br /> <br />Mexico of August 30, 1973 (Minute No. 242 of the International Boundary <br /> <br />and Water Commission, United States and Mexico). <br /> <br />In brief, Title I <br /> <br />provides the means of accomplishing Minute 242, which requires that water <br /> <br />delivered to Mexico have an average annual salinity of no more than llS <br /> <br />p/m ~ 30 p/m greater than the Colorado River water arriving at Imperial <br /> <br />Dam upstream of the United States-Mexican border.l/ <br /> <br />Title II of the Act provides for programs to control the salinity <br /> <br />of the Colorado River upstream from Imperial Dam. Title II also author- <br /> <br />ized the construction of four salinity control units--Paradox Valley and <br /> <br />Grand Valley Units in Colorado, Crystal Geyser Unit in Utah, and Las <br /> <br />Vegas Wash Units in Nevada. <br /> <br />Grand Valley Salt Loading <br /> <br />The Colorado River has eroded the Grand Valley from the Mancos <br /> <br />Formation, a sequence of thick beds of gray shale. The Mancos Formation <br /> <br />is a calcareous Marine Shale containing various soluble salts and is <br /> <br />fractured and jointed near the surface. These salts are leached from the <br /> <br />Mancos Shale and shale-derived soils by ground water that receives its <br /> <br />recharge from canal, lateral, and on-farm seepage. This leaching is the <br /> <br />major source of salt loading from the Grand Valley. <br /> <br />l/ p/m is a measure of the parts per million concentration of total <br />dissolved solids, an indicator of salinity concentration essentially <br />equivalent to concentrations expressed in mg/L. <br /> <br />20 <br />