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CHAPTER 2 - SALINITY OF THE RIVER Overview ~ o 00 OJ The Colorado River drains 246,000 square miles (approximately 157 million acres) of the western United States and a small portion of northern Mexico. Its waters serve some 4 million people within the United States' portion of the Colorado River Basin, and through export provides full or supplemental water supply to another 19 million people outside the Basin. The regional economy is based on irrigated agriculture, livestock grazing, mining, forestry, manufacturing, oil and gas production, recreation and tourism. About 3.5 million acres are irrigated within the Basin and hundreds of thousands of additional acres are irrigated by waters exported from the Basin. Hydroelectric power facilities along the Colorado River and its tributaries generate approximately 12 billion kilowatt-hours annually which is used both inside and outside of the Basin. The Colorado River also serves about 1.7 million people and 500,000 irrigated acres in Mexico. Salinity has long been recognized as one of the major problems of the river. For this Review, the tenns "salinity" and "total dissolved solids" (fDS) are used interchangeably, however IDS technically includes all of the soluble constituents potentially dissolved in the River, while salinity as defined in this Program and this Review includes only the combined concentration of the six major cations and anions (calcium, magnesium, sodium, carbonate, chloride, and sulfate) which together represent the bulk of TDS in the Colorado River. The current salinity control program is not designed to address trace minerals or any individual constituent that may be dissolved in the River, however these minerals may be removed as an incidental benefit of the Program. The Colorado, like most western rivers, increases in salinity from its headwaters to its mouth, carrying an average salt load of 9 million tons annually past Hoover Dam, the uppermost location at which numeric criteria have been established. In addition to total salt load which measures the total mass of salt carried in the River (tons/yr), this report also examines salinity in terms of concentration as expressed in milligrams per liter (mg/L). The salts in the Colorado River system are indigenous and pervasive. Many of the sediments of the basin were deposited in marine environments which were saline. Salts deposited with the sedimentary rocks are easily eroded, dissolved, and transported into the river system. The salinity control program is designed to prevent a portion of this abundant salt supply from moving into the river system. In a 1971 studl, the EPA analyzed salt loading in the basin and for convenience divided it into two categories: naturally occurring and human-caused. The EPA concluded that about half (47 percent) of the salinity concentration measured in water arriving at Hoover Dam is from 3The Minami Duality Prohlem in the Colorado River &~in, Summary Report, Environmental Protection Agency, Regions vm and IX, 65 pp" 1971. 2-1