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CHAPTER I INTRODUCTION "~~;\ for irrigation of most crops, although concentrations of 500 mg/L can have detrimental effects on salt-sensitive crops. The Environmental Protection Agency (EPA) secondary standards recommend that public drinking water supplies should contain less than 500 mg/L. Affected most severely by this salinity are municipal and industrial water users in the Las Vegas-Los Angeles-San Diego area, and irrigators in southern California and Arizona, who experience annual economic losses. 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 drinking water palata- bility. For irrigators, the higher concentrations cause decreased crop yields, altered cropping patterns, increased leaching and drainage require- ments, and increased management costs. The total direct and indirect damages attributable to salinity based on 1980 prices are estimated at $450,000 for each mg/L at Imperial Dam. Salinity not only has economic effects on water users in the Lower Basin States, but is also an important factor in international relations with Mexico, which is guaranteed an annual supply of 1.5 million acre-feet of Colo- rado River water by a 1944 treaty. In 1973 the United States and Mexico agreed that the water delivered to Mexico from the main stem of the river would have a salinity of no more than ll5+30 parts per million (ppm) greater than the average salinity of water at Imperial Dam. ~!fl~ The Seven-State Colorado River Basin Salinity Control Forum adopted in June 1975 numerical salinity criteria for three points on the lower main stem of the river, as proposed by the Basin States under Public Law 92-500 require- ments and EPA regulations. These criteria provide specific program objectives and if they are to be met, about 2.5 million tons of salt will have to be removed from the system each year. The criteria for the three locations are as follows: Below Hoover Dam Below Parker Dam Imperial Dam 723 mg/L 747 mg/L 879 mg/L In recognition of these facts, Congress passed the Colorado River Basin Salinity Control Act (Public Law 93-320) on June 24, 1974. Title II of that act authorized the Secretary of the Interior to construct, operate; and main- tain four specific salinity control units in the Upper Basin as the initial stage of the Colorado River Basin Salinity Control Project. In addition, the Secretary was authorized by Public Law 96-375, October 3, 1980, to prepare feasibility level reports on other units, one of which is the Big Sandy River Unit. Level of Detail The investigations will be conducted on a level of detail and accuracy sufficient to ident ify and evaluate al ternat ive plans, to select a recommended't,;jJ G 01 ('/7 3