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1 .' - ~ . ._/,_/.... v ) B:r:1.e:f:1.n.s Paper PROJECT: Yuma Desalting Plant Operation Study BACKGROUND AND PURPOSE: In accordance with a 1944 treaty between the United States and Mexico, the United States is obligated to deliver 1,500,000 acre-feet of Colorado River water to Mexico each year. Further, Minute No. 242 of the International Boundary and Water Commission requires that the United States shall adopt measures to assure that the water arriving at Morelos Dam have an average annual salinity of no more than 115 (~30) milligrams per liter over the average annual salinity of the water arriving at Imperial Dam. This is required to maintain water quality suitable for irrigation in the Mexicali Valley, Mexico. Public Law 93-320 (as amended by Public Law 96-336) signed June 24. 1974, authorized construction of the Yuma Desalting Complex, which was intended as part of the permanent solution to meet the salinity differentials as specified in Minute No. 242. Construction of the Yuma Desalting Complex began in November 1975. Its purpose is to bypass and/or desalt Wellton-Mohawk drainage returns, when required, to meet the specified salinity levels in the Colorado River, The last major construction contract for the Yuma Desalting Complex, at this time, is about half complete. Since the plant's inception, conditions on the river have changed dramatically. During the past 5 years, high flows have improved the quality of water arriving at Imperial Dam, and delivered to Mexico. During years of high flows and surplus water, plant operation is not needed to deliver water of usable quality to Mexico. When a series of dry years occur in the future, salinity will increase and possibly require extensive operation of the desalting plant. The need for full operation is projected to occur about 4 years in a 10-year period. During the other years, the desalting plant would be operated on a partial operation or standby basis. Annual operation, maintenance, and replacement (OM&R) costs, including energy, are projected to be in excess of $28 million, during years of full operation. These costs have more than doubled since the project was authorized. Costs are about $3 to $5 million during years when operation is not required. The purpose of this special study is to determine methods of reducing the OM&R costs of the Yuma Desalting Plant, and still provide Mexico with suitable quality water, without adversely impacting the usable water supplies of the Lower Basin States.