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e~.;531 Mexico 1.36 Maf of the 1.5 Maf at the Northerly International Boundary (NIB) about 1 mile downstream of the Desalting Plant discharge and upstream of Morelos Dam. The average annual salinity of water arriving at NIB could be no more than 115 p/m ~ 30 p/m greater than the salinity of water arriving at Imperial Dam. The remaining 140,000 acre-feet would be delivered at the Southerly International Boundary located near San Luis, Mexico. , , , AUTHORIZATION AND CONSTRUCTION The construction of the Desalting Plant was authorized under Title I of the Colorado River Basin Salinity Control Act (Public Law 93-320), as part of the permanent solution to meet the annual salinity differential (115 ~30 p/m) specified by Minute 242. The Desalting Plant would meet the salinity differential through the treatment of irrigation return flow from Wellton-Mohawk Irrigation and Drainage District (Wellton-Mohawk). Under the operation of the Desalting Plant, the salinity differential would be maintained and Wellton-Mohawk return flow that could not otherwise be delivered to Mexico under Minute 242, would be restored for delivery as part of the united States obligation. Delivery of the treated return flow would reduce upstream reservoir releases required to meet the Treaty obligations to Mexico, and save water for later use by the Lower Basin States. The first phase of construction, which began in November 1975, was based upon an installed capacity of 96 million gallons per day (Mgal/d). However improved irrigation efficiencies by Wellton-Mohawk resulted in less return flows and less Desalting Plant capacity being required. Studies recommended the 73 Mgal/d capacity plant which has been recently constructed and which is scheduled to begin operation in 1992. I -~ The completed Desalting Plant will desalt and/or bypass Wellton- Mohawk drainage return water as necessary to maintain acceptable salinity levels in the Colorado River and save water otherwise rejected into the Santa Clara Slough. During years of surplus Colorado River water the upstream reservoirs would be full, and operation of the Desalting Plant would not result in reject water being saved. As the salinity differential could be maintained by the release of excess water, the operation of the Desalting Plant would not be required. OPERATION Current projections indicate the Desalting Plant is expected to be operated about 60 percent of the years. However recent hydrology is indicating that the probability of Desalting Plant operation may be as high as 75 percent. Future discussions in this report will assume the established 60 percent probability of operation. 6