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The Protective and Regulatory Pumping Unit was developed to intercept part of the ground-water underflow that moves southward from the Yuma Mesa in the United States into Mexico. Before the PRPU was constructed, this underflow was increasing because of ground-water pumping in the Sonora Mesa Well Field, immediately south of the Southerly International Boundary in Mexico. The Colorado River Basin States expressed their concern about the pumping in their July 1973 letter to the President. Currently, 21 of the planned full complement of 35 wells and associated conveyance and energy facilities have been constructed. The wells are connected by a 15.3-mile pipeline and open concrete-lined canal that carries water by gravity across the Southerly International Boundary. With 35 wells, the PRPU would be capable of producing about 125,000 acre-feet of water per year. Ultimately, 125,000 acre-feet of water from the PRPU, combined with 15,000 acre-feet of water from wasteways and drains in the Yuma Valley, would furnish 140,000 acre-feet of Mexico's 1.5-million-acre-foot annual entitlement. The water would be delivered at the Southerly International Boundary near San Luis, Arizona. In addition, 35,000 acre-feet could be withdrawn by private wells and/or 242 wells for use on private land to equal the 160,000 acre-feet limit for pumping in the 5-mile zone. Currently, water from wasteways and drains in the Yuma Valley exceeds 15,000 acre-feet per year. Should these wasteway and drain flows diminish in the future, wells will be added to the PRPU, as needed, to ensure that approximately 140,000 acre-feet can be delivered at the Southerly International Boundary at all times. Additionally, as authorized by Title I, approximately 23,500 acres of private, State, and State-leased lands have been acquired within the 5-mile zone. The purpose of these acquisitions is to limit development and thus limit United States ground-water pumping to 160,000 acre-feet per year, as required by Minute No. 242. The acquisitions were completed in 1984. Yuma Desalting Plant The Yuma Desalting Plant is built on a 60-acre tract ofland 6 miles west of Yuma, Arizona. The purpose of the plant is to upgrade the quality of irrigation drainage water from the Wellton-Mohawk Irrigation and Drainage District (WMIDD) so that it can be returned to. the Colorado River and delivered to Mexico. The operational design parameters set up for the plant determined that a membrane desalting process was technically feasible and is economically suitable for the Yuma Desalting Plant operation. The factors included in this formula are the volume of water delivered to Mexico, the salinity differential required by Minute No. 242, the salinity of the Colorado River at Imperial Dam, the volume of drain water treated, the salinity of the drain water, a number of other factors related to the diffuse return flows below Imperial Dam, and plant operational factors. A study done in 1978 by the Advisory Committee on Irrigation Efficiency, Wellton-Mohawk Irrigation and Drainage District, recommended expansion of onfarm 45