General Description

The Desalting Complex Unit is one of three components authorized under Title I of the Colorado River Basin Salinity Control Act. The other two components are the Coachella Canal Unit and the Protective and Regulatory Pumping Unit, which are covered in separate brochures.

The objectives of the Desalting Complex Unit are to reduce the quantity and improve the quality of saline irrigation drainage water pumped from the shallow aquifer beneath the farmlands of the Wellton-Mohawk Division of the Gila Project. The purpose of improving the quality of this saline drainage water is to make it usable as part of the delivery of Colorado River water to Mexico in accordance with the treaty with Mexico of February 3, 1944, and the International Boundary and Water Commission's Minute No. 242 of August 30, 1973.

Pumped drainage from the Wellton-Mohawk Irrigation and Drainage District is currently transported to the Santa Clara Slough in Mexico without credit toward treaty deliveries. The primary objective of Minute No. 242 is to limit the average annual salinity of about 1,360,000 acre-feet of water delivered to Mexico, as measured upstream from Morelos Dam at the Northerly International Boundary located at the border of California and Mexico. The salinity of the water as it enters Mexico, according to Minute No. 242, must average no more than 115 ⁺30 parts per million annually over the annual average salinity of Colorado River water measured at Imperial Dam 27 miles upstream from Morelos Dam.

The Desalting Complex Unit consists of both structural and nonstructural measures to be implemented in the lower Gila and Colorado River valleys of southwestern Arizona and northwestern Sonora, Mexico. The structural features include the construction of what will be the world's largest membrane desalting plant, located about 4 miles west of Yuma. The plant will have a design capacity of 73 million gallons per day. Also included in the structural features are a 53-mile-long, concrete-lined bypass drain, which starts at the desalting plant and will carry plant reject water that has been separated by the desalting process to the Santa Clara Slough in Mexico; a buried reinforced concrete pipe siphon, which will replace a steel flume section of the Main Outlet Drain Extension (MODE); attendant works such as access roads, bridges, and electrical power transmission lines; and measures to mitigate adverse impacts on fish and wildlife habitat.

Nonstructural measures include an irrigation efficiency improvement program in conjunction with an acreage reduction program, both of which will reduce drainage flows in the Wellton-Mohawk Division of the Gila Project from about 214,000 to ultimately about 108,000 acre-feet per year.

Plan

The plan is to reduce the quantity and improve the quality of Wellton-Mohawk Division drainage so the majority of it can be credited toward treaty deliveries. The plan includes seven measures which either have been or are being implemented: (1) construction of the Yuma Desalting Plant; (2) construction of the bypass drain in the United States and Mexico (completed in 1977); (3) implementation of the Wellton-Mohawk Irrigation Efficiency Improvement Program; (4) implementation of measures to reduce Wellton-Mohawk acreage (completed in 1978); (5) acquisition of Painted Rock Reservoir land and modification of its operation schedule; (6) construction of the MODE siphon (completed in 1976); and (7) implementation of fish and wildlife mitigation measures.

Unit descriptions and facilities

The major feature of the Desalting Complex Unit is a reverse osmosis membrane desalting plant. This plant will reduce the salinity of pumped drainage water from the Wellton-Mohawk Irrigation and Drainage District before the water is returned to the Colorado River. The plant site is located about 4 miles west of Yuma, near the Arizona Public Service Company's Yucca Powerplant. Until officially named the Yuma Desalting Plant, it was known as the Yucca Site.

The desalting plant will have the capacity to treat about 97,300 acre-feet of feed water per year at about 2,900 parts per million of total dissolved solids. The plant will produce about 68,500 acre-feet of product water per year at about 295 parts per million and about 28,800 acre-feet of reject water per year at about 9,400 parts per million.

Wellton-Mohawk drainage is expected to total about 108,000 acre-feet annually when the desalting plant becomes operational. About 10,100 acre-feet of raw untreated MODE water will be blended with desalting plant product water to produce about 78,600 acre-feet per year of total blended water for return to the Colorado River. The total blend water will vary from year to year, depending upon actual flows that occur below Imperial Dam; but this flow, combined with other drainage and river flows below Imperial Dam, will result in 1,360,000 acre-feet of water being delivered to Mexico at the Northerly International Boundary. The desalting plant should ensure that the salinity of this water will average 115 parts per million more than the salinity of water arriving at Imperial Dam. This will bring the salinity level of the water entering Mexico into accordance with provisions of Minute No. 242.

Bypass Drain

The Main Outlet Drain Extension (MODE) transports Wellton-Mohawk drainage water to Morelos Dam. Reject water from the desalting plant will also be carried in the MODE for 2 miles from the desalting plant to Morelos Dam where the water enters the bypass drain. The bypass drain begins at the end of the MODE at Morelos Dam, and runs adjacent to the Yuma Valley Levee on the east side of the river through most of the 16 miles to the Southerly International Boundary. The drain crosses the boundary into Mexico near the Colorado River west of San Luis, Mexico, and then extends south for 37 miles through Mexico to the upper end of the Santa Clara Slough which feeds into the Gulf of California.

The entire length of the bypass drain in Mexico was constructed by Mexico at the United States' expense. This concrete-lined drain is a total of 53 miles long with a capacity equal to that of the MODE (353 cubic feet per second). In the bypass drain, with the desalting plant in normal operation, the flow will consist of desalting plant reject plus some raw drainage water from Wellton-Mohawk totaling a maximum flow of about 43.7 cubic feet per second. The actual flow will be highly dependent upon the amount of raw drainage flow that can be blended with plant product water from year to year.

MODE Siphon

A 12.5-foot-diameter semicircular metal flume section of the MODE has been replaced with a buried concrete pipe siphon. The flume was adjacent to the Colorado River in Yuma at the foot of Prison Hill. The siphon, completed in June 1976, is 0.65 miles long, 10 feet in diameter, and has a capacity of 353 cubic feet per second .

Wellton-Mohawk Irrigation Efficiency Improvement Programs

Irrigation efficiency improvement programs have been in progress in the Wellton-Mohawk Division to reduce drainage flows from irrigation. Wellton-Mohawk drainage water is pumped from the ground-water aquifer beneath irrigated lands to maintain a desired minimum water table depth of 8 feet. The programs are overseen by the Technical Field Committee (comprised of professionals from several Federal agencies) and include irrigation management services, on farm improvements, research and demonstrations, accelerated education, and land acreage reduction. By the time the desalting plant begins operations, these programs shall have improved the irrigation efficiency from 54 percent to a level near 70 percent. This will result in a reduction of drainage flows from about 214,000 to about 108,000 acre-feet annually.

Wellton-Mohawk Acreage Reduction

The irrigable land of the division has been reduced from 75,000 acres initially authorized for development to about 65,000 acres. The aim was to remove developed land with marginal irrigation efficiency from the district and prevent further development of undeveloped land within the district. Part of the land was in Federal ownership, and the balance was acquired by the Government from State and private owners. The net objective of the acreage reduction program was to reduce drainage return flows from the Wellton-Mohawk Irrigation and Drainage District.

Painted Rock Reservoir Land Acquisition and Operation Schedule Modification

Painted Rock Reservoir is located near Gila Bend, Arizona, and it is operated by the Corps of Engineers, who are presently developing a release schedule for Painted Rock Dam. This schedule will attempt to coordinate maximum releases that will be contained within the 10,000-cubic-foot-per-second capacity Gila River channel being developed through the Wellton-Mohawk Irrigation and Drainage District. The Corps has indicated that actual releases from Painted Rock Dam will be determined on a case by case basis, depending upon other releases being made from the Colorado River system and the impacts the releases will make on the Yuma area and Mexico.

Originally, Public Law 93-320, in Section 101 (j), authorized the acquisition of additional land in Painted Rock Reservoir which would be required for temporary water storage capacity due to modifications of the operation schedule. To date, however, the Corps has not obtained the legal authority to purchase lands behind Painted Rock Dam. The Corps has indicated that the purchase of these lands would not change their release plan for the reservoir because Painted Rock Dam was designed as a flood control structure, not a storage facility.

Fish and Wildlife

The development of the Desalting Complex Unit (and also the Protective and Regulatory Pumping Unit) will result in the loss of fish and wildlife habitat. Mitigation measures will replace about 65 percent of these losses. The mitigation measures were not originally authorized under Title I of the Colorado River Basin Salinity Control Act, but are included in amendatory legislation -- Public Law 96-336, dated September 1978.

Yuma Desalting Test Facility

The purpose of the Yuma Desalting Test Facility was to test pretreatment processes and membrane desalting equipment. The testing used Wellton-Mohawk drainage as feed water. The data obtained from the tests were used to determine the type of equipment needed for the Yuma Desalting Plant.

A mobile test facility was located at the test site in 1971, and the facility was expended in 1973 through the installation of a larger intake and pretreatment system. Following this expansion, desalting membrane manufacturers brought test units to the facility for experimental purposes. The resting of the desalting membrane units was done at each manufacturer's expense. The Government supplied the pretreated drain water.

Seven companies installed test units in 1973 and early 1974. Two additional units were put on line in late 1975. Nearly all units were membrane elements of commercial size and configuration, and they were generally representative in scale to the size and type that were purchased for the Yuma Desalting Plant.

The pretreatment system was similarly designed to be representative in scale to the system planned for the Yuma Desalting Plant. Several processes utilizing diatomaceous earth, potassium permanganate, manganese zeolite, alum flocculation, and partial Lime softening were tested starting in 1974. A partial lime softening process consistently provided the best and most reliable results.

In June 1982, testing at the facility was completed and subsequently discontinued. The test facility has since been disassembled.

Attendant Facilities

• A switchyard to provide power to the desalting plant was constructed at the plant site. Incoming power is at 161 kilovolts. A 161/13.8-kilovolt transformer was installed in the switchyard. A 3.8-mile-long transmission line was constructed from the Department of Energy's Western Area Power Administration's Knob Substation to the switchyard.
• Once the desalting plant is in operation, a potable water supply will be furnished for domestic use. This appropriately treated water will come from the plant's desalting membrane product line. Two 1,000-cubic-foot (7,500-gallon) storage tanks will be constructed, and used to control pressure. During periods when the plant is down, the water supply will be pumped from an onsite well about 130 feet deep.
• A 7,500-gallon-per-day sewage treatment plant will be constructed to treat the sewage effluent from the desalting building, visitor center, laboratory, and other buildings.
• An energy recovery system will recapture energy from the high pressure steam carrying reject water. The system will include impulse turbines directly connected to high pressure pumping units. The energy recovery equipment will be contained in a l.5-kilovolt NEMA III non-walk-in-type indoor unit.
• Chlorine equipment for control of bacteria and algae will be housed in the building containing the energy recovery system's equipment

Sludge Disposal Facilities

Studies of the sludge disposal facilities are in the advance planning stage. Data on these facilities are dependent upon final design.

The Pilot Knob waste disposal site in Imperial County, California, has been proposed, and the use of this site is still being negotiated. For the interim, an alternate site is located within the 5-mile Protective and Regulatory Pumping Zone. This disposal facility will contain the earthfill disposals of excess calcium carbonate wastes generated in the pretreatment process of the feed water at the desalting plant.

Initial facilities for 50 years of operation of the disposal site include three adjacent blocks of land with raised earth berms provided around the perimeters. Each block is about 1,800 by 4,350 feet and capable of containing up to 144 cells for disposals. The filled, earth-covered cells will be slightly larger than one acre. The individual blocks will be used at 15-year intervals, and the cells within each block will be used successively in sufficient number for 5 years of operation. All the cells will be lined to prevent the saline water contained in the sludge from percolating into the ground water.

The calcium carbonate wastes, at a concentration of about 60 percent solids, will be pumped by positive displacement pumps located at the desalting plant through a high-pressure steel pipeline to the disposal site. The pipeline will be 4.5 to 6.5 miles long, depending upon the block of land being filled.

Other facilities at the disposal site will include a 2,400-square-foot administration building, a potable water well, fuel storage with pumps, chain link fencing around each block, an access road, transmission and communication lines, and a visual and sound buffer of planted native trees along the north boundary of the site.

Development

The United States and Mexico signed a treaty in 1944 that guaranteed Mexico 1.5 million acre-feet of Colorado River water annually. Of that total, about 1.36 million acre-feet are delivered annually by the Colorado River to Mexico upstream from Morelos Dam. This treaty contains no specific provisions regarding the quality of the water to be delivered.

In 1961, the salinity of Colorado River water increased significantly. Two concurrent and principal factors led to this increase:

(1) The Wellton-Mohawk Irrigation and Drainage District began pumping saline waters (initially averaging about 6,000 parts per million) to lower the high ground-water levels below the crop root zone in the aquifer that underlies the Wellton-Mohawk Division of the Gila Project. This saline drainage was discharged into the Gila River, a tributary of the Colorado River, and consequently delivered to Mexico at the Northerly International Boundary.

(2) Excess Colorado River flows, which Mexico had received prior to 1961, were significantly decreased due to low runoff in the Upper Colorado River Basin, leaving less water for the saline Wellton-Mohawk drainage to be blended into.

In November 1961, as a result of the change in salinity, Mexico formally protested to the United States that the delivery of water that was harmful to the purposes stated in the treaty constituted a violation of the treaty. In response to this protest, the United States, in 1963-1964, began to modify Wellton-Mohawk drainage pumping and its river operations. In March 1965, an agreement (Minute No. 218) was reached by the two governments for a 5-year period. This agreement did not become effective until the MODE was completed and put into operation. Each country reserved its legal rights under the Minute, while providing for practical measures to further reduce the salinity of waters reaching Mexico. These measures consisted of the construction and operation of the 12-mile-long Main Outlet Drain Extension (MODE) from the end of the Wellton-Mohawk Main Outlet Drain (MOD) to Morelos Dam. The MOD was discharging Wellton-Mohawk drainage water into the Gila River near its confluence with the Colorado River. However, the MODE enabled the United States to discharge ail or part of the Wellton-Mohawk drainage water into the Colorado River either above or below Morelos Dam. When scheduled water deliveries to Mexico were at the treaty minimum, the United States discharged all Wellton-Mohawk drainage into the Colorado River below Morelos Dam without being credited for deliveries to Mexico. This amount of water that was bypassed without credit to treaty deliveries amounted to about 50,000 acre-feet per year. In 1971, Mexico requested that the United States discharge an additional 40,000 to 75,000 acre-feet of Wellton-Mohawk drainage flows annually into the bypass canal and into the Santa Clara Slough to further reduce the average salinity of water diverted to Mexico at Morelos Dam.

In 1972, the United States and Mexico agreed upon a permanent settlement of the salinity problem. To effect an immediate improvement in the quality of water delivered to Mexico above Morelos Dam, the two governments approved a new minute, No. 241, which was signed July 14, 1972. This minute provided for the bypass of 118,000 acre-feet annually of Wellton-Mohawk drainage water without credit as water delivered to Mexico by the United States (about twice the rate discharged below Morelos Dam by the United States under Minute No. 218). Minute No. 241 also provided for replacement of that bypassed Wellton-Mohawk drainage water with other water to he delivered to Mexico -- primarily from the Colorado River above Imperial Dam and also front wells on Yuma Mesa.

On August 30, 1973, joint recommendations were formally approved by the two governments and incorporated into Minute No. 242 of the international Boundary and Water Commission, thus terminating Minute NO. 241. Minute No. 242 provides that the 1.36 million acre-feet of treaty water, delivered annually by the United States to Mexico upstream of Morelos Dam. have an annual average salinity of no more than 115 +30 parts per million over the annual average salinity of Colorado River water arriving at Imperial Dam. The minute further provides for about 140,000 acre-feet of water to be delivered annually to Mexico, at the Southerly International Boundary at San Luis, with a salinity substantially the same as that of waters customarily delivered there. The minute also provided for the construction of the concrete-lined bypass drain from Morelos Dam to the Santa Clara Slough in Mexico.

The implementation of some provisions of Minute No. 242 were dependent upon construction or other measures which required expenditure of funds by the United States; these particular provisions became effective upon authorization by the Congress and subsequent notification by the United States to Mexico of such authorization. The authorization of funds to implement provisions of Minute No. 242 was included in Public Law 93-320, the Colorado River Basin Salinity Control Act of June 24, 1974.

Pending completion and operation of the desalting plant, an interim measure was adopted by the United States to effect the requirements of Minute No. 242. This measure consisted of discharging all Wellton-Mohawk drainage waters into the Colorado River immediately below Morelos Dam until June 23, 1977. Thereafter, these waters have been conveyed by the bypass drain to the Santa Clara Slough.

In March 1977, Mexico completed the 37 miles of drain in its territory. This was done to comply with: (a) Point 4 of Minute No. 242, which provides an extension of the concrete-lined bypass drain canal from Morelos Dam to the Santa Clara Slough at the expense of the United States; and (b) Minute No. 248, adopted by the International Boundary and Water Commission on June 10, 1975. In June 1977, the part of the bypass drain which is in the United States was completed by the Bureau of Reclamation. On June 23, 1977, the extension of the bypass drain was placed into operation. The drain carries a capacity of 353 cubic feet per second of Wellton-Mohawk drainage water flows from the previous terminus of the Main Outlet Drain Extension at Morelos Dam to the Santa Clara Slough at the Gulf of California.

Investigations

Investigations have been made by the Bureau of Reclamation, at the request of Congress, concerning the salinity problem of the Colorado River below Imperial Dam and finding a definitive solution to the international problems of Colorado River waters delivered to Mexico under the terms of the 1944 Treaty. These investigations led to the preparation of a special report in September 1973 on the Colorado River Salinity Control Project; they also led to the authorization of the Colorado River Salinity Control Project. The final environmental statement on the project was issued June 18, 1975. The Interim Sizing Study, Yuma Desalting Plant, Special Report, was published in June 1977.

In 1971, research and development studies were undertaken, and the Yuma Desalting Test Facility was established with the installation of a mobile test facility that was originally constructed and operated by the Department of the Interior's Office of Saline Water. This facility was expanded in 1974 and was operated for the Bureau of Reclamation by a contractor. A status report was prepared in April 1977 that presented findings of advance planning studies and project activities through the completion of the project.

A Reject Stream Replacement Study was authorized by the Colorado River Basin Salinity Control Act of 1974. This is a study to identify feasible measures which could replace the amount of water lost through reject from the Yuma Desalting Plant, which would have been applied to meeting treaty obligations. The study was initiated in 1975, and completed in June 1980.

A background plan and status report of the Colorado River Basin Salinity Control Project was issued in January 1979.

Authorization

The Desalting Complex Unit was authorized for construction by the act of June 24, 1974, (88 Stat. 266), Public Law 93-320, Title I (specifically, section 101).

Construction

The first major construction of the Desalting Complex Unit began in September 1975, by award of a contract for the construction of the Main Outlet Drain Extension concrete siphon and the relocation of the City of Yuma's standby municipal pumping plant. Installation of these structures and construction of the bypass drain was completed in 1977. Construction of the Yuma Desalting Plant started in 1980. Part of the pretreatment facilities and the electrical power switchyard have been completed. Construction of the balance of the pretreatment facilities is scheduled to be completed in 1984.

Benefits

Social and political benefits accrue that are outside the normal realm of economic quantification; however, in addition to meeting the salinity provisions of Minute No. 242, the unit will have the capacity to annually reclaim about 108,000 acre-feet of water that is presently being bypassed, and therefore, lost for further use.

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