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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.
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 this water was to make a portion of 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 (WMIDD) is currently transported to the Cienega de Santa Clara (Santa Clara Slough) wetlands in Mexico. This water bypasses the Colorado River and the Yuma Desalting Plant, and the United States does not receive any Treaty delivery credit for it. The primary objective of Minute No. 242 is to limit the average annual salinity of about 1.36 million acre-feet (AF) of water delivered to Mexico at that country's Morelos Dam just downstream of the Northerly International Boundary between California and Mexico. Minute 242 requires that the salinity of Colorado River water delivered to Mexico must average no more than 115 +/-30 parts per million (ppm) annually over the annual average salinity of Colorado River water measured at Imperial Dam 27 miles upstream from Morelos Dam. Any water that may be delivered to Mexico under the 1944 Treaty by means of the All-American Canal is considered as having been delivered upstream of Morelos Dam for the purpose of computing this salinity.
The Desalting Complex Unit consists of both structural and nonstructural measures implemented in the lower Gila and Colorado River valleys of southwestern Arizona and northwestern Sonora, Mexico. The structural features included the construction of one of the world`s largest membrane desalting plants located about 4 miles west of Yuma. The plant was built to desalt about 73 million gallons per day. The original design was about 25 percent larger, but the capacity was reduced because of budget restrictions and changes in the quantity and quality of the drainage water resulting from an irrigation efficiency program implemented by WMIDD.
Other structural features built as part of this Unit include a 50-mile-long, concrete-lined Bypass Drain, which extends from the end of the Main Outlet Drain 3 near Morelos Dam to the Cienega; a buried, reinforced concrete pipe siphon at Prison Hill, which replaced 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 impacts on fish and wildlife habitat.
Nonstructural measures include a program to improve irrigation efficiency and reduce acreage in the Wellton-Mohawk Division of the Gila Project to reduce drainage flows from about 214,000 acre-feet annually in the 1960’s to the 2004-2008 annual average of 106,897 acre-feet.
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The plan for the Desalting Complex Unit was to reduce the quantity and improve the quality of drainage waters from the Wellton-Mohawk Division of the Gila Project to allow more than 70 percent of those flows to be included in the Treaty deliveries to Mexico after treatment, and to improve the salinity of those deliveries. The authorized plan included seven measures: (1) construction of the Yuma Desalting Plant (completed in 1992); (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 (completed in 1981); (4) implementation of measures to reduce Wellton-Mohawk acreage (initial reductions were completed in 1978; additional lands were removed in 1992 as part of the Salt River-Maricopa Indian Water Rights Settlement); (5) acquisition of Painted Rock Reservoir land and modification of its operation schedule (completed, but it was subsequently determined that the operation schedule did not need to be changed); (6) construction of the MODE siphon (completed in 1976); and (7) implementation of fish and wildlife mitigation measures (completed).
The major feature of the Desalting Complex Unit is a reverse osmosis membrane desalting plant known as the Yuma Desalting Plant (YDP), located about 5 miles west of Yuma, Arizona.
Although the desalting plant is not currently operating, it could, as currently configured, recover most, but not all, of the current bypass flow of approximately 107,000 AF of water annually. The plant was designed to treat drainage water with a salinity ranging from about 2,850 ppm to 3,450 ppm of total dissolved solids. At the time the plant was essentially completed in 1992, the salinity averaged about 2,900 ppm. The target value for the salinity of the desalted (or product) water is 160 ppm, but acutal salinity would vary depending on water temperature, the age of the membranes used to treat the water, and the salinity of the pre-treated water. The dissolved solids (salts) removed during the desalination process would be discharged in the Main Outlet Drain Extension (MODE) downstream of the plant.
If the plant was to be operated at full capacity, 10,000 AF or more of untreated drainage water would be blended with 60,784 AF of product water from the desalting plant to produce about 70,784 AF annually of recovered water. This water would be discharged into the Colorado River just west of the plant for inclusion in Treaty deliveries to Mexico. The total amount of blended water would vary from year to year, depending upon actual flows that occur below Imperial Dam. This water, combined with other drainage and river flows below Imperial Dam, would result in 1.360 million AF of total water being delivered to Mexico at the Northerly International Boundary each year in accordance with the 1944 Treaty and Minute No. 242.
The Main Outlet Drain (MOD), MODE and Wellton-Mohawk Main Conveyance Channel carry drainage water from the WMIDD to the desalting plant and past it to the Bypass Drain in Mexico. The Bypass Drain begins at the end of the MODE near Morelos Dam, approximately two miles south of the YDP. From there it runs west of adjacent to the Yuma Valley Levee on the east side of the Colorado River for approximately 16 miles to the Southerly International Boundary between the United States and Mexico, crossing the border near San Luis Colorado, Mexico. It then extends south for 37 miles through Mexico to the upper end of the Cienega de Santa Clara. Concentrate (the water that is left after the desalting process) from the YDP would be carried to the Bypass Drain via the concrete-lined the MODE.
The portion of the Bypass Drain in Mexico was constructed by Mexico with funding provided by the United States. This concrete-lined drain is a total of 53 miles long with a capacity equal to that of the MODE, MOD and Wellton-Mohawk Conveyance Channel (353 cubic feet per second). With the desalting plant in normal operation, the flow in the Bypass Drain could consist of concentrate plus some raw drainage water from WMIDD. The volume and salinity of that flow would be highly dependent upon the amount of raw drainage blended with concentrate from year to year, as well as other factors.
A 12.5-foot-diameter semicircular metal flume section of the MODE was replaced with a buried reinforced-concrete pipe siphon 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 (cfs).
Irrigation efficiency improvement programs were conducted in the Wellton-Mohawk Division of the Gila Project to reduce drainage flows from irrigation. Drainage water from the WMIDD is pumped from the groundwater aquifer beneath irrigated lands to maintain a desired minimum water table depth of 8 feet. The programs were overseen by the Technical Field Committee (comprised of professionals from several Federal agencies), and included irrigation management services, on-farm improvements, research and demonstrations, accelerated education, and land acreage reduction. By the time the desalting plant was completed, these programs improved the irrigation efficiency in the Wellton-Mohawk Division from 54 percent to approximately 70 percent, reducing the drainage flows from about 214,000 AF to about 107,000 AF annually in the 1990s.
The irrigable land in the Wellton-Mohawk Division was reduced from 75,000 acres initially authorized for development to about 65,000 acres. The acreage reduction program was accomplished to remove developed land with marginal irrigation efficiency from the WMIDD, to remove some of the most saline groundwater, and to prevent further development of undeveloped land within WMIDD. Part of the land was in Federal ownership; the balance was acquired by the Federal government from the State of Arizona 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 Dam and Reservoir, located near Gila Bend, Arizona, is operated by the U.S. Army Corps of Engineers, which developed the flood release schedule for the dam. This schedule ws developed in an attempt to coordinate maximum releases that would be contained within the 10,000 cfs capacity Gila River channel that runs through the WMIDD. Actual releases from Painted Rock Dam are determined on a case-by-case basis, and generally depend on the amount of water being released from Colorado River reservoirs, and the impacts the releases will have on the Yuma area and Mexico.
Section 101 (j) of Public Law 93-320 authorized the Secretary of the Interior, through the Corps, to acquire additional land in Painted Rock Reservoir for temporary water storage capacity needs resulting from modifications to the operation schedule due to Minute 242 requirements. The Corps 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; thus, these lands were not acquired.
It was anticipated that some loss of fish and wildlife habitat would occur as a result of the development of the Desalting Complex Unit and the Protective and Regulatory Pumping Unit. Mitigation measures were implemented to replace about 65 percent of the losses that occurred. The mitigation measures were not originally authorized under Title I of the Colorado River Basin Salinity Control Act, but were included in amendatory legislation -- Public Law 96-336, dated September 1978.
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 extended in 1973 with 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 testing of the desalting membrane units was done at each manufacturer`s expense, while the Federal 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.
A state-of-the-art advanced water treatment research center is now located at the Yuma Desalting Plant Complex. The Water Quality Improvement Center (WQIC) is one of six National Centers for Water Treatment Technologies and the only center focused on inland brackish water, both surface and groundwater. Research is conducted by the Bureau of Reclamation in partnership with academia, state and local governments, and the private sector.
A switchyard to provide power to the desalting plant was constructed at the plant site in 1979. The switchyard is connected to the Department of Energy`s Western Area Power Administration`s Pilot Knob Substation by a 3.8-mile-long transmission line.
A sewage treatment plant was also constructed to treat the sewage effluent from the desalting building and other on-site facilities. The sewage system was modified in 1991, 1992, and 1999 by the addition if a life station, an upgrade to the sewage treatment plant, and the addition of another septic tank leach field system. The sewage system has an Aquifer Protection Permit issued by the Arizona Department of Environmental Quality in 2000 for the life of the Yuma Desalting Plant.
An energy recovery system was constructed to recapture energy from the high pressure concentrate pipes. The system includes impulse turbines directly connected to high pressure pumping units.
Operation of the YDP would generate biosolids from the pretreatment process. The biosolids, consisting primarily of water and calcium carbonate, would be transported under high-pressure through a steel pipeline to a dispostal site called the "A-22 evaporative ponds" located approximately 20 miles southeast of the plant. To reduce the volume of the biosolids at the A-22 site, the water will be allowed to evaporate, and the dry biosolids would then take up a smaller area. Currently, the A-22 site is being used to store and dispose of by-products from testing carried out at the Water Quality Improvement Center adjacent to the YDP.
The A-22 site was designed to provide for 50 years of full-scale continuous operation of the YDP. The portion of the site constructed to date consists of three sets of evaporation ponds. The first set consists of four cells built as test units to verify the earthen berm design slopes and liner data. The second and third sets each consist of 32 cells -- each cell is an individual, lined evaporation pond with associated piping, with a surface area of approximately one acre. After a cell is filled, the liner will be extended over the top of the cell and covered with soil to create a permanent closure. All the cells are lined with polyvinyl chloride (PVC) liner to prevent the saline water or any of the constituents contained in the biosolids from percolating into and possibly contaminating the groundwater. Five monitoring wells located throughout the site are used to monitor the groundwater.
Other facilities at the disposal site include an administration building, a potable water well, an access road, and transmission lines.
The Bureau of Reclamation owns and maintains the Yuma Desalting Plant.
The United States and Mexico signed a treaty on February 3, 1944 that guaranteed Mexico 1.5 million acre-feet of Colorado River water annually. Of that total, about 1.36 million AF are delivered annually upstream of Mexico's Morelos Dam, which is located west of Yuma, Arizona. 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 occurrences were the principal causes of this increase:
(1) The WMIDD began pumping saline waters (initially averaging about 6,000 ppm) to lower the high groundwater levels below the crop root zone in the aquifer under the Wellton-Mohawk Division of the Gila Project. This saline drainage was carried through the Wellton-Mohawk Main Outlet Drain (MOD) and discharged into the Gila River, a tributary of the Colorado River, near the confluence of the two rivers. It then flowed into the Colorado River and was subsequently delivered to Mexico as part of the Colorado River requirement 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 and the filling of Lake Powell. This left less Colorado River water for blending with the saline Wellton-Mohawk drainage.
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 and constituted a violation of the treaty.
In response, the United States began to modify its river operations and Wellton-Mohawk drainage pumping in 1963. In March 1965, an agreement (Minute No. 218) was reached by the two governments for a 5-year period. 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 agreement did not become effective until the MODE was completed and put into operation.) The MOD could discharge Wellton-Mohawk drainage water into the Gila River near its confluence with the Colorado River. However, the MODE enabled the United States to discharge all 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. The amount of water that was bypassed without credit to treaty deliveries amounted to about 50,000 AF annually. In 1971, Mexico requested that the United States discharge an additional 40,000 to 75,000 AF of Wellton-Mohawk drainage flows annually into the Bypass Danal for conveyance to the Cienega de Santa Clara 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 AF annually of Wellton-Mohawk drainage water without credit as water delivered to Mexico by the United States; this was 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 AF 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 AF of water to be delivered annually to Mexico, at the Southerly International Boundary at San Luis Rio Colorado, Mexico, 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 Cienega de Santa Clara 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 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 Cienega de Santa Clara.
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 Cienega 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 Cienega at the Gulf of California.
At the request of Congress, the Bureau of Reclamation conducted investigations concerning the salinity problem of the Colorado River below Imperial Dam, to find 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, as well as eventual authorization of the 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, the Yuma Desalting Test Facility was established east of Yuma by the U.S. Department of the Interior's Office of Saline Water to test pre-treatment processes and membrane desalting equipment. This facility was expanded in 1973, and was operated for the Bureau of Reclamation by a contractor. Following this expansion, desalting membrane manufacturers brought test units to the facility to test desalting membranes at their own expense. Seven companies installed test units in 1973 and early 1974. Two additional units were put on-line in late 1975. Nearly all the units were membrane elements of commercial size and configuration. In June 1982, testing at the facility was completed and subsequently discontinued. The test facility was then disassembled.
A Reject Stream Replacement Study was authorized by the Colorado River Basin Salinity Control Act of 1974. This was a study to identify feasible measures which could replace the amount of water lost as concentrate 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.
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), and amended by P.L. 96-336 of September 4, 1980.
The first major construction of the Desalting Complex Unit began in September 1975 with the 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, and was essentially completed in 1992.
Shortly after the plant was completed in 1992, it was commissioned for operation at one-third capacity. However, plant operations ceased in 1993 when a flood on the Gila River damaged the Main Outlet Drain Extension (MODE) intake canal that provides feed water to the plant. This damage was subsequently repaired, but for the next several years, high flows on the Colorado River made operation of the YDP unnecessary. The plant has been maintained in a "ready reserve" state so it would be capable of operating if needed. Since 1993, multiple technological improvements have been made to the YDP, primarily to stay current with advances in water treatment, and some of the design deficiencies identified during the 1992-93 test operation have been resolved.
In 2007, Reclamation conducted a demonstration run of the plant, operating it at about ten percent of full capacity for three months. The purposes of the run were to acquire current operational data, test equipment already replaced to address design deficiencies, and conduct research applicable to the resolution of remaining design deficiencies.
Drought conditions, population growth, and the continuing need for water for municipal, environmental, and recreational uses in the lower Colorado River Basin have created further demand on the already stressed Colorado River water supply. In 2008, Reclamation was contacted by the major municipal water supply entities in Arizona, southern California and southern Nevada regarding the need to obtain information about the plant's capability and operational readiness. This information can only be obtained through actual operation of the facility. Without this real-time information, Reclamation would not be able to determine whether the YDP could reliably operate on a long-term basis in the future, or determine what, if any, improvement to the facility may be necessary to ensure the most efficient, cost-effective and reliable long-term operation.(Long-term operation is not currently under consideration.)
To obtain the necessary data, the plant needs to be operated at a scale and for a duration which covers seasonal variation when chemical use and power consumption are highly variable. Reclamation proposes to obtain this data through a pilot run of the YDP, during which the plant would be operated at up to one-third capacity of the original design for 365 operating days during a 12- to 18-month period. If conducted, the pilot run would begin in 2010.
Under the proposed pilot run, flows in the Bypass Drain would be reduced by approximately 29,000 AF, while salinity levels would be increased by about 540 ppm. The Environmental Assessment (and associated appendices A-D and E) for this proposed action focused on this potential impact on the environmental resources of the Cienega. While the statutory provisions of the National Environmental Policy Act and the Council on Environmental Quality implementing regulations do not require assessment of environmental impacts in the sovereign territory of a foreign nation, Reclamation, through the International Boundary and Water Commission, initiated consultation with Mexico regarding the proposed pilot run.
The outcome of this consultation was a Joint Report dated July 17, 2009. The United States, Mexico, and a partnership of non-governmental organizations, as stated in commitment letters from each party and further outlined in the Joint Report, will each arrange for 10,000 AF of water (for a total of 30,000 AF) in connection with the reduction in flow and increase in salinity level if the pilot run proceeds. Furthermore, these entities have committed to working through the Colorado River Joint Cooperative Process, pursuant to Minute 306 of the Treaty, to continue to address long-term approaches to maintain the environmental values of the Cienega.
In addition to meeting the salinity provisions of Minute No. 242, and reclaiming the water that is being bypassed from the Wellton-Mohawk District to the Cienega de Santa Clara, the Yuma Desalting Plant has social and political benefits accrue that are outside the normal realm of economic quantification.
Research conducted under Title I of the 1974 Colorado River Basin Salinity Control Act develops technologies and methods to economically improve water quality. These improvements make more water available for agricultural, municipal, or environmental use in the United States and Mexico. Technology developed through research is also used to treat agricultural drainage for return or reuse. These new technologies and methods provide Reclamation with more options to manage groundwater and meet salinity obligations to Mexico in accordance with Minute No. 242.
The Water Quality Improvement Center (WQIC) is located adjacent to the YDP and was constructed as part of the Yuma Desalting Plant Complex Unit. The WQIC was originally called the "Test Train" until it was commissioned in 1997 as a National Center for Water Treatment Technology initiated by the Bureau of Reclamation, the National Water Research Institute, and the U.S. Army. The WQIC provides the potable water used at the YDP site, which is also the location of Reclamation's Yuma Area Office. The WQIC was originally designed as a 1-to-100 scale model of the YDP to be used for equipment and process testing and improvements prior to alterations to the desalting plant itself. The 14,000 square-foot WQIC houses membrane water treatment research equipment from bench-scale to full-scale, serving as a field site to investigate new and improved water treatment technologies, including pretreatment associated with desalination.
WQIC research goals include: (1) identify technologies and processes to advance the state of water treatment technology and reduce the costs to treat impaired waters by conducting united research efforts between government, academia, and private industry through Cooperative Research and Development Agreements, and (2) conduct research under the YDP Cost Reduction Program, as authorized under the Act, to promote the identification of technologies and processes that will reduce the cost of operating the YDP.