General Description

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

The Protective and Regulatory Pumping Unit is located within a 5-mile-wide strip of land along the United States/Mexico border in southwestern Arizona. The strip of land extends about 13 miles eastward from the vicinity of San Luis, Arizona.

The objectives of the unit are to manage and conserve the United States ground-water resources for the benefit of the United States, and to provide obligated water deliveries to Mexico.

The unit has been developed by constructing a well field and delivery system, called the 242 Well Field and Lateral, to intercept part of the ground-water underflow that is moving southward into Mexico from Yuma Mesa in the United States.

Prior to enactment of Public Law 93-320 (which authorized the unit) and Minute 242 (which effects pumping limitations), this ground-water underflow was being increased by withdrawals of ground water in Mexico from the San Luis Mesa Well Field immediately south of the Southerly International Boundary, the boundary between Arizona and Mexico near San Luis, Arizona, and Sonora, Mexico. The development of the unit in the United States will reduce the amount of ground-water underflow to Mexico.

The ground water recovered by the unit is collected in a conveyance system (the 242 Lateral) and is delivered to Mexico by the United States as a portion of the treaty obligations of Colorado River water.

Major features of the unit consist of the field of 35 wells, the 242 Lateral and other connecting laterals, a 34.5-kilovolt transmission line, and attendant facilities.

Plan

There are no natural or constructed surface drains to carry irrigation drainage water from Yuma Mesa. Some of the Colorado River water used for irrigating the mesa's agricultural lands infiltrates into the underlying aquifers in the United States and flows south into Mexico. Since much of this water is being lost to Mexico without being credited as treaty deliveries from the United States, interception and pumping of the ground water underflows by the 242 Well Field will permit the recovery of part of this valuable water resource. As this water is credited through surface deliveries to Mexico, an equal amount of water can be retained in storage upstream for beneficial use in the United States.

In accordance with the agreement of Minute No. 242, each country is limited to pumping no more than 160,000 acre-feet of ground water per year within its 5-mile zone. In Mexico, ground water is pumped by the 63 wells of the San Luis Mesa Well Field. The water is then collected in a canal and conveyed to agricultural lands.

In the United States, the unit well field is planned with a maximum total pumping capacity of 125,000 acre-feet of ground water per year. This, coupled with about 15,000 acre-feet per year of drainage return flow, will permit the United States to deliver about 140,000 acre-feet of water per year into the Mexican canal system near the Southerly International Boundary. This is in combination with 1.36 million acre-feet per year of drainage and regulatory flows of water delivered to Mexico upstream at the Northerly International Boundary. The legislation also provides that private water users within the 5-mile zone may also use an additional 35,000 acre-feet of water per year, part of which may be supplied by 10 additional wells which could bring the total to a possible 45 wells. These additional wells may be required primarily for peaking capacity; that is, at any given moment the short-term demand may exceed the capacity of 35 wells.

Unit descriptions and facilities

The existing unit wells are located in a continuous line, spaced at 0.5-mile intervals along the southern portion of Yuma Mesa next to the Southerly International Boundary. This field now consists of 21 wells and is to expand to a possible 35 wells.

Plans for the well field development include completing the 14 additional wells in a line 1 mile north of the initial group. The rate of installation of these wells will be determined by the rate of decrease in drainage flows, the amount of change in the ground-water table, and the rate at which private irrigation and municipal users withdraw ground water by private wells. Maximum ground-water pumping within the 5-mile zone will be limited to 160,000 acre-feet per year.

The pumping of ground water through 1990 would ensure the capability of the United States to meet the 140,000 acre-feet per year obligation of water customarily delivered to Mexico at the Southerly International Boundary. Excess pumping capability from the wells will be available to supply additional water requirements for irrigation, municipal, and other uses.

Hillander "C" Irrigation District, which is a private development adjacent to the well field, has entered into a temporary water contract with the U.S. Government for pumping up to 4,000 acre-feet of water per year from the 242 Lateral, and a permanent contract is being developed. This district includes about 3,440 acres, with about 2,300 acres currently under irrigation.

The 242 Well Field, along with pumping in Mexico, may have a notable impact upon the existing private wells by lowering the ground-water levels. Domestic users in the 5-mile zone, other than Hillander "C" Irrigation District, are pumping about 4,000 acre-feet of water annually for municipal and irrigation use. The 242 wells are drilled to a depth of about 600 feet; the lower 300 feet are screened. Each well is designed to pump an average of 7.5 cubic feet per second. The discharge of each well is collected in the 242 Lateral, which is a conveyance system that carries water westerly to the downstream side of the existing Boundary Pumping Plant, a facility of the Valley Division of the Yuma Project.

The 242 Lateral is designed as an open and closed system consisting of a pipeline on each end and an open concrete-lined channel in the center. The reinforced concrete pipe ranges from 27 to 72 inches in diameter, and its design capacity increases as it passes each well. The concrete-lined open channel has a lining thickness of 2.5 inches and a bottom width that ranges from 4 to 6 feet. Water depths range from 2.8 to 5.8 feet with water velocities from 3.6 to 4.8 feet per second. Two settling basins have been constructed within the open channel portion of the lateral to provide storage capacity for the accumulation of sediment, mostly windblown sand. A stand box, a vertical open structure which serves as a relief vent, is located at each well site along the pipeline to eliminate water hammer. The end of the 242 conveyance channel, the terminal discharge pipeline, is a 72-inch reinforced concrete pipe designed to carry 220 cubic feet per second of pumped water.

Attendant Facilities

A 12-mile-long access road was constructed along side the 242 Lateral and the well sites. This road serves the construction, operation, and maintenance of the unit facilities.

The Department of Energy's Western Area Power Administration (WAPA) has constructed a 69-kilovolt transmission line east of Yuma to WAPA's Sonora Substation, located about 18 miles south and slightly west of Gila Substation. The Sonora Substation is also located about 2.5 miles north of well 242-22, the eastern end of the well field. To distribute power to the existing well pumps, a 34.5-kilovolt transmission line connects Sonora Substation with the 34.5-kilovolt transmission line constructed next to the access road and conveyance system. The 34.5-kilovolt electrical distribution system also connects to the Yuma County Water Users' Association's system at the western end of the well field. The WAPA facilities were completed in 1983.

Water deliveries to Mexico at the Southerly International Boundary will be coordinated with deliveries to the Northerly International Boundary located between California and Mexico. This will minimize overdeliveries to Mexico.

Fish and Wildlife

The development of the Protective and Regulatory Pumping Unit (and also the Desalting Complex 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.

Operating agency

All unit works are operated and maintained by the Bureau of Reclamation.

Development

During the negotiations between the United States and Mexico that were to resolve the salinity problems of the Colorado River water delivered to Mexico (Minute No. 242), the United States Government brought to the attention of the Mexican Government that the ground water underlying the United States was being withdrawn by Mexican pumping. This was due to operation of a well field that Mexico had installed immediately south of the Southerly International Boundary. It was recognized by the United States that this withdrawal of ground water would significantly affect the United States in several areas, particularly since Mexico expressed the intention to continue pumping from the well field to supply irrigation water to Mexican agricultural lands. Among other things, Minute No. 242 stipulates that the United States and Mexico will limit ground-water pumping within each country to 160,000 acre-feet annually within 5 miles of the Arizona/Sonora border. By pumping these waters, Mexico withdraws ground water from the United States without credit for delivery of water from the United States to Mexico in accordance with treaty obligations.

Yuma Valley agricultural drainage and irrigation waste are delivered to the Mexican canal system and are credited toward the 1.5 million acre-feet-per-year delivery requirement of Colorado River water to Mexico. Historically, these annual flows have been about 125,000 acre-feet of drainage returns and 15,000 acre-feet of regulatory waste (unused irrigation water) from the Yuma Canal system. Pumping by Mexico and the United States has lowered the ground-water elevations and is reducing the amount of drainage underflows from Yuma Valley. The annual combined flow of about 140,000 acre-feet per year at the Southerly International Boundary will gradually decline to only 15,000 acre-feet per year. The Protective and Regulatory Pumping Unit is intended to maintain deliveries near the 140,000 acre-foot level.

To maintain the treaty flow to Mexico, any reduction in deliveries at the Southerly International Boundary would have to be made up at the Northerly International Boundary by increased deliveries of water from other sources. Under present conditions, this can be done only by releasing additional stored river water not now committed to Mexico.

In compliance with Minute No. 242 and Section 103 (a) of the Colorado River Basin Salinity Control Act of 1974, a memorandum report was prepared on protective and regulatory ground-water pumping in November 1974. Presented in this report were alternative plans for a protective and regulatory ground-water pumping field, which set the framework for development of the Protective and Regulatory Pumping Unit.

Authorization

The unit was authorized by the Colorado River Basin Salinity Control Act, Title I, of June 24, 1974 (88 Stat. 266), Public Law 93-320. Section 103 (a) of the Act authorized the Secretary of the Department of the Interior (consistent with Minute No. 242) to construct, operate, and maintain well fields capable of furnishing about 160,000 acre-feet of water per year for use in the United States and for delivery to Mexico in satisfaction of the 1944 Mexican Water Treaty. The Act also established a 5-mile protective pumping zone along the Southerly International Boundary.

Construction

Construction of the unit features began in 1977. Installation of the access road and the 242 Lateral, along with construction of 14 wells and associated features, was completed in 1978. By 1983, a total of 21 wells had been completed. Completion of the remaining wells has been deferred until after the high surplus flows that began in 1983 have receded and until the wells are needed to help meet the treaty obligations.

Benefits

Social and political benefits accrue that are outside the normal realm of economic quantification; however, the Protective and Regulatory Pumping Unit will use the United States ground-water resources in the Yuma area for the benefit of the United States and will provide water deliveries to Mexico, thereby conserving upstream Colorado River water.

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