WSP09892

• File Number: 8278.10
• Description: Title I - Yuma Desalting Plant
• Basin: Colorado Mainstem
• Date: 1/1/1986
• Author: USDOI/BOR
• Title: Yuma Desalting Plant Background and Briefing
• Water Supply Pro - Doc Type: Project Overview

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Yuma Desalting Plant

The Colorado River is a 14 hundred-mile-long ribbon of life for over 15 million people in the
southwestern United States and Mexico,
For years it's been known as a river of too much or too little. Today the Colorado is one of the
world's most regulated rivers. But regulation necessary to ensure a sufficient quantity of water
for the users has also exacted a price in the quality of the water available.
As the Southwest was being developed during the early part of this century, the big question
was "will there be enough water?' Today people also ask, "How good will the available water be?'
Salinity is a naturally occurring phenomenon in rivers. Before the beginning of this century,
the salinity of the Colorado River water at its source high in the Rocky Mountains was about 50
parts per million.
Where the river crossed the border into Mexico in the early 1900s, the level was about 400
parts per million, In regulating the Colorado, two things have occurred since the beginning of this
century that have now almost doubled the salinity of the water arriving at the Mexican border-
to around 800 parts per million.
Construction of over 20 reservoirs in this century has tremendously increased evaporative sur-
faces.
When water evaporates, the dissolved salts are left behind, increasing salinity in the same
manner that water evaporates from the oceans.
The development of numerous irrigation and water user districts along the river has also in-
creased salinity.
Irrigation water from the river is routed to farmlands and is applied to crops, Concentration of
salts also occurs from evaporation in the fields and from transevaporation from crops, The water
not absorbed by crops percolates through the mineral-rich soils of the southwest U.S. These natu-
rally occurring minerals dissolve in the water, making it saline, as it percolates down into the
groundwater, These minerals include sodium chloride (or table salt), sodium bicarbonate (or bak-
ing soda), and calcium sulfate (or gypsum),
To keep the groundwater table from building up to the root zone of the plants and drowning
them, this saline groundwater is naturally drained or sometimes pumped out of the ground and
returned to the river system.
In southwestern Arizona, the Wellton-Mohawk Irrigation and Drainage District, east of Yuma,
was one of the last districts developed in the 1950s. The saline return flows, or irrigation drain-
age from these farmlands east of Yuma had a marked impact on river salinities during times of
low river flows. When this drainage water was put into the lower end of the river in the 1960s
near Yuma above Morelos Dam, river salinities sometimes exceeded 12 hundred parts per million,
In 1961 Mexico expressed concerns that the increased salinity was harming crops in the Mexi-
cali Valley.
During 12 years of negotiations with the Mexican government, various interim agreements
were ironed out. A permanent and definitive solution was reached in 1973 through the efforts of
the Brownell Commission. This was a group of representatives from the U.S. that coordinated
with the seven basin states and negotiated with Mexico.
This 1973 international agreement conformed with terms of the earlier 1944 Treaty between
Mexico and the U.S. that essentially provided for the delivery of one and a half million acre-feet
of Colorado River water per year to Mexico.
In the 1973 international agreement, called Minute No. 242, the United States agreed to a sa-
linity level for water being delivered to Mexico at Morelos Dam. The salinity level for water deliv-
ered at Morelos Dam would be determined by the quality of water arriving 27 miles upstream at
Imperial Dam. Specifically, the salinity of the water delivered to Mexico at Morelos Dam must av-
erage no more than 115 (+ or minus 30) parts per million annually above the average salinity of
Colorado River water arriving at Imperial Dam.
This agreed-upon salinity level would be achieved by constructing a desalting plant. Enough of
the salts would be removed from irrigation return flows to make the water acceptable for dis-
charge into the river and later delivery to Mexico,
Authorization to begin work was provided by the Colorado River Basin Salinity Control Act,
passed by Congress in June 1974.