A yearlong test run of the idled and aging Yuma Desalting Plant on the Arizona-Mexico border exceeded expectations, proving to federal and state water officials that the plant can turn large volumes of salty agricultural runoff into new freshwater supplies in the drought-stricken Lower Colorado River Basin.

But the plant's success may have come at the expense of an important wetland near the plant.

The 15,000-acre wetland called the Cienega de Santa Clara is an important stopover for migrating birds and provides habitat for more than 95 species, including the endangered Yuma clapper rail.

About 25 percent of the flow that feeds the wetland was diverted to the plant during the pilot project, and while much of that water has been replaced under a binational agreement, just how well those safeguards protected the wetland remains to be seen.

The desalting plant, which ran at one-third capacity for about a year, treated 29,000 acre-feet of irrigation runoff to help meet treaty obligations that require the United States to send 1.5 million acre-feet of potable Colorado River water to Mexico each year. The amount of treated water released into the river is enough to meet the needs of about 116,000 people for one year.

The Yuma Desalting Plant on the Arizona-Mexico border could provide millions of gallons of potable water to thirsty Southwestern cities, but its operation also may compromise a vital wetland south of the border. Photo courtesy of the Bureau of Reclamation.

The Bureau of Reclamation, in collaboration with the Metropolitan Water District of Southern California, the Central Arizona Water Conservation District and the Southern Nevada Water Authority, orchestrated the trial run -- the first major test of the 19-year-old plant -- to evaluate the plant's performance and operating costs.

The water agencies provided funding for the $23 million project and in return received water credits. With treated agricultural runoff from the Yuma plant contributing to Mexico's share of Colorado River water, Reclamation was able to leave that amount of water in upstream reservoirs, where it will be used to augment supplies in Arizona, California and Nevada.

Reclamation officials said no major problems arose during the entire test run, even after an earthquake that hit the area last spring. "Throughout the operation, the Yuma Desalting Plant performed above expectations," said Lorri Gray-Lee, Reclamation's Lower Colorado regional director.

The agency had allowed 12 to 18 months for the test run, but the plant's desalination system proved so effective that officials were able to reach their target of 29,000 acre-feet much sooner and at a lower cost than expected, said Jennifer McCloskey, manager of Reclamation's Yuma Area Office in Arizona.

"We ended about six weeks early," she said. "We started in May and ended in March. That's because the water recovery from the plant ended up being better than expected."

A few leaks occurred during the test run, but officials were able to fix them without taking the plant offline, she added.

Officials decided to conduct the long-term, high-volume pilot run after a 90-day demonstration run at 10 percent capacity in the spring of 2007 showed encouraging results (Land Letter, May 6, 2010).

The water authorities are hoping that the plant, which was mothballed shortly after it came online in 1992 due to flood damage, can be resurrected.

Drought, population growth and the impacts of climate change in the Southwest have placed unprecedented stress on the Colorado River, which supplies water to almost 30 million people in seven states.

Several studies, including one by the U.S. Center of the Stockholm Environment Institute in February, predict shrinking water supplies in the Southwest over the coming decades (Land Letter, Feb. 24).

McCloskey said that while the long-term pilot run exceeded expectations, so far there are no plans to move ahead with resuming permanent operations. Reclamation is preparing a report on the test run, which will be available to the public, she said.

"I have no funding or direction to consider any future operation at this time," she said. "We're just assessing the information that we have."

Wetland implications

The Cienega de Santa Clara benefited greatly from the closure of the plant, which lies about 5 miles west of Yuma. Highly saline agricultural runoff that would have been diverted to the facility for treatment was rerouted into the Colorado River Delta, bypassing the Lower Colorado River and expanding the wetland, which grew to about 40,000 acres.

The wetland typically receives about 107,000 acre-feet of agricultural runoff water per year, but while the plant was operating during the test run, it lost about 25 percent of those flows.

Under a much-touted agreement signed before the test run began, however, the U.S. and Mexican governments, along with Environmental Defense Fund and ProNatura Noroeste, committed to providing "replacement" water for the wetland. The United States sent overflows from rainstorms to the wetland last fall, McCloskey said, and Mexico is close to fulfilling its part of the agreement.

About half of the water from the environmental groups, obtained by purchasing water rights from Mexican farmers, has yet to be delivered, but there is still time to complete those deliveries within the 18-month time frame of the agreement, said Jennifer Pitt, director of EDF's Colorado River Project.

"I am very confident by the end of that time period the full complement of water will have been replaced," she said. "Considerable amounts of replacement water [have] shown up."

The binational agreement that ensured the wetland's water supply while the plant was operating marked a turning point in U.S.-Mexico relations on water policy, Pitt added. She said she hopes that collaborative spirit will lead to further progress on Colorado River Delta issues in the future. "I feel like we've come such a long way," she said.

While scientists are still studying the effects of the trial run on the wetland, Pitt said she believes the chances are good that the replacement water kept the cienega from suffering any major ecological damage. "We'll have to wait and see. But since we weren't decreasing the flows, my hope is that impacts are minimal."

Reese writes from Santa Fe, N.M.