Desalination, an Emerging Water Resource Issue

In coverage of the current water events of the day desalination has lately been gathering greater national attention. Recent newsworthy events include the construction and operation of the Western Hemisphere’s largest desalination plant in Tampa. Meanwhile California is experiencing a surge of interest in desalination, with plants proposed or planned at 13 sites along the coast.

Nor is California limiting its desalination operations to coastal areas. The Calleguas Municipal Water District plans to remove salt from groundwater under the Simi and Conejo valleys, with plants converting up to 30,000 acre-feet a year of brackish groundwater into potable supplies.

From these and other developments from around the country it is obvious that desalination is shaping up to be an important 21st century water resource issue, with an expanding role in the perennial quest for additional water supplies. Although its water supply potential is getting most of the recent attention, desalination also can be used to improve water quality of current supplies.

What significance does this emerging issue have for Arizona? What salinity issues confront the state? What commitments or activities demonstrate the state’s concern about salinity? What resolution does the state seek to resolve its salinity problems?

The situation in Arizona shows desalination to be a multifaceted issue, with more at stake than just reducing the cost of the treatment process to increase water supplies. Arizona confronts certain political and water quality and supply issues that ensure some variations to the desalination issue as it plays out in other states.

One of the more obvious differences, one shared by many other states, is that landlocked Arizona will not be desalinating seawater. The value of inland brackish water as a water supply is being increasingly recognized. It is a resource that previously had been overlooked since other higher-quality, more potable water supplies were available. With supplies of such waters now more limited brackish water is attracting attention as a potential drinking water source.

It is Arizona’s involvement in the politics of salinity at the international level that distinguishes the state’s situation. In this arena, Arizona’s involvement with salinity and salinity control predates the current interest. Through its participation in the Colorado River Basin Salinity Control Forum Arizona has been active in desalination strategies since 1975.

The seven basin states created the forum in response to the political crisis arising when Mexico objected to the salinity levels of the Colorado River as its waters entered the country. The forum’s intent was to reduce salinity and recommend water quality standards for various points along the Colorado River. The issue was water quality, and the goal was to reduce the amount of salt entering the river from the upper basin states. Strategies included encouraging on-farm irrigation efficiencies to control salinity and taking measure to prevent saline groundwater from entering the river.

Recently Arizona’s interest in the Colorado River salinity issue has acquired a new focus due to its concern about ensuring the state’s supply of Colorado River water. Unlike other areas of the country where new desalination plants are being proposed, at issue in Arizona is the operation of a plant that was constructed over ten years ago. The state is urging the U.S. Bureau of Reclamation to put the Yuma desalination plant on line. To Arizona, desalination in this situation is a water supply issue.

The plant was constructed to reclaim drainage water from the Wellton-Mohawk Irrigation District, to ensure that water delivered to Mexico is suitable for beneficial use. Per treaty obligation, the United States is to ensure delivery of 1,500,000 acre-feet of water each year. Drainage from the irritation district averages more than 100,000 acre-feet per year which, because of its high salinity, was unsuitable for delivery to Mexico. In what was to be a temporary solution to the problem, drainage water was bypassed around the Mexican diversion at Morelos Dam, with the 100,000 acre-feet not figuring into the Mexican Colorado River allocation.

That the wastewater was being bypassed was not a critical issue at one time since there were abundant flows on the Colorado River during most of the 1990s, and the Lower Basin States were not using their full 7.5 million acre-feet allocation. Operation of the plant therefore, which was completed in 1992, was not critical. Times have changed, however, with water supplies back to normal and lower basin states’ water demands in excess of compact apportionments. Meanwhile the upper basin demands continue to grow.

Arizona argues that the bypassed water is in a sense a lost opportunity, its flows not credited as part of Mexico’s entitlement. It says that delivery of water in excess of treaty requirements will result in an increased frequency of water supply shortages in the United States. Arizona water users stand to be hurt the most since the Central Arizona Project is junior to all other water users in the Lower Basin.

The federal government is balking at starting upthe plant, offering instead to retire Arizona and California farmland to free up additional water supplies. Arizona doubts this a workable strategy and is urging the operation of the plant. This is a very controversial issue since the bypassed water has environmental value in maintaining the Cienga de Santa Clara in Mexico. If the plant were operating, the reduced wastewater then flowing to the cienga would greatly increase in salinity, posing a threat to the wetlands.

While desalination of Colorado River water is debated officials in central Arizona are pondering their salinity concerns and the possible role of desalination. The area is plagued by a salt loading problem, with the diversion and use of surface water resulting in an accumulation of salts. Phoenix area water officials have organized the Central Arizona Salinity Study to investigate the worsening problem.

Steve Rossi, the principal water resources planner for the City of Phoenix, says phase one of the study has been an evaluation of the problem. He says, “We studied the salt balance in the valley and asked how much salt is coming into the area vs. how much is leaving. We found that about a million tons per year of salt are coming into the valley and staying.”

Central Arizona Project water and water from the Salt and Verde rivers are responsible for much of the salt. Also contributing to the salt loading is reclaimed water. Water reclaimed for use from a treatment plant increases salinity. Estimates are that one cycle of municipal use increases the salt content of water by 200 to 400 milligrams per liter. Sanitary discharges and water softeners contribute to the salinity of reclaimed water.

Rossi asks, “How does the elevated levels of salinity in our source waters affect our ability to use the water down the road? The study is looking at that but also looking at the economics; for example, what does this high salt content mean for consumers and utilities?”

He says the shorter life span of fixtures and appliances and the cost to industry was estimated to be about $30 million per year. “That is a rough number, mostly direct costs, but it is a starting point.”

Groundwater in the southwest portion of the valley has especially high salinity, with water in the Buckeye area at 2,500 milligrams per liter and even more at times. (Brackish water is generally considered to be over 1,000 mg/l.) Rossi says some of this water could be treated to serve future water demands in the area. He says, “We are in some very preliminary discussions — very preliminary — about a possible regional brackish water desalination plant somewhere in the west valley. It could take this brackish water, irrigation tail water and water from the end of the Salt River Project system and convert it to potable supplies.”

Unlike areas in Southern California the Phoenix area is not overly strapped for water supplies to require immediate adoption of desalination. Rossi says, “From a water resource planning perspective we are not in a critical situation in the city, but we also realize that our growth path is phenomenal. Desalination of brackish groundwater is not something we need to do soon, but it is part of our long-range plan.”

Tucson also is participating in the Central Arizona Salinity Study. The city faces future water quality decisions that may involve desalination. In introducing CAP water to its customers, Tucson Water has been delivering a blend of groundwater and CAP water. Eventually, as increased amounts of CAP water are included in the blend, the salinity of water delivered to customers will increase, although it is unlikely to be more than 600 to 650 mg/l.

Water quality has been at ticklish issue in Tucson, with the troubled introduction of CAP water contributing to citizens’ wariness in matters of water quality. As a result, the utility is concerned about the public’s reaction to increased salinity. It intends to hold public hearings to solicit citizen input about what options to pursue. One option would be to build a desalination plant to ensure a high quality drinking water. This would be an expensive proposition that would increase the cost of water. This is not an immediate issue, but more of a down–the-road concern, to be considered as part of long-range planning.

In a sense, Tucson’s consideration of desalination is to improve the aesthetics of water. Drinking water at 600 to 650 mg/l is not unpalatable, although water users whose supply was once total groundwater and then a blend might object. In Phoenix, where water supplies tend to be of higher salinity than those in Tucson, citizens drink water of about 600 mg/l.

With desalination on the national water resource agenda, many areas now look to the treatment process to help maintain sustainable water supplies. Arizona at this point is not among them. Improvements in desalination technology, however, will benefit the state. Central Arizona will be better able to confront its salt loading problem, and communities like Tucson can improve the quality of current drinking water supplies.

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