Photos by Jeffry Scott / Staff Colorado River water will be recharged and blended at the $75 million Clearwater Renewable Resource Facility in Avra Valley.

Central Arizona Project water seeps into the ground in a 20-acre recharge pond at Tucson Water's Clearwater facility west of the city. The process removes organic matter and plant parts.

* Now: A blend of local ground water and the more mineral-laden river water will be funneled into the city utility's drinking water distribution pipelines.

Then: Unblended river water, intended to serve as a substitute for our dwindling underground reserves, was sent to about half of the customer base.

* Now: Delivery will not be limited to specific areas, and the direction of flow through the distribution pipes will remain consistent.

Then: When 58 percent of Tucson Water's connections - about 84,000 customers - received the river water, the utility reversed flow in some pipes as it shut down several ground-water wells. The flip-flop stripped rust from aging pipes.

* Now: The blend will be disinfected with chlorine, as is the current ground-water flow. Last time, chloramine, a combination of chlorine and ammonia, was used.

Then: Since chloramine can be toxic to human blood cells in kidney-dialysis as well as to pet fish, dialysis patients were advised to consult their doctors about how to remove it from water used in dialysis. Fish owners were pointed to pet stores for water-neutralizing fixes. No such warnings will be issued this time.

Changes in the blend's drinking water quality will occur at a trickle pace.

This spring, it will be dominated by ground water, with river water making up less than 5 percent of the flow. But within eight years or so, it will consist of a near half-and-half mix, which utility officials say can be maintained until at least 2021.

Click on graphic to enlarge

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Total dissolved solids -- a measure of minerals dissolved in water - average about 250 parts per million in local ground water. The local average for Colorado River water is 650 parts per million. The blend's limit is set for 450 parts per million. Tucson Water officials say customers found that level palatable during a 1998 taste test.

Tucson Water Deputy Director Marie Pearthree says the blend is needed to wean the city from dependence on ground water. Strain on aquifer reserves diminishes water quality and quantity, dries up wildlife-rich streams and causes subsidence - a sinking of land that can disrupt buried pipes and weaken building foundations.

"Tucson's lucky in that it has two systems, so there's always a backup water supply," Pearthree says.

Without the river water system, she says, the utility would likely be forced to eventually step up ground-water pumping in the the outskirts of the metropolitan area. Persistent pumping in central Tucson over the last five decades has caused the water table to drop by up to 200 feet in some spots.

The backup system will lift a summertime threat of mandatory water restrictions. Still, Tucson Water spokesman Mitch Basefsky says the annual call for voluntary measures will continue.

"Barring a systemwide problem, we're no longer going to run out of water," Basefsky says. "But the more we conserve, the more wells we can turn off in the central well field."

Last May, sweltering customers nearly maxed out Tucson Water's ready supply of ground water by using 130 million gallons a day. To help meet the demand, the utility cranked into operation four wells in its central well field that had been turned off.

This summer, in an attempt to spur replenishment of underground reserves, the utility aims to shut down 26 wells in that well field. By mid-2003, one-third of the central well field, about 85 wells, is slated for rest.

The idea to augment Tucson's ground-water supply with Colorado River water was kicked around for several decades before 1968, when Congress approved construction of the Central Arizona Project, the 336-mile aqueduct that delivers river water to Pima, Maricopa and Pinal counties.

In 1989, when construction of the CAP's string of canals, pumping stations and storage facilities finally stretched into Southern Arizona, the Tucson City Council called for a fast-paced switch to CAP delivery.

But - and other things, followed by claims asking for the city to pay the repair bills.

Two years later, City Hall turned off the river water system and returned Tucson Water to reliance on ground water. In the aftermath, utility officials conceded that they botched the debut.

Among the most glaring errors was the utility's fumbling of the chemical treatment process for pH, which left the CAP water acidic and corrosive. Consequently, it ate into older mains and caused leaks.

In the city's haste to start CAP water delivery and grapple with budgetary constraints, Tucson Water scrapped a plan to test it on various mains. Testing would have found that in some cases old galvanized-steel pipes were held together only by calcium deposits - similar to the white stuff at the bottom of a teapot - that had built up over the years under the flow of ground water.

In 1995, city voters passed the Water Consumer Protection Act, which bans direct delivery of CAP water unless it's purified to match the quality of top-notch local ground water. Tucson Water officials say an expensive membrane filtration operation would be needed to produce water of that quality.

The utility has spent the last several years mending the delivery system and customer trust, Pearthree says. Among other things, it has replaced 169 miles of worn-out galvanized iron mains and relined 43 miles of structurally sound cast-iron mains. And last year, it conducted a $1.9 million Ambassador Neighborhoods program, through which volunteer homes received a simulated sampling of the CAP-ground water blend.

"We feel like we're really ready" for the reintroduction of CAP water, Pearthree says.

This second try will revolve around the $75 million Clearwater Renewable Resource Facility in Avra Valley, about 15 miles west of city limits, where CAP water will be recharged and blended. Altogether, the city has poured about $250 million into its river delivery system, Pearthree says.

Over the next several months, Clearwater will produce an average of 18 million gallons of the CAP-ground water mix a day. When running full-throttle in spring 2003, it will yield 54 million gallons daily and 19.5 billion gallons a year -- about half of Tucson Water's annual customer demand.

Tucson Water serves an estimated 675,000 people, who collectively use an average of 101 million gallons daily.

Through the site's recharge process - a natural underground filtration - organic matter such as bacteria and plant parts are removed from the river water. About 3 1/2 years ago, the utility started piping river water from the nearby CAP canal to Clearwater's shallow recharge basins, which are kept wet for two weeks then dry for 10-day periods, during which algae die off and the basin's floor cracks and becomes more porous.

CAP water can reach the ground-water table - 375 feet below the surface - within six to eight months. But Tucson Water will pump the blend from depths of 580 to 1,000 feet, where ground water now dominates the mix.

After about two decades of infiltration, however, the harder-tasting CAP water could make up an estimated 86 percent of the blend, Pearthree says, pointing out that recharge does not lower mineral content. To maintain a near half-and-half blend, she says, Tucson Water plans to siphon ground water from another area in Avra Valley.

From Clearwater, the blend will be piped 11 1/2 miles to the Hayden-Udall treatment plant, which was built to handle direct delivery of unblended CAP water but mothballed in 1994. There, chlorine will be used as a disinfectant rather than chloramine because much of the organic matter will be screened out by recharge.

Chlorine wasn't used last time because its mingling with organic material in straight-up CAP water can yield trihalomethanes, a byproduct known to cause cancer in laboratory animals.

Federal law and an even stricter city ordinance, however, set health safety limits for trihalomethanes. Under the Environmental Protection Agency's cap, the theoretical risk in drinking tainted chlorine- or chloramine-treated CAP water over a lifetime is minuscule.

* Contact Maureen O'Connell at 807-7789 or oconnell@azstarnet.com.

* Monday, April 30: Arizona's competition and uncertainty on the Colorado River.

* Tuesday, May 1: How much growth can our water supply support?

* Wednesday, May 2: How Tucsonans are preparing for the return of the CAP.

* Thursday, May 3: Answers to your questions about the issue.

* Friday, May 4: Coverage of Thursday's reintroduction of CAP water.

Find out more about the CAP at its own Web site.

Tucson Water is the delivery source for CAP water, and more.

You might also be interested in StarNet's coverage of the Sonoran Desert Conservation Plan.