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Water Resources

Arizona Using Underground Water Storage

August 27, 2004

For the past 10 years, billions of gallons of water have been spilled onto Arizona’s dry riverbeds and flat, undeveloped desert by the state’s two largest water suppliers-the Salt River Project (SRP) and the Central Arizona Project (CAP)-where it seeps into underground storage reservoirs called aquifers. The intentional discharges are known as water recharge or water banking. These water recharge projects allow Arizona to collect its full share of Colorado River water, even when it doesn’t need it, and to then protect it from the stresses of growth and drought for future use.

The SRP opened the first water bank in 1994, and to date has recharged close to 770,000 acre-feet of water in the Granite Reef Underground Storage Project. The CAP has stored more than 400,000 acre-feet of water in five projects throughout the state. All told, the state’s water banking program has stored more than 1.8 million acre-feet. (An acre-foot consists of 325,851 gallons, enough water to serve a five-person household for one year.)

The Granite Reef project, situated on 200 acres in the dry Salt River, is an ideal location for a recharge basin. Riverbeds have layers of soil, sand and gravel above the aquifer that allow water to percolate quickly resulting in reduced water lost to evaporation-Granite Reef loses less than 2 percent a year. The project works much like any other bank. Some of the water deposits made are saved for the long-term when water supplies run short, while other withdrawals are made soon after they’re deposited at other locations.

The City of Chandler put some of its SRP and CAP allocations into the Granite Reef project.  It received a credit for the deposit which allowed it pump water from one of the municipal wells without violating the state’s strict groundwater regulations. The City of Glendale has entered into an agreement with SRP to use part of a new recharge project, slated to open in 2005, to recharge effluent from its treatment plant located in close proximity to the new project.

Some western water managers, however, are not sold on the idea that aquifer recharging is a beneficial use of surface water.  One western state has even hinted that if the Colorado River is unable to meet demands in future years, the state will challenge the water bank and force Arizona to leave water in the river. Other states are not so critical and are even studying the Arizona design as a way to help states make more efficient use of increasingly rare water surpluses by banking supplies for future use.

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