Safe-yield: A Balancing Act For Arizona’s Aquifers
By Warren Tenney
The goal of safe-yield has motivated Arizona's cities to maximize their use of renewable water supplies - such as river water or recycled wastewater - and to minimize pumping groundwater. Safe-yield means the amount of water pumped out of the ground is the same as what goes back into underground aquifers. The efforts of Arizona's cities to use renewable supplies and to protect groundwater have helped the Phoenix metropolitan area to weather a 16-year drought. It is also why we are able to sustain long-term economic strength and enjoy our desert lifestyle. Arizona’s innovative groundwater law—known as the Groundwater Management Act of 1980—designated safe-yield as a goal for the Phoenix Metro area.
Groundwater is rain and snowmelt that has settled over thousands of years between rocks and gravel beneath the surface of the Earth. Groundwater that settles in aquifers is a finite source of water. Before 1980, uncontrolled pumping was depleting central Arizona’s groundwater, resulting in a lower water table, fissures in the ground, and cracked building foundations. The Groundwater Management Act required farmers and cities to stop the unsustainable pumping of groundwater and encouraged them to use more water from the Colorado, Salt and Verde Rivers. Today only about 7 to 10 percent of the water used by AMWUA cities is made up of groundwater.
To help meet the safe-yield goal of the Phoenix area, cities financed the construction of treatment plants and other infrastructure necessary to use river water and recycled wastewater. Cities also have invested in storing water underground to meet and sustain safe-yield.
How To Recharge An Aquifer
Storing water underground is known as “recharging” the aquifer. Cities recharge water for at least two purposes: (1) to build up a bank account of water underground for times of drought, and (2) to offset groundwater that a city pumps and delivers to businesses and citizens. There are several ways to recharge or store water in an aquifer. Here are a few.
- Recharge Wells: Cities build two main types of wells to recharge water into an aquifer. The first is a well dug 150 to 200 feet into the ground to direct water back into the aquifer. These are called “vadose zone wells” because they are sunk through the vadose zone or the upper part of the aquifer. The second type of well is called an “aquifer storage and recovery well” or ASR. This is a deeper well that goes 600 to 1,000 feet into the middle of the aquifer. These wells allow cities to both pump water directly back into the aquifer and pull it back out when needed.
- Recharge Basins: Some cities create basins filled with recycled wastewater or river water. The basins are built to help the water easily percolate back into the ground. These are known as recharge basins. Recharge basins come in a variety of sizes, from large regional facilities that allow multiple entities to store water at one location, to smaller recharge basins often owned by individual cities. Some cities create recharge basins on the site of their treatment plants, while others turn portions of them into recreational areas. For example, the Town of Gilbert turned some of its recharge basins into a 110-acre wildlife habitat called the Riparian Preserve at Water Ranch, located near Guadalupe and Greenfield roads.
- Managed Recharge: Some cities receive permits to discharge river water into normally dry streambeds, where the water filters back into the aquifer.
Granite Reef Underground Storage Project is a regional storage facility. PHOTO: Salt River Project
How Cities Make It Happen
Like many Valley cities, the City of Scottsdale recharges water into its aquifers as part of its commitment to contribute to the region’s goal of safe-yield. The City recharges most of this water within the city limits through vadose and ASR wells, and it also recharges water at regional recharge basins owned by Salt River Project and the Central Arizona Project.
Along with water from the Salt and Verde Rivers delivered by SRP, Scottsdale receives water from the Colorado River through the Central Arizona Project (CAP). Each October, Scottsdale Water places an order for its annual allotment of 80,000 acre-feet of Colorado River water with Central Arizona Project. One acre-foot of water serves an average of three Arizona families for a year. Scottsdale’s current demand for Colorado River water is about 60,000 acre-feet per year. This leaves 20,000 acre-feet available to store.
Scottsdale sends the remaining 20,000 acre-feet of Colorado River water to one of two regional recharge facilities in Maricopa County. These recharge facilities provide two advantages. First, the facilities allow the city to return the water to the aquifer, which allows Scottsdale to do its part towards the safe-yield goal for the Phoenix metro area. Second, it allows the city to store or bank additional water it doesn’t need immediately for future use.
The state's water agency, Arizona Department of Water Resources, tracks the amount of water each city pumps out and recharges back into the ground. The agency does this to monitor if a city is contributing to the regional goal of safe-yield. The Department also tracks the amount of water each city returns to the aquifer as storage and gives the city a “long-term storage credit” for each acre-foot it stores, minus a 5 percent donation to the aquifer. This will allow cities to withdraw the correct amount of water when it is needed.
What It Means To You
In 1980, wells in Maricopa, Pinal and Pima counties pumped out 2.3 million acre-feet more groundwater than could be recharged naturally. In 2013, the groundwater overdraft in Maricopa, Pinal and Pima counties had been reduced to about 382,000 acre-feet due to the Groundwater Management Act, including efforts to reach safe-yield.
The Groundwater Management Act only applies to the state’s five most populated areas called Active Management Areas. In recent years, the Phoenix Active Management Area has met its safe-yield goal. The challenge is to continue to sustain safe-yield and ensure we have water to meet our future demand. AMWUA cities will continue to seek and promote solutions to ensure that central Arizona achieves safe-yield to protect our most valuable resource—water.
For 46 years, Arizona Municipal Water Users Association has worked to protect our member cities’ ability to provide assured, safe and sustainable water supplies to their communities. For more water information visit www.amwua.org.