Suburban Growth is Pressuring Groundwater Supplies in a Dry Western State

Population growth in suburban areas lacking abundant water supplies, years of drought conditions in the western U.S., and high demand for water from one of the region’s most important rivers.

These are some of the major factors that a new report identifies as contributing to risks and uncertainty for an Arizona program that is meant to ensure residential communities in the state will have enough drinking water in the decades ahead.

“It’s an important program and a lot of homeowners are dependent on it,” said Sarah Porter, executive director of The Kyl Center for Water Policy at the University of Arizona, and a co-author of the report. “It's important to shore it up and make sure that it’s working.”

The report explains how for nearly 40 years the state prohibited subdivision lot sales in its most urban areas, unless lots had access to a “100-year assured water supply”—primarily from naturally renewing sources of surface water like rivers.

But this changed in 1993. State lawmakers at that time established a framework where groundwater deposits, which often recharge on their own at very slow rates, could count toward the water supply requirement, with the understanding that surface water would later be used to replenish the underground supplies.

A state entity known as the Central Arizona Groundwater Replenishment District was formed and charged with acquiring the water that would be used to replace the groundwater.

Since the district was created the number of communities that have enrolled in the program has far surpassed initial expectations. In 1995, the district served 184 homes located in Arizona subdivisions. By 2013 that number had swelled to 263,707, the report says.

The places depending most heavily on the district are mainly suburban subdivisions. 

Phoenix, on the other hand, has enough surface water available to accommodate growth without aggressively tapping aquifers that would require replenishment under the state’s guidelines. Tucson depends heavily on the Colorado River for water.

"There are lots of cities in Arizona that have plenty of water supplies for a large amount of growth,” Porter noted. “This isn't about growth or not. It’s about growing where there’s capacity for growth on renewable supplies, and ongoing good water management.”

“This is really about where, where the growth takes place,” she added.

When the Groundwater Replenishment District was created officials believed that—at least through 2046—there would be enough water available to refill aquifers covered by the program from what’s known as the Central Arizona Project.

The Central Arizona Project is a 336-mile system of canals and other infrastructure that diverts water from the Colorado River to central and southern parts of the state. 

But recent years have been tough for the Colorado River basin, which covers much of Arizona. The basin experienced its driest 19-year period in over a century between 2000 and 2018. 

Amid these parched conditions, demand for water from the river system has remained high among farmers who depend on it for irrigation, as well as from people living in fast-growing cities. The river provides water for about 40 million people and 5 million acres of farmland across the western U.S. and Mexico.

Climate change is stirring additional questions about how much water will be available from the river in the years ahead.

Porter said that there’s a more defensible case to be made for the Groundwater Replenishment District program if it were able to rely on excess Colorado River water from the Central Arizona Project. Turning to alternative sources could drive up costs for the district and homeowners, while posing other complications as well.

“I think everybody is in agreement that we shouldn't be counting on extra Colorado River water,” Porter said.

Most of the district’s costs are covered by annual assessments charged to its member communities. These rates have been on the rise over the past two decades, according to the report, shooting up to around $727 per acre-foot of water in the Phoenix area from around $154.

The district’s consultants, the report points out, have warned of a potential “financial catastrophe” if more and more homeowners seek to avoid the assessments as they go up.

There are environmental concerns, too. The report raises the prospect that some aquifers might run dry. Land subsidence, which refers to when land sinks, can occur when groundwater is deeply depleted. This can damage roads, water mains and other infrastructure.

If aquifers collapse too much, they can become increasingly difficult to replenish. The report also notes that the likelihood of poor water quality due to both manmade contaminants and naturally occurring pollutants increases with deeper wells.

As it stands, subdivisions covered by the district can pump water from 1,000 to 1,100 feet from underground, the report says.

Its authors offer a number of recommendations that lawmakers and other state officials might consider. 

For example, undertaking a more rigorous examination of how much water is realistically available to replenish aquifers and decreasing the depth from which communities that qualify for the replenishment district program can pump groundwater.

Requiring the district to refill water in the same locations where it is depleted is another suggestion.

Arizona’s Department of Water Resources, which approves the district’s operations plan every 10 years, said Monday it would not comment on the report until it had more time to review it.

Porter pointed out that Arizona had recently engaged in somewhat tense discussions related to a multi-state drought contingency plan for managing the Colorado River. 

There were other sticky situations over water as well during the past few years, like a plan that would have involved the replenishment district acquiring water rights tied to farmland in the Mohave County area, in northwest Arizona. That idea was dropped after it drew opposition from residents in that area.

Porter acknowledged that homebuilding is an important part of Arizona's economy, but also emphasized that the stakes are high when it comes to water management in such an arid state.

“Arizona really does understand,” she said, “we don’t have as much room to mess up with water as other places.”