Arizona has been banking groundwater for decades. Experts urge caution about using it
The Colorado River has been in severe decline for years, in part, because a historic drought has reduced rainfall and snowpack. These are all factors above the river.
However, the groundwater lurking beneath the Earth’s surface also plays a big role in the river’s levels. Groundwater can be stored for thousands of years, according to Fred Tillman, a research hydrologist with the Arizona Water Science Center at the U.S. Geological Survey.
“So if there's a stream that is flowing all year long, meaning after it has rain drain, and after the water has run off into the stream, and that stream is being fed by groundwater. So that groundwater and surface water interaction is a huge important component of perennial year-round stream flow in streams” he said.
Once the water is pumped, it takes a lot of effort to get water back into the ground.
“In Arizona, most of our groundwater is ancient. In other words, it was physically stored underground and passed geologic ages eight to nine thousand years ago” said Kathleen Ferris with the Kyl Center for Water Policy at Arizona State University. “And it is not renewable."
Once you pump it, you have basically used it. In order to recharge these aquifers that can be depleted, water is injected into the ground artificially. A whole agency has even been created for it: The Arizona Water Banking Authority.
Tillman said, “They will put that excess Colorado River water out in these detention basins and allow it to infiltrate down, essentially recharge the groundwater systems there. So in that way, they're banking.”
The banking authority was created in 1996. Through 2021, the agency has stored over 4 million acre-feet for cities throughout Arizona. It oversees 28 storage facilities that can recharge aquifers. And as Ferris says, groundwater maintenance affects more than just water supplies. It affects overall land composition.
“Groundwater holds open the pores between the rocks and the gravel in which it is stored," Ferris says. "And if you take it out, then those rocks and gravel collapse into each other.”
This is a scientific process known as subsidence. When this happens the land sinks and it can damage critical infrastructure, such as sewer pipes and canals.
In 2017, the state Department of Water Resources found that 3,400 square miles around Arizona were at risk of sinking. In Willcox, just east of Tucson, there is an electric pole with a sign marking how much the land has subsided. The markings run the entire length of the pole, starting from the top in 1969 down to where it is now. Land near Luke Air Force Base, west of Phoenix, has already dropped more than 19 feet since 1950.
"Phoenix geologically is kind of a bowl. So we're kind of at the bottom of the bowl. The likelihood of having those kinds of impacts in the city of Phoenix and in the service area, we would really, really have to be pumping a lot of groundwater,“ said Cynthia Campbell, water resources management advisor for the city of Phoenix.
The state did not begin actively controlling any of its groundwater usage until the landmark Groundwater Management Act of 1980 was passed. That created five management zones in urban areas, including one in Phoenix. Each zone was asked to reduce its reliance on groundwater by 2025 to a safe yield level. That’s when the amount of water being withdrawn is the same as the amount being replenished.
Campbell says since the 1980 act, Phoenix has altered its water usage strategy.
“And so we completely changed the system around, we decommissioned a lot of wells. We gave up using groundwater almost in total” Campbell said. “And today, we would consider ourselves a surface-water system.”
According to Campbell, Phoenix receives 58% of its water from the Salt and Verde rivers. And only 40% from the Colorado River. Of that 40%, only about 2% comes from groundwater. Campbell says that’s a negligible amount because it nearly equals the amount the city needs to draw from wells to keep them operational.
Ferris says this number is a low number but it is important to keep an eye on.
“Relying on groundwater from only 2% of your water supply is pretty darn good. But as soon as Colorado River shortages hit, more and more groundwater will be pumped,” she said.
And with Arizona already slated to lose 21% of its water allocation from the Colorado River in 2023, water managers are already having to adjust.