WATER POLITICS

The aquifer in the Gila Bend Basin — where pumping isn’t regulated — fell faster from 2000 through 2020 than all but two others in the U.S. and all but 46 others among 1,693 groundwater basins around the world analyzed by the researchers.

But the same study gives high marks to Tucson Water’s efforts to replenish the aquifer here with renewable water brought by the Central Arizona Project canal from the Colorado River. That’s resulted in some of the fastest-rising water levels among those analyzed.

Specifically for the Tucson area, the study found that:

— The water table in the basin including most of Tucson, which the study calls the Upper Santa Cruz Basin, rose faster since 2000 than all but 18 of the aquifers studied worldwide.

— Two individual monitoring wells in the Avra Valley where Tucson Water is replenishing the aquifer ranked among the top 500 fastest-rising such wells in that period, among about 170,000 total wells examined.

But in Gila Bend, by contrast, Elizabeth McClure, a longtime resident who has raised concerns about water levels, said she’s “shocked” the basin made a global list of rapidly declining wells.

“It’s incredibly crazy to me. You wouldn’t think Gila Bend would be on a list like that,” said the 67-year-old. “For me, it makes the issue even more critical than what I thought.”

“Declines can be slowed”

The massive study analyzed groundwater level trends in more than 40 countries.

Groundwater level declines were widespread worldwide. Overall, 1,201 basins, or 71% of all studied, showed declines. Worse, the study found that in 30% of the 1,693 aquifers, the rate of decline has accelerated since 2000, compared to the 20th century’s last two decades.

The declines were especially large in aquifers located in arid climates where a large portion of the landscape is under cultivation for farming. Overall, the study said irrigation worldwide is estimated to account for 70% of global groundwater withdrawals. A similar proportion of Arizona groundwater is used by farmers.

The Gila Bend Basin fits that category. Its local economy depends heavily on growing cattle feed that’s used in dairies there and in central Arizona in general. Its use of groundwater is unregulated, which many critics say dramatically increases the rate of groundwater pumping compared to what nature puts back into the aquifer through rainfall.

But the experience of rising water levels and monitoring wells in the Tucson area is one example of what the researchers cited when they wrote: “Our analysis of groundwater levels suggests that long-term groundwater losses are neither universal nor inevitable.”

Water levels in the aquifer in much of Tucson have climbed steadily since Tucson Water started putting most of its Colorado River supply into the ground to replenish the aquifer at the beginning of the 21st century, both the new study and a Tucson Water map show.

The study also cited the experience of Albuquerque, where groundwater levels have been rising since about 2008 because of its increased use of surface water. That water also comes from the Colorado River Basin, through a smaller-scale version of the CAP known as the San Juan-Chama Basin project.

“In 16% of the aquifer systems, groundwater level declines reversed — defined as cases in which groundwater levels declined in the late 20th century but rose in the early 21st century.” said the study, prepared by researchers from the U.S., South Africa, the United Kingdom and Switzerland.

“Declines can be slowed, stopped, and even reversed, but much work needs to be done to address groundwater depletion, because of its prevalence,” researcher Scott Jasechko told the Arizona Daily Star. One of the study’s eight co-authors, Jasechko is a hydrology professor at the University of California at Santa Barbara.

While considerably more aquifers around the world are in bad shape than are okay, “I think it would be wise to learn from the positive cases where things have turned around, so we can think about implementing those kinds of solutions in other places where groundwater levels are declining,” he said.

Farms, state regulators at odds

The study’s release last month came around the same time the Arizona Department of Water Resources announced it was considering imposing a management system on the Gila Bend Basin for the first time.

The basin’s wells have fallen about 4 feet a year since 2000, the department found, which matches the global study’s findings.

Continued declines on that scale can hurt water quality, jack up pumping costs, and trigger land subsidence, which ADWR has already documented to have increased significantly in the Gila Bend Basin over the past decade.

The Gila Bend Basin is mainly low desert, interspersed with mountains that reach 3,200 feet high. The town of Gila Bend has around 1,850 residents, who rely on groundwater from town wells for drinking. Precipitation tops out at 8 inches on the valley floor and 12 inches in the mountains — far less than Tucson and Mount Lemmon get in a typical year.

The Gila River runs through the basin but only carries water there after rains and has been plagued by drought. And unlike Tucson, the basin does not have access to water from the Colorado River.

Sixty-four farms grow crops on more than 50,000 acres in the Gila Bend Basin, the U.S. Agriculture Department reported in 2023.

Most commonly grown are forage crops such as alfalfa, barley, wheat and corn, which are used to feed dairy cattle, said Robert Van Hofwegen, manager of the Paloma Irrigation District within the basin.

The basin has six dairies, which furnish almost 20% of all Arizona’s milk production, he said.

The state’s measurements of water level decline were taken in 18 “index wells” around the basin that the department checks at least annually, said Ryan Mitchell, ADWR’s chief hydrologist, at a recent public meeting.

The Jan. 30 meeting was called to discuss possibly creating a state-run Active Management Area for the basin, which would allow the state to impose conservation requirements on the farmers and require them to install water meters on their wells.

That’s an action many farmers there strenuously oppose.

Besides having the fastest average water level decline in the state, the Gila Bend Basin had the well that fell the farthest from 2000 to 2020 of any state-measured index well — 185 feet, ADWR said in an April 2023 report.

Notably, the Gila Bend Basin’s declines since 2000 exceeded even those in aquifers in California’s Central Valley, long notorious for its chronic over-pumping of the aquifer by farms, the new global study shows.

In some cases, Central Valley wells have fallen faster than 4 feet per year during droughts, but then rebounded during California’s frequent events of very heavy rainfall and flooding — storms that strike Arizona much less often.

Some individual Central Valley wells are probably falling at rates comparable to or even faster than those in the Gila Bend Basin, but “an individual well is not the same as an aquifer,” Jasechko said.

Why is it falling so fast?

Some Gila Bend Basin farmers say their groundwater levels have been heavily impacted over the last few decades by other forces besides pumping.

They include the raising of Roosevelt Dam east of Phoenix in the late 1990s on the Salt River, a Gila River tributary, to store more water for Phoenix.

Another factor was diversion of a lot of sewage effluent from the Gila River starting in the 1980s for use at the then-new Palo Verde Nuclear Generating Station in suburban Buckeye.

“We used to get a significant amount of effluent from Phoenix,” said Van Hofwegen, of the Paloma Irrigation District, which delivers water to 15 customers growing crops on about 30,000 acres. “Over time, people above us with rights are taking it. We’re not seeing what we used to.”

Also, the Phoenix cities reuse much more effluent than before, to extend their water supplies, meaning less effluent leaves that area, he said. “Many people want to say it’s wasted water when it leaves (the sewage) treatment system,” he said of the effluent. “That’s anything but the truth. If water goes down the river, it recharges an aquifer.”

Then there’s the drought. Last year, the Gila River carried water through the basin for about 90 days. But that was only the fourth time the river has brought in any water in the last 20 years, Van Hofwegen said.

Another possible factor is since 2000, people drilled or deepened 51 wells within the basin either to irrigate crops or water their livestock, a database from Arizona State University’s Kyl Center for Water Policy shows. The database draws from ADWR well records.

That’s a lot fewer than the 269 wells drilled or deepened for farming and livestock watering since 2000 in the Willcox Basin southeast of Tucson. There, dozens of homeowners have reported their wells drying up.

But the average new and deepened well depths in the Gila Bend Basin haven’t been much less than in the Willcox Basin — 1,200 feet versus 1,500 feet in recent years.

“Additional wells generally mean more demand, which contributes to the water table’s decline, but this is kind of a chicken-or-the-egg question. Wells are drilled deeper for a reason: It’s costly to drill wells — the deeper the well, the higher the cost,” said Sarah Porter, the Kyl Center’s director. “And because of the additional energy required, pumping from greater depths typically results in more costly water.”

“In any event, what happened in the 1990s is only part of the story,” Porter said of the farmers’ concerns.

“There are now more and deeper wells, all 18 index wells have declined over the last 20 years and the subsidence rate is increasing,” Porter said, referring to the sinking and cracking of land due to pumping. “Because there are no metering requirements for rural areas, we don’t have withdrawal data to give us a more complete picture.”

While the farmers’ concern about upstream diversions may be accurate, “it must be realized that that likely was never their water — other parties have the right to it,” Porter said.

In fact, the river flows could decline even further because the Salt River Project is about to do a pilot project to boost the amount of water it stores in Roosevelt Dam upstream of Phoenix, she said. If that becomes a permanent change, the Gila Bend area will get even less river water.

“The aquifer is being depleted due to increasing groundwater demand or persistent groundwater demand,” Porter said. “It’s not going to be replenished.”

McClure, the longtime Gila Bend resident worried about water levels, said she thinks the farmers are trying to discredit ADWR’s findings. “They’re trying to insinuate that the numbers are not as serious as ADWR says.”

Gila provides minimal recharge

Responding to questions from the Star, ADWR responded mostly in general terms, saying only, “When the outflows exceed the inflows, you will see groundwater level declines. The Gila Bend Basin is experiencing a significant discrepancy in inflows and outflows.”

The department also declined to respond directly to farmers’ concerns that upstream depletions lowered the aquifer.

ADWR downplayed the benefits to the basin’s aquifer from river flows that recharge it. In rivers and streams like this section of the Gila River that only carry water after storms, the flows “only provide minimal aquifer recharge,” allowing no more than 10% of the surface flow to seep into the aquifer, the department said.

Van Hofgren said “quite a few” wells in his district rose after last year’s prolonged river flows amid the long drought. “If last year’s Gila River flows repeat themselves for a couple more years, “we could see significant recharge in the basin.”

But ADWR said that of 18 index wells it measured in late 2023, only four showed water level rises.

What now?

“These challenges are manageable by the right approach,” Van Hofwegen said.

“Those wells that rose are in a very porous, productive part of the aquifer, where it is easier to pump the water but also easier to recharge that water,” he said. The basin’s ability to recharge water is “very unique” for Arizona groundwater basins, he said.

“Our current challenges are not necessarily permanent challenges. With recharge we could recover.”

But how will that happen?

“If that was easy to answer, water would not be such a hot topic,” he said.

Today, two competing bills in the Legislature offer different approaches to how pumping in rural basins like this one would be regulated.

A bill favored by Senate Republicans would authorize creation of groundwater basin management areas and require some conservation, but it would establish a complex, lengthy process for creating them that has sparked sharp criticism in some quarters.

A bill pushed by House Democrats would make it simpler to create state-run management areas in rural basins (Tucson and Phoenix already have them) and would require stricter conservation, but opponents say it gives too much power over appointing the basins to the governor.

What happens to the Gila Bend Basin could well depend on how this clash ultimately plays out.

Colorado River cuts loom large

Tucson was dependent for groundwater for decades in the 1900s and its wells dropped precipitously.

But today, most if not all of the water delivered to the 730,000-plus residents of Tucson Water’s service area is CAP water from the Colorado. The utility started recharging that water into the aquifer in 2001 and since 2012 has put almost of that supply into the ground and pumped it out as needed.

As a result, the increases in some well levels here are among the largest in the world, the new study found.

In midtown Tucson and on much of the east side, the water table rose more than 3 feet a year annually from 2000 through 2022, a Tucson Water map shows. Water levels have risen at around 7 feet per year in that period on a section of the south side near where interstates 10 and 19 meet.

All of those areas were heavily pumped for drinking water starting around World War II but aren’t pumped much anymore, if at all.

The study analyzed water levels in monitoring wells across the entire Upper Santa Cruz Basin, which covers both city and suburban areas from Oracle Junction north of Tucson to Green Valley south of it.

Because the basin includes many suburban areas where groundwater still is being pumped for drinking, the average reading of 48 monitoring wells analyzed for the study showed a much smaller rise — of about 1.15 feet a year from 2000 through 2020.

In the Avra Valley, the Tucson Water map and the new study showed a maximum increase of 208 feet since 2000 at a monitoring well less than a mile south of the Southern Avra Valley Storage and Recovery Project. That’s one of two sets of recharge basins in the valley where CAP replenishes groundwater.

That increase was the 117th fastest rise of any of the 170,000 monitoring wells globally that the study reviewed, researcher Jasechko said. Before recharge began, that well’s water level had fallen 54 feet from 1954 to 2004, he said.

At a second monitoring well farther north, the water level rose 148 feet from 1997 through 2021. That ranked as the 478th fastest rising monitoring well studied, Jasechko said. That well lies about a mile north of the second set of city recharge basins, known as the Central Avra Valley Storage and Recovery Project.

Tucson Water recharges CAP into those basins for two purposes. It places the large majority of its annual CAP supply of 144,191 acre-feet into the aquifer for water treatment and pumps it back out each year for delivery to the utility’s customers. The rest is left in the ground to be stored for future use if the city’s annual supply is cut back due to Colorado River shortages.

Tucson has been able to store nearly six years’ worth of drinking water supplies in recharge basins.

But the future looks muddier. That’s due to the likelihood of CAP delivery cutbacks to Tucson and other Arizona cities because of Colorado River shortages.

Last year, for instance, Tucson agreed to leave more than one-third of its CAP supply in Lake Mead to raise the depleted reservoir’s water level.

In 2024 and 2025, due to leaving water in Mead, Tucson Water estimates barely a fourth, at most, of what the city has historically stored in its recharge basins “may be available for long-term storage,” utility spokeswoman Natalie DeRoock told the Star.