Sonora ranchers warn imperiled San Pedro aquifer will impact Arizona, too

EJ. EMILIANO ZAPATA, Sonora — The scarcity of water in Sonora’s portion of the binational San Pedro River aquifer is sounding an alarm for activists trying to save the arid Southwest’s last free-flowing river and its national riparian conservation area in Arizona.

Reports of a dwindling water supply from northern Sonora ranchers and conservationists are also calling into question long-held assumptions about how much water moves from the headwaters of the San Pedro, near Cananea, Sonora, and flows north into Arizona.

On the sprawling ejido lands northeast of Cananea, rancher Francisco Valenzuela Quijada says he’s devoting ever more time to trucking water out to his thirsty cattle that graze on pastures above the San Pedro River basin, especially over the last three years.

As streams have dried up, and previously reliable watering holes are now empty, Valenzuela Quijada, 53, says it’s up to him to ensure his family’s 60 cattle have enough to drink. Five times a week, he spends the day filling two 300-gallon water-storage tanks from a community water supply, and driving them out to water troughs he’s installed in the pastures.

It takes three hours to drain the tanks and two more to get back home, said the father of four.

“I don’t have any other option. Time that I could invest in repairing fences and reforestation has been spent giving water to the cattle,” Valenzuela Quijada said in Spanish, speaking in front of his home outside Cananea in Ejido Emiliano Zapata, one of the region’s seven collective-use land parcels, known as ejidos, granted to the people by the government in 1959.

Citing a falling underground water table, ranchers here say their animals’ food supply is also in peril. The nutrient-rich native grasses that used to cover the landscape are yellowing and sparse, outcompeted by drought-resistant species like mesquite trees, chollas and invasive grasses.

Water levels in residential wells are also dropping, forcing some to spend thousands of dollars to deepen them.

Ranchers say the falling water table is not only due to climate change and an ongoing drought in the region; they also fault what they call excessive groundwater pumping by Grupo Mexico’s Buenavista del Cobre copper mine.

And they warn that downstream, Arizonans will ultimately feel the effects in the Sierra Vista sub-watershed of the same aquifer, especially as the mine’s operations continue to expand.

“The ambition of Grupo Mexico is not going to stop,” Valenzuela Quijada said.

Too little is known about how water flows through underground soil and rock around the border, experts say. But recent plans for Arizona and Mexican researchers to collaborate on what would be the first jointly developed groundwater model of the aquifer could finally shed light on previously unanswerable questions.

“That would be huge,” said Holly Richter, hydrologist and former conservationist with The Nature Conservancy who now runs her own consulting firm, Resilient Rivers. “We’ve had conversations about this in the past that never came to fruition, so the fact that this is back on the table is really encouraging.”

In the meantime, activists in Sonora say Arizona water managers should pay attention to what’s happening on the binational aquifer south of the U.S.-Mexico border. In Cananea, mining giant Grupo Mexico has expanded its copper production four-fold over the last three decades and appears to be seeking new sources of water to sustain its operations.

“The need for water required by a company of this magnitude is very large,” said Gerardo Carreón Arroyo, conservationist with Sonora nonprofit Naturalia. “We are talking about a binational basin. What happens in Mexico will surely have repercussions in Arizona.”

Grupo Mexico’s Buenavista del Cobre copper mine is by far the biggest user on Sonora’s San Pedro basin, having secured permits allowing it to pump about 20 million cubic meters of water each year from the aquifer.

That’s 68% of the total volume of annual groundwater pumping on the San Pedro that’s permitted by Mexico’s national water commission, Conagua.

It’s also more than three times the volume of the aquifer’s water deficit, which Conagua estimates is 6.7 million cubic meters. That’s how much more groundwater extraction is allowed every year, compared to how much is expected to be replenished by rainfall and other sources.

The total volume of groundwater extraction permitted on the San Pedro hasn’t increased since 2005, Conagua told the Arizona Daily Star in August, though a 2021 Colegio de Sonora report said the mine’s portion of that concession has increased.

Locals say the mine’s recent efforts to move its wells south on another nearby aquifer, on the Sonora River, indicate the San Pedro is being depleted and that the mine is searching for new sources of water.

Asked if the Buenavista mine is struggling to maintain an adequate water supply, a Grupo Mexico spokesperson said via email that the characterization “does not correspond to reality.”

Conagua did not respond to a series of questions submitted on Sept. 16. A spokesperson said agency engineers are busy preparing for the Oct. 1 change in presidential administration.

Impact on Arizona’s San Pedro

Arizona-based experts say so far, it’s been hard to conclude how groundwater use south of the border affects Arizona’s Upper San Pedro Basin, which includes the federally protected 57,000-acre San Pedro Riparian National Conservation Area, in Cochise County.

Groundwater models of the Upper San Pedro Basin — which aim to simulate how water moves through variable geologic features, hidden underground — suffer from a lack of data from Sonora on groundwater-pumping volumes and water-table depths.

“The reality is all models are only as good as the data they’re based upon,” Richter said.

Laurel Lacher is one of the leading hydrologists studying the San Pedro. Her recent efforts have focused on improving the simulation of groundwater conditions at the Arizona-Mexico border.

In building her recent groundwater model of the Upper San Pedro, the typical data-gap challenges were “amplified a million times in Mexico, because the data are very sparse,” Lacher said during an Aug. 21 online presentation to the Upper San Pedro Partnership, a nonprofit group representing 15 federal, state and local agencies, as well as a major developer and some conservation groups.

The partnership aims to preserve the San Pedro without resorting to mandatory cuts to water use or limits on population growth, strategies supported by other conservationists.

If less water than assumed is flowing from Sonora into Arizona’s San Pedro aquifer, that would mean long-held assumptions about the annual water deficit in Arizona’s aquifer are wrong, activists say.

Arizona water managers have spent years trying to reduce the water deficit in Arizona’s San Pedro, providing updates on the deficit status in annual reports to Congress through 2011, part of an unsuccessful effort to eliminate the deficit by that year.

While the deficit has declined greatly since the 1990s, the most recent estimate is a deficit of 3,800 acre-feet per year, or about 4.7 million cubic meters. Experts warn that the river could dry up if the deficit isn’t reduced to zero, the Star has reported.

But Arizona’s deficit calculation has assumed a consistent underground inflow of water from Sonora, at a rate of 3,000 acre-feet per year, a figure estimated in the 1990s by Arizona Department of Water Resources researchers which multiple experts called “uncertain.”

It’s been the best estimate available, until recently.

In response to an August inquiry from the Star, Arizona Department of Water Resources researchers used their latest groundwater model of the San Pedro, released in February, to generate a new estimate of inflow from Sonora’s San Pedro.

The result was 527 acre-feet per year, nearly 2,500 acre-feet less than the previous estimate, said Emily LoDolce, groundwater modeling section manager with ADWR.

If Upper San Pedro Partnership’s advisory scientists decide to update their water budget with the new figure, that would add nearly 2,500 acre-feet to the annual water deficit, bringing it to 6,273 acre-feet.

Hydrologists and researchers had differing opinions on the significance and reliability of the new estimate. The ADWR’s model has been criticized for data gaps and unsubstantiated methodology, and in June a state judge ordered the agency to develop an addendum to address the concerns.

But water activist Robin Silver, co-founder of Tucson’s Center for Biological Diversity, welcomes any acknowledgment that water flow from Mexico may have been overestimated for years.

“It’s huge news,” said Silver. He has won several court cases over the past 20 years overturning federal biological opinions allowing the U.S. Army’s Fort Huachuca to keep operating at current levels despite the threat to several endangered species living near the river.

Silver said a more realistic water-deficit figure is crucial for those working to save the San Pedro, as state and local water managers are eager to factor in any reductions to the deficit, while downplaying impacts that increase the deficit.

In an emailed statement from ADWR, attributed only to “ADWR hydrologists,” the agency said the new estimate is based on historical data going back to the 1940s, and that the 2,500 acre-feet difference could reflect changes in methodology compared to earlier studies.

“Developing a water budget is more involved than swapping out one number for another,” the ADWR statement said. “...We’re not saying one (estimate) is better than the other.”

The San Pedro is the last free-flowing, undammed desert river in the arid U.S. West, Silver said.

“Not only is it incredibly invaluable in itself, but it also represents the last migratory bus stop for the millions of neo-tropical song birds that move south to north in the spring, and north to south in the fall,” he said. “But for the survival of the San Pedro River, we’d be losing millions of song birds.”

Ejidos see falling water table

Back to homepage

Subscriber Login

Keep reading with a digital access subscription.

Subscribe now

In Cananea’s neighboring ejidos, ranchers report cutting back on the number of cattle they own as water and food supplies dwindle, both due to drought and groundwater pumping that doesn’t allow the water table to recover. Farmers are no longer seeding fields that they believe can’t survive current conditions.

The scarcity of water has become a “crisis” that’s changing the economics of the cattle industry, said Carreón Arroyo of conservation group Naturalia.

“It is no longer sustainable to have the number of cattle that ranches once had,” he said in Spanish, “and for this reason, many ranchers have retired from the activity, sold their ranches, or simply abandoned them.”

The Buenavista del Cobre copper mine has at least 45 wells on ejido land over the San Pedro, property the mine can access because of controversial right-of-way agreements, signed decades ago, allowing the mine to take water in exchange for financial compensation and other benefits.

But in Ejido José María Morelos, just west of the San Pedro River and north of Cananea, residents have vowed “never” to allow the mine access to their wells, said Gilberto Rendón, 55, treasurer of his ejido’s elected council.

Nevertheless, they’re suffering the effects of groundwater extractions upstream, in Ejido Zapata and Ejido Zaragoza, plus the impact of climate change, he said.

Rendón said on his ranch, they’ve invested 200,000 pesos — about $10,000 — in deepening the well from a depth of about 80 feet to 200 feet. That was a little deeper than necessary to reach the water table, but he wanted to avoid having to deepen the well again in a few years.

Rendón recalled swimming in the San Pedro as a kid growing up on its banks.

“It was very beautiful. In those times, there was a lot of water in the river, but now it’s dry,” as of about seven years ago, he said in Spanish. “There isn’t even enough water for the few cows that used to drink there.”

One challenge for Mexico in regulating water use is that Conagua gets funding from the permits it issues — known as concessions — allowing groundwater pumping, said Jeffrey Banister, director of the University of Arizona Southwest Center.

“There’s a kind of pay-to-play dynamic that’s going on in Mexican water politics,” he said.

Conagua used to be part of a cabinet-level ministry, but “since the ’90s it’s a much smaller agency and owes much of its budget to fees from water concessions,” Banister said. While the issue has been “well-documented” by Sonoran researchers, “it’s a really big issue that not many people talk about.”

Another clue that the San Pedro’s water table in Sonora is dropping comes from The Nature Conservancy’s decades-long effort to document how much surface water exists along the San Pedro River at the the driest time of the year. The binational “wet/dry mapping” effort involves dozens of volunteers in Sonora and Arizona who, in June, walk along the San Pedro River and note where water is present at the surface.

In Sonora, those reports show that the percentage of “wet” riverbed has decreased from an average of 52% between 2007 and 2009, to an average of 13% in the last three years of mapping, according to The Nature Conservancy’s records.

Cananea residents point out that between 2007 and 2010, a worker strike at the Buenavista mine meant the region experienced a few years free of the mine’s groundwater extractions. In those years, the San Pedro basin’s groundwater levels bounced back noticeably, Carreón Arroyo said.

Mine’s production rising

Grupo Mexico purchased Cananea’s Buenavista del Cobre mine in 1990. Since then copper production has surged and with it, water usage, experts say. Copper production at the Cananea mine increased from 100,000 tons in 1990, to more than 400,000 tons in 2019, according to a 2021 report by Colegio de Sonora professor José Luis Moreno.

Meanwhile, the mine’s total water concession, including other aquifers such as the Bacoachi River, has more than doubled since 2007, climbing from about 30 million cubic meters to nearly 64 million cubic meters in 2020, the report said, citing federal data.

Last year, the mine opened a zinc plant in Cananea, with a capacity of 100,000 tons of zinc and 35,000 tons of copper, according to a Grupo Mexico investors’ report.

Reports of residential wells going dry don’t necessarily mean the water table has dropped precipitously, said David Pratt, supervisory hydrologist in the Tucson office of the USGS’s Arizona Water Science Center. Even a minor reduction in the water table could mean shallower wells no longer reach the underground water table, he said.

But Sonora residents say there are indications that Grupo Mexico, even with its deep industrial wells, is also struggling to access an adequate water supply from the San Pedro aquifer.

Residents south of Cananea, along the smaller Sonora River aquifer, have been protesting against the mine’s efforts to drill new wells further south on the aquifer. The communities felt the impact on their wells almost immediately, after the mine began using water trucks to transport water to the mine earlier this year, they said.

“The mine wouldn’t be doing that if they weren’t struggling,” said Humberto de Hoyos, a Cananea native and one of the organizers of the citizen protest. “The mine is pumping all the water it can because it knows that water is running out.”

The citizens’ highway blockade has successfully prevented the mine from continuing its effort to truck water from the Sonora River basin to the Buenavista mine, the Star reported in August.

For Arizona hydrologist Lacher, it’s a troubling indication that the mine could seek to drill more wells closer to the U.S. border, increasingly the likelihood of impacts to Arizona’s watershed.

“Over the long term, you may see more and more tendency for (the mine in) Mexico to want to exploit the aquifer downstream, like they’re doing in the Rio Sonora right now,” she said in the Aug. 21 presentation. “And that would be worrisome.”

In July, Cananea residents submitted a complaint to Sonora Gov. Alfonso Durazo’s office, as well as Conagua and its state-level counterpart CEA, alleging the mine was covertly taking water from a new public water tank in the hilly neighborhood of Fortín. That tank had a new pipeline attached to it which residents say led directly to the mine’s new zinc plant, according to the complaint.

“They were stealing the city’s water for industrial use, leaving us without water,” said Omar Lugo Patron, one of the complainants. He said the governor’s office hasn’t yet responded.

“The government is protecting Grupo Mexico,” he said in Spanish. “This reservoir was built with public resources. It’s the city’s, but they (the mine) have control.”

In an emailed response, a Grupo Mexico spokesperson simply called the allegations “unfounded.”

A statement from Durazo’s office said even though the water tank is on the mine’s property, the mine doesn’t have access to it.

“The state authority (CEA) is the only one that enters to carry out maintenance work for optimal supply to the population,” the statement said, adding that drops in water pressure to surrounding neighborhoods are due to improvement work.

Durazo’s office said it’s launched a “justice plan” for Cananea, investing 198 million pesos — about $10 million U.S. dollars — in social programs, housing and water-system upgrades. The goals are “rehabilitation of the urban landscape and to compensate for the damage and historical abandonment of the community,” including compensation to former miners for historic labor-rights violations committed by the mining company with its union-busting tactics.

Clearer picture on horizon, experts hope

Researchers in Arizona and Sonora are optimistic that a binational collaboration will illuminate previously unanswerable questions, through an effort of the Transboundary Aquifer Assessment Program, or TAAP, established by the International Boundary and Water Commission in 2009.

For the first time, they hope to jointly craft a binational groundwater model, informed by experts and data from both sides of the border, and hopefully calibrated with more recent groundwater-level data from the San Pedro’s Sonora sub-watershed.

On Sept. 10 and 11, the IBWC and its Mexican counterpart CILA hosted a workshop in Hermosillo where researchers from the University of Arizona and Universidad de Sonora, as well as federal water agencies from both countries, discussed the collaboration and agreed on the need for it.

Where it goes from here will depend on funding but for now, TAAP’s technical teams are developing scope-of-work proposals and making plans for a new round of water-table measurements in Sonora.

The most recent groundwater measurement data from Sonora dates back to 2011, said Elia Tapia, a member of the TAAP team and professor at the Universidad de Sonora. As a graduate student in 2011, Tapia participated in the last data survey of the San Pedro, even getting access to the mine’s private wells, she said.

The 2011 data was used in the last binational study of the San Pedro, in 2016, which aimed to collect all the data that was available at the time on the aquifer. But momentum to use that data to create a joint groundwater model stalled, as TAAP turned its focus to projects related to the Santa Cruz River.

Thirteen years after the 2011 data-gathering effort, the prospect of exploring groundwater levels in Sonora’s San Pedro aquifer is “awesome,” Tapia said. “That’s a huge step for both nations.”

Biological corridor

Conservationist Carreón Arroyo wants more people to understand the critical importance of the biological corridor that the San Pedro River provides.

Nautralia has monitored species in the San Pedro for 15 years, including mountain lions and pumas, mule deer and white-tailed deer and beavers. It provides a resting place for neo-tropical birds during migration and a nesting site for American bald eagles, he said.

While Naturalia hasn’t yet observed them, “we know that jaguars use this corridor to reach the wild areas of Arizona, such as the Huachucas or Catalina mountains,” he said.

So many species rely on the continued presence of water here, and its protection is vital for the entire Southwest, he said.

“That is the magnitude of the importance of this basin, and of what happens to it in Sonora and in Arizona,” he said.