For the first time, U.S. and Mexico take stock of the underground water they share

The two countries have tried for years to thoroughly assess aquifers along the border.


An unknown number of aquifers dot the border along the U.S. and Mexico, groundwater both sides use for agriculture, irrigation, and cities. Likewise, how much border communities rely on them and the ways they are managed by either country remain largely unclear. 

For a decade, researchers have attempted to study these transboundary aquifers, but limited funding from the U.S. government and a dearth of information have hampered their efforts. Now, though, the first leg of research is wrapping up on two major water systems: the San Pedro and Santa Cruz aquifers along the Arizona-Sonora border. Researchers can now present a more unified picture of groundwater systems and collaborate on their management – an increasingly pressing challenge in the age of climate change, shifting precipitation patterns and extended drought.

The San Pedro National Conservation Area in southern Arizona.

In 2006, the two countries signed the Transboundary Aquifer Assessment Act, which allowed for assessment of major aquifers along the borders of Texas, Arizona and New Mexico: the Hueco Bolson and Mesilla Basin aquifers below El Paso and Ciudad Juárez and Arizona’s San Pedro and Santa Cruz aquifers. Congress authorized $50 million from 2007 to 2016 for the project, but ended up providing only a fraction of that. 

Despite the limited budget, the research team from the U.S. Geological Survey, Mexico’s National Water Commission, the University of Arizona and the University of Sonora in Mexico finished a report about the San Pedro Aquifer. The San Pedro study, a preliminary version of which was released in January, focused on land ownership, water and soil quality, and precipitation. It marked the first time the two countries collaborated aquifer research. The complete version will be published later this year, and a similar study on the Santa Cruz Aquifer is expected early next year. 

“Phase one was a long phase – it never went dormant, but activities halted for a while in some areas,” says Sharon Megdal, director of the Water Resources Research Center at the University of Arizona. “Maybe in phase two, where we’re realizing there’s interest in both countries, it’s time to take a look at some questions. Should there be assessment of additional aquifers? What kind of studies can we do? Can we meet the intentions of the original U.S. legislation?”

A map of the San Pedro and Santa Cruz Aquifers along the U.S.-Mexican border.
University of Arizona

Megdal says these reports are only the first step toward understanding the cross-border aquifers. Researchers don’t even know how many aquifers cross the border. One estimate reported 16, another said up to 36. Sorting out how transborder water management will work isn’t possible without more complete information.

Though the U.S. and Mexico have been collaborating for years with the International Boundary and Water Commission to understand groundwater supply, U.S. policies have complicated those efforts. Each state has different groundwater rights regulations, and in the case of Arizona, laws vary within the state on different aquifers.

Still, the countries can look to the successful 2012 agreement between the U.S. and Mexico on Colorado River Delta restoration, which was decades in the making. The Minute 319 pact allowed for a one-time pulse flow of water released at the Morelos Dam near Yuma, Arizona to reinvigorate the Delta floodplain and reach Mexico. “We’ve had some very good cooperation,” Megdal says. “I think the processes and involvement of NGOs and different players in addressing the Colorado River transborder issues could extend into aquifers – but it’s a largely a different set of players.”

Lyndsey Gilpin is an editorial fellow at High Country News. She tweets

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