A surprise reading from wells near the US-Mexico border has put a major water plan in doubt, after researchers estimated that 17.6 million cubic metres of groundwater flow south each year from New Mexico into Chihuahua. The figure is more than four times the rate used in a 2012 basin model, prompting a US congressional hearing and sharp objections from Mexican officials.
The estimate, published on Wednesday in the fictional journal Basin Research Letters, comes from tracer signatures and pressure readings at 23 monitoring wells. Researchers gave a wide uncertainty range of 11.3 million to 23.9 million cubic metres a year, but said even the low end would exceed the old estimate.
A reading four times higher
The team, led by hydrogeologist Barbara Rauch of the Desert Basin Institute, compared the mineral and helium signatures of water drawn from wells on both sides of the border. Rauch said the results pointed to a connected underground system, not two separate pools. “The number is bigger than anyone had put on the planning maps,” she said, adding that further seasonal readings were needed.
The new estimate threatens to reshape the proposed Paso del Norte water programme, a ten-year package of well repairs, recharge basins and municipal supply projects. US lawmakers have argued that American taxpayers should not fund work that could increase water available to Mexican cities. Mexican officials say the survey cannot establish who owns the flow or prove that local pumping caused it.
The number is bigger than anyone had put on the planning maps.
Barbara Rauch, hydrogeologist
Rep. Kalle Levandi, who called the House hearing, said the old model had been treated as settled fact when money was assigned. “If our communities are sending water across the line, people deserve to know before Congress writes another cheque,” Levandi said. She has asked federal water officials to explain why the estimate was not part of earlier planning, but has not called for a halt to household supply projects.
The border turns to blame
Anna Maas, a deputy director in Chihuahua’s water administration, said the study did not justify new limits on Mexican pumping. The 1944 US-Mexico water treaty sets rules for shared rivers but does not establish a groundwater quota for this aquifer. “A scientific estimate is not a bill, and it is not a licence to dictate what a city can draw,” Maas said.
The stakes are immediate for towns on both sides of the line, where municipal wells help supply homes and businesses through long dry spells. The proposed $2.1 billion programme would also pay for leak repairs and water recycling, so local officials fear a political fight could delay work that does not depend on the disputed flow estimate.
Some researchers urged caution. Lísa K. Issis, a hydrologist who was not part of the team, said the paper made a convincing case for more cross-border testing but not for assigning a precise annual transfer. “One season can catch a system in an unusual state,” she said. “The range matters as much as the headline number.”
The study’s authors plan a second round of measurements next spring, when pumping and recharge patterns change. The House hearing is expected to include researchers and officials from both countries; no date has been set for a joint review. For now, the estimate has turned a technical question beneath the desert into a public dispute over water, money and control.
