Gerardo Medina’s plumbing has never worked well. Most of the time, water arrives at his home on the outskirts of Mexico City dirty…or it doesn’t come at all. And although the Mexico City Water System (SACMEX) repeatedly sends people to fix the pipes, the lines always fail again, Medina said. “It’s a never-ending story.”
In Medina’s neighborhood, Ampliación Santa Martha, broken pipes, cracked streets, and curved windows warn of a (not so) hidden problem: Mexico City is sinking.
Scientists agree that groundwater extraction is a contributing factor to this subsidence, although estimates of the extraction rate vary. Authors of a new study published in Geophysical Research Letters used satellite data to narrow these estimates. They found that between 1 and 13 cubic kilometers (0.2 and 3 cubic mile) of groundwater have been pumped each year since 2014 to serve the 22 million residents of the Mexico City Basin. (For reference, that’s enough water to fill up to 5 million Olympic-sized swimming pools.)
The Mexico City Basin is like a wet sponge. It is composed of lava flows, ashes, clays, and sands. Water flows in the pore spaces between these sediments. Though pumping has caused the surface to sink by about 35 centimeters (14 inches) per year, subsidence is not uniform, according to study authors.
When water is removed, the sediment compacts, causing subsidence in some areas and cracks in others, explained Dora Carreón Freyre, a geological engineer at the National Autonomous University of Mexico (UNAM) and member of the United Nations Educational, Scientific and Cultural Organization’s Land Subsidence International Initiative (LASII) who was not involved in the new research.
Mexico City’s urban growth also blocks precipitation from reaching the spongy sediments by increasing the amount of land covered by impervious surfaces like roads and parking lots. These impervious surfaces prevent increasingly scarce rains from recharging underground aquifers. “It’s a perfectly disastrous formula,” Carreón Freyre said.
Although previous studies did consider pumping to be a factor causing subsidence in Mexico City, some researchers argue that the main trigger is long-term compaction of an ancient lake bed. The city was built on Lake Texcoco, which filled with silt in the 17th century after Spanish conquistadores began draining the lake. Since then, the weight of the city’s development has caused the silt to steadily pack more tightly, making the ground shrink and sink. Authors of a 2021 study pointed to steady subsidence that did not fluctuate with groundwater pumping rates as evidence of the lake bed’s compaction as the dominant influence on the phenomenon.
Seeking to further explore the mechanisms responsible for Mexico City’s subsidence, Mohammad Khorrami, a geotechnical engineer at the Virginia Polytechnic Institute and State University (Virginia Tech), and his colleagues took to the skies to calculate how much water had been pumped from the basin below.
With satellite data from the Gravity Recovery and Climate Experiment (GRACE), the researchers measured a slight decrease in the basin’s gravity since 2014 due to changes in the mass of water near the surface. Modeling groundwater storage changes along with interferometric synthetic aperture radar (InSAR) data—which record slight changes in surface elevation—between 2014 and 2021 indicated that groundwater pumping is the main driver of subsidence, Khorrami said.
The satellite data indicated that because of groundwater loss, Earth’s surface is sinking in some areas but rebounding in others, Khorrami explained. In some places surrounding Mexico City, the ground is rising 2 centimeters (0.7 inch) per year. The research offers “one of the first InSAR observations of rebounding of the ground as a result of the huge amount of groundwater mass loss or extraction,” Khorrami said.
Enrique Cabral, a geological engineer with UNAM, is skeptical about the interpretation of these results. Using satellite gravity data is the right thing to do for this type of analysis, Cabral said, but these tools are intended for studying much larger, subcontinent-sized basins. “The margin of uncertainty is large,” he said. “Although Mexico City is big, the basin area is not of those dimensions.”
Carreón Freyre, too, expressed some skepticism. The new study reflects a good use of remote measurement and modeling techniques, she said, but the volume of withdrawn water and supposed uplift do not make much sense considering the context of Mexico City’s water management. “Groundwater has been extracted little by little over decades,” she explained, so a scenario in which the surface rebounds as if all the water were removed at once is far from realistic.
A Long-Standing Problem
One of the great challenges surrounding Mexico City’s subsidence is the availability of groundwater level data, said Manoochehr Shirzaei, a geophysicist at Virginia Tech and a coauthor of the new study.
UNAM geological engineer Darío Solano agreed. Although public authorities such as SACMEX and the National Water Commission (CONAGUA) have a history of opacity in publishing this type of data, he said, the information exists and can be accessed by direct request through online data transparency systems.
Continued research into Mexico City’s subsidence is important to keep attention on a problem that often remains neglected, Cabral said. The effects of subsidence, such as a limited water supply and damage to infrastructure, are long-term in scope. As a result, many residents and policymakers do not perceive the current level of pumping as an immediate problem and are less likely to conserve water, he explained.
“All research that…indirectly puts the magnifying glass on this [subsidence] process and on the fact that we must do something to address it is very welcome,” Cabral said.
—Humberto Basilio (@HumbertoBasilio), Science Writer