The ground in Mexico City is sinking at a rate of almost 50 centimeters (20 inches) per year, and it’s not stopping anytime soon, nor will it rebound, say Chaussard et al. in a new study.
Combining 115 years of ground-based and 24 years of space-based measurements, the team of U.S. and Mexican scientists has concluded that wide swaths of ground beneath the city are steadily compacting after long ago being drained of water. The ground will continue to compact for about 150 years, they forecast, adding up to 30 meters to what is already several meters of subsidence during the 20th century.
Unlike subsidence seen in many other cities of the world, Mexico City’s subsidence does not seem to reflect local groundwater pumping rates, as would be expected. Instead, it reflects the steady compacting of the ancient lake bed on which the city was built.
That lake bed was once Lake Texcoco, home of the Aztec city Tenochtitlán. As water extraction drove groundwater deeper underground, the 100-meter-thick, salty, clay-rich lake bed was left high and dry. Its very fine mineral grains have since been steadily repacking themselves more tightly, causing the ground to shrink and subside.
That kind of compaction is irreversible, the researchers report, and it’s responsible for the appearance of extensive fractures that damage buildings, historical sites, sewers, and gas and water lines in the city. The fracturing is also opening up the ground to contaminated surface waters, which could make access to clean water in the city, already difficult, even worse.
“The stage is set for a dual water and subsidence crisis if no drastic water management actions are implemented,” the researchers write. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1029/2020JB020648, 2021)
—Larry O’Hanlon (@earth2larryo), Science Writer