Mexico City is sinking irreversibly and increasingly fast

https://www.iflscience.com/environment/mexico-city-is-sinking-faster-than-ever-and-theres-no-hope-it-can-be-reversed/

Mexico City Is Sinking Faster Than Ever And There’s No Hope It Can Be Reversed

For over 100 years, it has been known that Mexico City is sinking due to the removal of water from the ground it’s built on. This sinking has now reached an alarming rate of 50 centimeters (20 inches) per year and a new study published in JGR Solid Earth suggests that there is no hope of reversing it.

Mexico City is built over the Aztec city of Tenochtitlan and Lake Texcoco, a system of salt and fresh water lakes. The Aztec had dikes to separate the fresh water and stop floods but those were destroyed during the Spanish colonial invasion and the siege of the city in the 1500s. Afterward the Spanish drained the lake with only a small section remaining.

In the 1900s, the city was sinking at a rate of 9 centimeters (3.5 inches) a year. Since the late 1950s, when that rose to 29 centimeters (11.4 inches) a year, the amount of water that could be drilled from the ground in the area was capped. While that helped to slow down the sinking for a while, it did not stop or reverse the process. For while it went back to 9 centimeters, but in the last two decades it rose to a sinking rate of 50 centimeters in some parts of the city. Now, the team believes that the amount of water removed no longer affects the level of subsidence.

The team used 115 years of leveling data combined with more modern approaches such as Interferometric Synthetic Aperture Radar (InSAR) collected over the last 24 years and GPS data from the last 14 years to show “no significant elastic deformation exists, demonstrating that the subsidence is almost fully irreversible.” The clay layers below the city have so far been compressed by 17 percent and it is unlikely to bounce back.

The team stresses that the compression of these layers continues and according to their model, the whole layer will end up compressed by 30 percent which could lead to additional subsidence of up to 30 meters (99 feet) in 150 years.  

“These subsidence rates will persist unless water levels are brought back up to shallow depths. Even if water levels were to be raised, there is no hope for recovering the great majority of the lost elevation and the lost storage capacity of the aquitard,” the researchers write.

With a population of over 21 million people, Greater Mexico City is the eleventh largest metropolitan area in the world and the largest in North America. Over 70 percent of its drinking water comes from groundwater extraction wells located throughout the basin.

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