It’s a vicious cycle. Mexico City is always short of water, so it keeps drilling deeper for more, weakening the ancient clay lake beds on which the Aztecs originally built their city. The more water is pumped out, the deeper the city sinks. And now there’s no going back.
We have known for over 100 years that Mexico City is sinking as the water is removed from the ground it’s built on. By today, the sinking has reached an alarming level of 50 centimeters (20 inches) per year and according to a new study published in JGR Solid Earth, there’s no hope of reversing the process.
The city is built over the Aztec settlement of Tenochtitlan and Lake Texcoco, a system of salt and fresh water lakes. To stop floods and separate the fresh water, the Aztecs built dikes which were destroyed by the Spanish conquistadors during the city’s siege in the 1500s. The Spanish then drained the lake, leaving only a small section.
Back during the 1900s, the city was sinking at a rate of 9 centimeters (3.5 inches) a year. By the late 1950s, that rose to 29 centimeters (11.4 inches) a year, so the amount of water pumped out from the ground in the area had to be capped. That helped to slow down the sinking for a while, but it definitely did not stop reverse or even stop the process. It did go back to 9 centimeters for some time, but during the last two decades it rose to an alarming 50 centimeters per year in some parts of the city. According to the research team, the amount of water removed no longer affects the level of subsidence, which means that the city is doomed to sink.
Using 115 years of leveling data combined with modern approaches such as Interferometric Synthetic Aperture Radar (InSAR) from the past 24 years and GPS data from the last 14 years, the scientists were able to show that “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 are unlikely to ever bounce back. According to the model created by the team, as the process continues the layers will end up compressed by 30 percent, which could lead to an additional sag 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 note.
Over 70 percent of Mexico City’s drinking water is gained through groundwater extraction, from wells located throughout the basin. Since Greater Mexico City has a population of over 21 million people, this means that an insane amount of water continues to be pumped out from those clay layers day by day. The city’s future is doomed.