Mexico City, one of the largest and most populous cities in the world, is facing a serious threat from the ground beneath it. The city is built on a lake bed that has been drained of water for centuries, causing the clay sheets underneath the city to compress and crack. The result is that the city is sinking at a rate of up to 50 centimeters a year, and this will continue for about 150 years until the clay reaches its maximum compaction.
The sinking ground threatens the infrastructure and water security of millions of people, as it damages pipes, roads, buildings, and monuments. Some of the city’s most iconic landmarks, such as the Metropolitan Cathedral and the Angel of Independence, are tilting and cracking due to the uneven subsidence. The city’s drainage system is also affected, as it relies on gravity to move wastewater out of the basin. As the city sinks, the pipes have to work harder to pump the water uphill, increasing the risk of floods and sewage spills.
There is no hope for restoring the lost elevation and storage capacity of the aquitard, a layer that restricts groundwater flow.
In the northeastern part of the city, an area that remains largely undeveloped and where sinking rates have gone unnoticed, researchers have discovered that the land is subsiding at a rate of up to 50 centimeters annually.
“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 authors write. An aquitard is a region that restricts groundwater flowing from one aquifer to another.
“If you put heavy buildings on that kind of ground, and use shallow foundations, the soil compacts,” geotechnical engineer Eddie Bromhead from Kingston University London told The Guardian in 2004.
“So that, along with removing the water, is why Mexico City is such a mess.”
Scientists initially observed Mexico City’s sinking in the early 1900s, at a pace of about 8 centimeters annually. By 1958, this rate had surged to 29 centimeters per year, prompting a decision to limit the water extraction from wells in the city center.
Following this measure, the sinking rate decreased to less than 9 centimeters per year. However, recent higher-resolution data has unveiled a steady rate of up to 40 centimeters per year in the historic downtown area over the past two decades.
The city faces a paradoxical situation of having too much water and too little water at the same time, as it suffers from floods, droughts, and contamination. The city receives abundant rainfall during the wet season, but most of it goes to waste as it runs off the paved surface or mixes with sewage. The city also depends on groundwater for more than 70% of its water supply, but this is unsustainable as it depletes the aquifer and worsens the subsidence. Moreover, the water quality is poor due to pollution from industrial and agricultural activities.
Some of these measures are already being implemented by various actors in the city, such as Isla Urbana, a social enterprise that installs rainwater harvesting systems in low-income households.
However, these solutions are not enough to address the magnitude of the problem. The city needs a comprehensive and coordinated plan that involves all levels of government, civil society, private sector, and academia.
Relevant articles:
– Mexico City Is Sinking at an Alarming And Largely Unstoppable Rate, New Data Finds, ScienceAlert, 07 May 2021
– Climate change is making Mexico City sink, MIT Technology Review, 23 December 2021
– Mexico City Subsidence, Landscape Sinking Faster Than Expected, Science Times, 17 May 2021
– How a city that floods is running out of water, BBC Future, 14 May 2018