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 recovering the lost elevation and storage capacity of the aquitard, which is a layer that restricts groundwater flow. “The subsidence is permanent, irreversible and will continue even if we stop using groundwater” said Dr. Dora Carreón-Freyre, a geologist at the National Autonomous University of Mexico. She added that “we have records from 1900 to date, and there is a very clear relationship between the extraction of water and land subsidence”.
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.
“We need to change the paradigm of water management in Mexico City. We need to move from a supply-side approach to a demand-side approach” said Dr. Gonzalo Hatch, a hydrologist and director of Agua Capital, a non-governmental organization that works on water issues in the city. He suggested that the city should implement measures such as rainwater harvesting, water conservation, leakage reduction, and wastewater treatment to reduce its dependence on groundwater and improve its resilience.
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. “The city has been trying to deal with two problems: flooding and drought. And both are related to climate change” said Dr. Enrique Lomnitz, an urban planner and founder of Isla Urbana. He claimed that rainwater harvesting can help solve both problems by capturing and storing rainwater for domestic use.
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. “The city is not going to collapse into the lake. But there are going to be serious consequences for human activities” warned Dr. Effie Liu, a geoscientist at Cornell University who conducted a study on Mexico City’s subsidence using satellite data. She urged that “we need to act now before it’s too late”.
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