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Why Mexico’s water crisis is making it sink

MEXICO CITY — Visitors recently strolled around Mexico City’s Metropolitan Cathedral, the oldest in Latin America and one of the largest. Walking from room to room, tourists took pictures of spectacular altars, columns and soaring sculptures. But there is another unintentional detail that stands out: the cathedral is leaning.

“I feel the slope now,” one visitor told a friend, walking from a side room to the main entrance hall.

This subsidence, called land subsidence, is occurring all over the world. Although it may be subtle in many places, it grows land about an inch or two per year across much of the United States – rates in Mexico City are among the highest in the world.

Some areas of Mexico City have retreated 20 inches per year in recent decades, researchers say. Overall, clay layers beneath the ground have compressed by 17 percent over the past century.

According to researchers, one of the causes of Mexico City’s irregular sinking is the pumping of groundwater. The extraction of water allows the porous soil to compact and become depressed. Since more than half of the city’s water supply comes from underground aquifers, its leaders have struggled to solve the problem.

“We have recorded some 120 years of subsidence in our city,” said Enrique Cabral-Cano, a geophysicist at the National Autonomous University of Mexico and author of the research. “All we do is just kick the ball.”

The buildings of Mexico City’s historic center, where century-old streets and buildings tilt and curve so much that they become a talking point on guided tours, reveal just how far the city has sunk. But researchers say it is also visible at the airport terminal and on runways, in above-ground subway stations and on some streets, requiring costly repairs.

“It’s a pretty expensive issue,” Cabral-Cano said. Looking at the cost of the structure, damage from satellite data, building codes and more, he said his team’s initial studies “highlight that this is a fairly expensive process and that rivals any great earthquake or hurricane.”

Most of this subsidence is irreversible, researchers say, because the Earth is still compacting and reacting to long-ago water drainage methods. Cabral-Cano and his colleagues predict that the land will sink another 100 feet over the next 150 years. Today, tap-drying water shortages, compounded by low rainfall, climate change and poor infrastructure, continue to increase reliance on groundwater pumping to meet the city’s water demand.

As Mexicans elect a new president on Sunday, leading candidates have proposed plans to tackle the country’s water crisis. Front-runner Claudia Sheinbaum, who also served as mayor of Mexico City, focused on cracking down on water-intensive agricultural industries and improving irrigation systems.

But it’s unclear whether policymakers will pay attention to subsidence issues and address the underground source.

“The subsidence hasn’t really caught on among politicians and policymakers,” Cabral-Cano said. “It’s not on their radar, even though everyone knows it happens.”

Why Mexico is more likely to sink

One of the reasons the city is dark is the soft ground it was built on. Present-day Mexico City sits atop drained lake beds, built on elastic clay soils that can easily compact if water is removed.

As a growing population depleted natural springs, the city began digging wells and pumping groundwater to meet the demand for water. The ground began to contract and compress the earth, like a drying kitchen sponge.

The sinking got worse over time, researchers have shown over the years. In the 1900s, the city was sinking at a rate of about 3.5 inches per year. By the late 1950s, subsidence rates reached 11.4 inches per year. Soon after, rates briefly improved to 3.5 inches as downtown pumping wells were capped.

But over the past two decades, parts of the city have seen 15 to 20 inches of subsidence.

Part of the problem is the demand for water from a large population. Today, about 70 percent of the city’s water supply comes from pumping groundwater from city wells. In this city of nearly 22 million people, water is pumped faster than it can be recharged.

A study found that a huge amount of groundwater – up to 5 million Olympic-sized swimming pools – has been pumped out every year since 2014 to meet growing demand, depleting groundwater and causing subsidence.

In addition to the high demand for water, the city’s concrete and asphalt prevent some of the rain from seeping into the porous part of the aquifer.

Climate change is also straining our planet’s ability to cool aquifers. Mexico has warmed about 1.6 degrees Celsius (2.9 degrees Fahrenheit) since pre-industrial times, although the city is showing even greater warming due to heat-absorbing materials like concrete. The large accelerates the evaporation of surface waters and affects precipitation patterns.

While the city is at the start of the rainy season, the situation today looks bleak. As of April 30, the entire Federal District was suffering from a “severe” drought. As of May 27, NASA data shows that groundwater levels in the Federal District are about 2 percent below the long-term average for this time of year.

“The solution is not to get water from underground, but we need it to live,” said Darío Solano-Rojas, a geological engineer at the National Autonomous University of Mexico. “We need to think about ways to change water use. That’s just too much water we’re getting from underground.

Centuries earlier, before the Spanish entered the scene and before modern population growth created an outsized demand for water, indigenous communities exploited the lakes around them as a water source, building dikes and canals to control flooding.

There are still remnants of what the city looked like before the lakes dried up. More than an hour from the city center, the municipality of Xochimilco is home to an ecological reserve with famous floating gardens called chinampas, where food and plants grow.

“It’s not that we don’t have water. It’s just not well administered,” said Cassandra Itallé Garduño Mendoza, owner of one of the area’s chinampas.

But in April, next to the preserve, a construction crew was installing a new pipe to extract water from several hundred feet deep, again threatening the lake level.

Dealing with the consequences

As aquifers become depleted, water developers begin to dig in new places and deeper underground. But that only expands the problems, Cabral-Cano said. In the 1950s, he said the city was much smaller and most of the wells were located in the city center, but they have since been moved.

“One of the decisions was to move the water wells away from the city, so that the city center would not be subject to this subsidence process,” Cabral-Cano said. “The fastest rate of subsidence is around the Mexico City metropolitan area.”

An afternoon, Solano-Rojas traveled east of the city to Valle de Chalco, which was built on the dry bed of Lake Chalco. By analyzing satellite data, he knows that this area has experienced high rates of subsidence. He points to a large historic building with walls completely split and tilted on different levels. which he describes as a clear example of land subsidence.

But other people in the area don’t seem to notice. One resident, who lives down the street from the crumbling structure, said she knew nothing about the subsidence issues. She remembers the bursting of a nearby underground water pipe, which Solano-Rojas suspects was indirectly linked to the subsidence. He also notices cracks on the exterior walls of his house, a telltale sign of uneven flooring.

These signs may be more subtle than the leaning buildings of historic downtown Mexico City, but Solano-Rojas said they can affect people’s lives even more.

“We all get water from the aquifer, but the consequences are not evenly distributed across the city,” Solano-Rojas said.

At this point, Solano-Rojas and Cabral-Cano have a trained eye for these signs of subsidence, which many might overlook. Data shows high rates around the main airport, which can be felt by a bumpy runway and uneven foundations in one of the terminals. Uneven roads and rickety subways could also be linked to land subsidence.

But there are limited solutions that each resident can tackle individually, Cabral-Cano said. Some people may try to repair cracks or reinforce the structure of their home, but he said the local or federal government will have to address the underlying water issues.

“There’s only so much you can do as a person,” he said. “I hope your home is not significantly affected.”

washingtonpost

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