MEXICO CITY — On a recent morning, visitors wandered around Mexico City’s Metropolitan Cathedral, Latin America’s oldest — and one of its largest. Walking from chamber to chamber, tourists snapped images of dramatic ceiling-high altars, soaring columns and sculptures. But there’s another unintended detail that stands out: the cathedral is leaning.

“I do feel the slope now,” a visitor said to a friend, walking from a side chamber to the main entry hall.

This sinking, which is known as land subsidence, crops up across the world. While it can be subtle in many places — it pushes land down around an inch or two a year in much of the U.S. — the rates in Mexico City are some of the highest in the world.

Some areas in Mexico City are slipping as fast as 20 inches a year in recent decades, according to researchers. Overall, the clay layers under the soil have compressed by 17 percent in the last century.

A culprit for the uneven sinking in Mexico City, researchers say, is pumping water from underground. The water extraction enables the porous soil to compact and depress. Since more than half of the city’s water supply comes from underground aquifers, its leaders have struggled to tackle the problem.

“We’ve had 120-something years of recorded 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 in Mexico City’s historic center, where streets and centuries-old buildings slope and bend so much that it becomes a talking point in guided tours, expose how much the city has sunk. But researchers say it’s also apparent at the airport terminal and runways, aboveground metro stations and just driving down some streets — which require costly repairs.

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

Most of this sinking, the researchers say, is irreversible because the Earth is still compacting and responding to the ways water was drained a long time ago. Cabral-Cano and his colleagues project the land is going to sink another 100 feet over the next 150 years. Water shortages today that are running taps dry, worsened by low rainfall, climate change and poor infrastructure, continue to reinforce reliance on groundwater pumping to meet the city’s water demand.

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

But it’s hard to know if decision-makers will pay attention to the subsidence issues and tackle the underground source.

“Subsidence hasn’t really percolated into the politicians and decision-making persons,” said Cabral-Cano. “It’s not on their radar, even though everybody knows that it happens.”

Why Mexico City is more susceptible to sinking

One reason the city is sinking is the soft ground on which it was built. Present-day Mexico City lies on top of drained lakebeds, built on elastic clay soils that can easily compact if water is removed.

As a growing population depleted natural springs, the city began to dig wells and pump groundwater to meet water demands. The soil began to contract and compress the land, like a kitchen sponge drying up.

The sinking worsened over time, researchers have shown over the years. In the 1900s, the city was sinking at around 3.5 inches per year. In the late 1950s, subsidence rates rose as high as 11.4 inches per year. Shortly after, rates briefly improved to 3.5 inches as pumping wells in the city center was capped.

But in the last two decades, some parts of the city have experienced 15 to 20 inches of subsidence.

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

One study found that a tremendous amount of groundwater — as much as 5 million Olympic-size pools — have been pumped each year since 2014 to meet the growing demand, depleting groundwater and causing sinking.

In addition to the high water demand, the city’s concrete and asphalt prevents some rain from filtering down to the porous part of the aquifer.

Climate change is also stressing our planet’s ability to refresh the aquifers. Mexico has warmed around 1.6 degrees Celsius (2.9 degrees Fahrenheit) since preindustrial times, although the city shows even more warming because of heat-absorbing materials like concrete. The great speeds up evaporation of surface water and affects rainfall patterns.

While the city is at the beginning of the rainy season, the situation looks grim today. As of April 30, the entire federal district was suffering from a “severe” drought. As of May 27, NASA data show groundwater levels of the federal district are at about 2 percent compared to the long-term average for this time of the year.

“The solution is don’t get water from underground, but we kind of need it for a living,” said Darío Solano-Rojas, a geological engineer at the National Autonomous University of Mexico. “We need to think of ways of changing usage of water. It’s just too much water we’re gaining from the underground.”

Centuries earlier, before the Spanish entered the picture, and before modern population booms created an outsize demand for water, the native communities leveraged the lakes around them as a water source, building levees and canals to control flooding.

There are still remnants of what the city looked like before the lakes were drained. More than an hour away from the city’s center, the Xochimilco borough holds an ecological reserve with famous floating gardens called chinampas, where food and plants can grow.

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

But, in April, next to the reserve, a construction crew was installing a new pipe to extract water from several hundred feet below, threatening the lake levels there as well.

Dealing with the consequences

As aquifers deplete, water operators start digging into new places and deeper underground. But that just spreads the problems out farther, Cabral-Cano said. In the 1950s, he said the city was much smaller and most wells were located in the downtown area, but they have since been relocated.

“One of the decisions was to move the water wells further away from the city, so that the downtown area wouldn’t be subjected to this subsidence process,” Cabral-Cano said. “The fastest subsidence rate is on the surroundings of Mexico City’s metropolitan area.”

One afternoon, Solano-Rojas drove east of the city to Valle de Chalco, which was built on the drained lake bed of Lake Chalco. Analyzing satellite data, he knows this area has experienced high rates of subsidence. He points to a large historical building with its walls completely split, sloping at different levels what he describes as an obvious example of sinking land.

But others in the area don’t seem to notice. One resident, who lives down the street from the crumbling structure, said she doesn’t know anything about subsidence issues. She recalled an underground water pipe bursting nearby, which Solano-Rojas suspects was indirectly subsidence-related. He also notices cracks on the outside walls of her house, a telltale sign of unlevel ground.

These signs may be more subtle than the leaning buildings in Mexico City’s historic city center, but Solano-Rojas said these can affect people’s lives even more.

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

At this point Solano-Rojas and Cabral-Cano have a trained eye for these signs of subsidence, which many could miss. Data shows high rates around the main airport, which can be felt in a bumpy runway and an uneven foundation in one of the terminals. Uneven roads and wobbly metros could also be linked to the sinking ground.

But there are limited solutions any individual residents can tackle, Cabral-Cano said. Some people can try to repair cracks or beef up the structure of their house, but he said the local or federal government will have to address the underlying water issues.

“As a person, there’s not much you can do,” he said. “Hope that your home will not be significantly impacted.”



Source link

Leave a Reply

Your email address will not be published. Required fields are marked *