| | |  News Around the Republic of Mexico 
Mexico's Capital is a Sinking Metropolis
 Chris Hawley - USA Today
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April 09, 2010
| | Construction machines excavate bedrock of at the bottom of a 200-foot pit in Teoloyucan, Mexico on Monday as part of the Eastern Drainage Tunnel project. Crews are rushing to finish the massive, 37-mile tunnel because other drainage channels are beginning to flow backwards as Mexico City sinks into the ground. (Sergio Solache) | |
Mexico City - Deep underground, tunneling crews are in a race against time as they try to save the world's third-largest metropolis from catastrophe.
Above them, the Mexican capital is sinking into the earth at a record rate, tilting the city's sewage tunnels so they are actually running backward.
Crews are rushing to build a 37-mile drainage tunnel to save much of the city from future floods.
"Imagine the Congress, the stock exchange, the country's biggest airport, everything under water," said Ariel Flores, water-reuse manager for the National Water Commission. "It would paralyze the economy of the entire country. It would be a total disaster."
Across this city of 18.7 million, crews have started a flurry of projects to shore up areas that are sinking by as much as 8 inches a year. They're renovating a key intersection, filling in holes under a commuter-train line, strengthening churches in the historic center, rehabilitating another major drainage tunnel and dredging the city's above-ground sewage canals.
Flooding poses the most danger, and there are already signs of trouble. In February, the Remedios River, a sewage canal, backed up and broke through its dike, flooding 4,000 homes with raw waste. In Mexico City, waste and rainwater run through the same sewer system.
After the flood, the city installed five more massive pumps to force water out of the sinking metropolis.
"The goal: to reduce the risk of flooding as much as we can in our city," Mayor Marcelo Ebrard said during a visit to a drainage project on Monday. "With luck, we'll be in time."
Mexico City's sinking problem dates back centuries. The Aztecs had built their capital city, known as Tenochtitlan, on a flat island in the middle of a lake. The city flooded frequently during the June-to-October rainy season.
After the Spanish defeated the Aztecs in 1521, Spanish colonizers began building drainage canals in an attempt to control the floods. One flood, in 1629, left the city underwater for five years.
As the water disappeared, the city settled into the mud of the lakebed, creating a hump in the drainage canals. The city built additional drainage tunnels to get through the higher ground, but as the city sank, they, too, have begun tilting backward.
In recent decades, the city's population soared, forcing authorities to pump more drinking water out of underground aquifers, which worsened the sinking.
Mexico City and its suburbs make up the world's third-most-populous urban area after Tokyo and New Delhi, India, according to the United Nations.
Much of the wastewater no longer flows naturally out of the city. Pumps are used to get it over a rise called the Sierra de Guadalupe.
The sinking has reduced Mexico City's drainage capacity by 30percent since 1975, even as its population has nearly doubled.
In a few years, the current system of pumps will not be enough to get the water out, the National Water Commission says. If just one key drainage tunnel, known as the Central Emitter, were to be blocked, it would flood most of northeast Mexico City and two suburbs in adjacent Mexico state, studies show.
The area includes more than 4 million people, the central tourist district, a cathedral, an airport, several subway lines, many government offices and dozens of corporate headquarters. It also includes Highway 150, the city's main connection to southern Mexico.
As the ground sinks, landmarks that are anchored to the bedrock seem to thrust skyward. Workers have had to add 14 steps to the base of the Independence Monument since it was built in 1910, and a water pipe that was installed at ground level in 1934 now juts 27 feet in the air beside the Revolution Monument.
The Insurgentes Traffic Circle, a main intersection built on underground pylons in 1970, is now 12 feet higher than the streets feeding into it. Cars have to gun their engines up a slope just to make the turn.
On a recent afternoon, construction crews were tearing up the asphalt to make way for a new ramp so that traffic could make the grade.
"We can't stop the sinking, so we just have to adapt the streets to it," engineer Carlos Pixor said as he watched a city bus mount the hill.
In the city's colonial center, the federal government is injecting columns of wet concrete 115 feet beneath the Holy Trinity Church in an attempt to shore it up.
The 17th-century church is listing to the side and sinking faster than surrounding streets. Worshipers have to descend a 7-foot-high stairway to get in the door.
Crews finished a similar project at the nearby Santo Domingo church in January.
But the biggest project is the Eastern Drainage Tunnel, a 23-foot-wide, $1.1 billion pipeline that will run north for 37 miles from Mexico City to lower ground in the Mezquital Valley.
Six tunneling machines, each as long as a football field, will burrow simultaneously through 6-mile sections. At its deepest point, the tunnel will be 495 feet underground.
Excavation began in June 2008 and will finish in 2012, the National Water Commission says.
The tunnel will release water and reduce the risk of flooding. But it won't stop the sinking, said Ramón Domínguez, a hydraulic engineer at the National Autonomous University of Mexico.
To stop the ancient lakebed from contracting farther, the city has to stop pumping from its aquifer or inject water back into the ground, he said.
Before injecting it, the water has to be cleaned, and the city currently does not have enough water-treatment plants for that. Only 6 percent of Mexico City's wastewater is treated; the rest is used to irrigate fields in the Mezquital Valley.
Even if the water could be returned to the aquifer, it's unlikely the ground could be "reinflated," Flores said. Once the soil of the lakebed is compacted, it will never return to its original volume, he said.
"You can't raise the city again," Domínguez said. "The only hope is to stop it from sinking further."
Reach the reporter at chris.hawley(at)arizonarepublic.com.
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