Journal: Mexico City Sinking Into Depleted Aquifer


MEXICO CITY -- In a plaza next to one of this city's most important shrines, the colossal Monument to the Revolution, a humble water pipe has become a curious monument of its own to what is, literally, Mexico City's continuing collapse.

Flush with the ground in 1934 when the Monument to the Revolution was built, the water pipe now soars 26 feet into the air. Why? Firmly anchored in a hard layer of subsoil beneath the city's shallow aquifer, the pipe has stayed put in the last six decades while the city has fallen away.

Mexico City is sinking. So much water has been pumped out from the aquifer beneath it to satisfy the metropolitan area's 18 million residents that the ground is collapsing underfoot at a stunning rate.

Many cities have experienced subsidence. The most famous, Venice, has sunk about 9 inches during the 20th century as its water table has dropped. But from here Venice's problems seem marginal. Mexico City has sunk 30 feet.

"The sinking of the soil in Mexico City is one of the biggest engineering problems any city has faced, anywhere," said Ismael Herrera Revilla, a mathematics professor at the National Autonomous University who led a binational scientific panel in a five-year study of the city's water crisis.

In 1519, when the Spaniards conquered the Aztec capital, Tenochtitlan, there was plenty of water; Mexico City originally straddled two lakes. But the conquistadors who built their own city adjacent to the Aztec one brought engineers to drain the lakes.

Early in this century the fast-growing city exhausted its natural springs. Well-digging began, and as the city pumped more and more water, the soil began to give way. In the early decades of this century, annual sinkage in the city center averaged about 2 inches, but when it peaked at mid-century the soil was collapsing away at the astonishing rate of 19 inches a year.

Because the subsidence is not uniform, it has caused hundreds of millions of dollars in damage to buildings and other structures over the years, especially in the colonial-era city center.

Because of cracking and the threat of worse, engineers have put up scaffolding to support the ceiling and walls of the National Cathedral, the largest and oldest in Latin America, and are carrying out a complex and costly effort to shore up its foundations. Across the central Zocalo plaza, the sinking has caused the National Palace to list dangerously, and architects are working to keep one wing attached to the rest of the building.

Striking evidence of the sinking is visible along Line 2 of the city's Metro, or subway, which runs above ground alongside Tlalpan Avenue south of the city center. Absolutely horizontal when first constructed in the mid-1960s, the tracks now look like a roller coaster.

After the subsidence shattered hundreds of irreplaceable colonial churches and mansions, the city stopped pumping water in the city center, instead drawing from wells at the periphery. In recent years, this has slowed the sinking of the city center to about an inch annually. But some suburbs with many wells continue to sink 18 to 24 inches each year.

Today the most serious damage is occurring underground, where the collapsing subsoil continues to rupture sewer lines, subway tunnels, and potable water pipes.

One recent day, Cesar Buenrostro, the city's new Public Works director, stepped onto an elevator at a construction site on Mexico City's eastern edge, descending 80 feet through a cement tube to show two reporters a vast drilling machine with 20-foot teeth that is chewing a new wastewater tunnel through the city's clay subsoil.

The tunnel is one of the last segments of a new 124-mile network of vast, very deep sewers, designed to carry rain and waste out of the city. The $870 million system is needed not only to keep up with city growth, but also because sewage no longer flows by gravity into the Grand Drainage Canal, which was for a century the main sewer outlet.

The city's 30-foot drop has forced the installation of vast pumping stations to elevate the sewage to the level of the canal entrance, greatly increasing operation costs. The new, deep sewer will reduce the reliance on pumping to drain the city.

"Soil sinkage is a huge problem, but unfortunately we can't drastically reduce the pumping of the aquifer now, because the city needs the water," Buenrostro said. "Our basic problem is the concentration of political power, industry and most of our cultural treasures and educational institutions in the capital. We want to change that, but for now we have to provide water to our people."

The city has tried to exploit alternative water sources, building vast pipelines to bring in water from faraway rivers. Eight percent of its water, for instance, is now pumped nearly 4,000 feet up, at enormous cost, from the Cutzamala River 80 miles to the west.

But 70 percent of the water used in the metropolitan area, which includes the capital and all or portions of 17 surrounding counties in the State of Mexico, is still drawn from the aquifer.

The water difficulties have become a vicious circle: as the city grows, more water is pumped from the aquifer. As more is pumped, the city sinks further. The sinkage ruptures more underground water pipes, sending fresh water gushing into the sewers, aggravating the shortage, requiring more water to be pumped from the aquifer, and so on.

Mexico City has one of the world's leakiest water distribution systems. About a third of every gallon of fresh water pumped into the system leaks out, enough water to supply the cities of Guadalajara and Monterrey, which have a combined population of 2.7 million.

Repair brigades race through Mexico City streets plugging 40,000 ruptures a year, yet many leaks go unreported by neighbors until water pressure drops inside houses.

Some leaders have begun trying to convince the citizenry of the precious nature of the city's beleaguered aquifer. Jorge Legorreta, an architect appointed last month as borough president for the city's central district, spoke out in a radio talk show about the sinking city and the water crisis that is causing it.

He urged city residents to put empty buckets out in their patios to collect rain during the stormy season, but he said later that he got little response.

"Changing attitudes about water is difficult," Legorreta said. "Mexico has been living for too long off of water that has accumulated through the ages."

Thursday, January 29, 1998

Copyright 1998 The New York Times