Mexico City sinks as aquifer is exhausted Subsidence of 30 feet during this century

THE BALTIMORE SUN

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. 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 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 city exhausted its natural springs. Well-digging began, and as the city pumped 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 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.

Because of cracking and the threat of worse, engineers have put up scaffolding to support the ceiling and walls of the National Cathedral 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.

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

After the subsidence shattered hundreds of 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 Public Works director, entered a construction site on Mexico City's eastern edge, descending 80 feet through a concrete tube to show reporters a drilling machine with 20-foot teeth that is chewing a wastewater tunnel through the city's clay subsoil.

The tunnel is one of the last segments of a 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.

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 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.

Pub Date: 2/01/98

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