Why is Mexico City sinking? (original) (raw)

One bad decision after another. Travel back far enough through history and, if you are feeling uncharitable, you can lay the blame at the door of the Aztecs, or Mexica. It was they, after all, who decided to build the city in the middle of a lake that had formed in a volcanic crater. Easy to defend, it may have been. Great to build on, it was not.

The magnitude of Mexico City's problems, which made the news this week, are undoubtedly alarming. While Venice is sinking into its lagoon at a leisurely centimetre or so a year, Mexico is in relative freefall, having plunged some 10 metres in the past century.

The Aztec's unusual choice of location for their fine city was just the start of the problems. When the Spaniards took over in 1519, they filled in the lake and sank deep wells to bring water up from underground aquifers. "Everywhere they pumped up groundwater from the boreholes, the ground sank. Without the water there, the sediments that the city was built on compressed a lot more," says Eddie Bromhead, a geotechnical engineer at Kingston University.

As the city's population exploded, demand for water rose sharply, and the aquifer beneath the city is now believed to be in danger of collapsing.

The final mistake was to go rapidly to high-rise buildings when a sudden influx of jobseekers swelled the city's population to around 20m as it stands at today. "If you put heavy buildings on that kind of ground, and use shallow foundations, the soil compacts," says Bromhead. "So that, along with removing the water, is why Mexico City is such a mess."

For a city that sprawls around 40km east to west and 60km north to south, solving the problem is not going to be easy.

"They could move the whole city somewhere else, but that's hardly practical. Or they could underpin every building but that's not practical either," says Bromhead. "As the years pass, they could gradually replace badly founded buildings with better ones. Or, of course, they could just get on with it and live with crooked buildings."

In an attempt to get their water usage under control, a series of pumping stations have been installed to draw in water from nearby rivers. But with water demand standing at more than 10m gallons a day, Mexico City is a thirsty place. The electricity bill for the pumps is phenomenal, enough to meet all the needs of a city of more than a million inhabitants.

Mexico City's sinking into the soil is causing other problems too. While once the city's waste simply drained away into the far-off Gulf of Mexico, now it is pumped out of the hole that Mexico City is slowly settling into.