News Notes

Urban hazards
Sinking of a titanic city

Mexico City is growing at a rate of 350,000 new people per year, but this city of roughly 20 million is facing graver problems than its exponential growth. The city is sinking, and quickly running out of water.

Mexico City has sunk about 30 feet in the last century. The Monumento a la Independcia, built at ground level in 1910 to celebrate the 100th anniversary of Mexico’s War of Independence, now requires 23 additional steps to reach its base as the ground has sunk below it. Damage has plagued the city’s infrastructure, including building foundations, wells, streets and the sewer system. Flooding has become a problem as the city now lies 6 feet below nearby Lake Texcoco. Instead of marking walls to measure their growth, children mark their height on neighborhood well casings to see if they are growing faster than the ground is sinking.

The land has been subsiding at a rate of one to three inches per year. Mexico City is subsiding because water is being drawn from the ancient, underlying aquifer that supplies 72 percent of the city’s groundwater, and being drawn faster than it is replenished.

The city is built on layers of clay and highly permeable sand and gravel that easily compress when fluid is withdrawn. “Inelastic compaction is not recoverable,” says geologist Stan Leake, a groundwater scientist with the U.S. Geological Survey. Even if the aquifer was completely recharged and the water table brought back to its original level the land would not recover, he says. The clays have also begun to crack. Leake states that earth fissures, which seem to be “related to geologic structure adjacent to compressible sediments that are compacting,” are already occurring around the city.

[At left: This building in Mexico City was built on piles to prevent it from settling into the soft lakebed deposits. The underlying aquifer has compacted beneath the building and the building appears to be “rising” out of the ground. Mexico City is one of the few subsidence areas where this effect is observed. Picture taken in 1977 and supplied courtesy of Thomas Holzer of the USGS]

According to Alan Morgan, a geologist at the University of Waterloo who studies Mexico City subsidence, water in the main aquifer is being pumped at a much higher rate than it can recharge, causing the water table to drop roughly 3 feet per year. The precipitation rate is lower than the evaporation rate in this arid region so groundwater recharge is extremely limited. Unlimited pumping will ultimately deplete the aquifer, forcing Mexico City to bring in almost all of its water at “some great economic and environmental cost,” Morgan says. Although it is estimated that the quantity of groundwater in storage in the Basin of Mexico is 240 to 350 times the current annual extraction, the usable life of the aquifer cannot be reliably predicted.

Both the Aztecs and the Spanish had expansive systems of aqueducts for bringing their water in from the mountains surrounding the city. Water was brought into the valley until 1846, when residents first discovered the artesian aquifer that underlies the city. As more and more water was withdrawn, subsidence began. Subsidence was first noticed in 1891 in the old part of the city but not studied until 1925. In 1948 it was proved that groundwater withdrawal was causing the subsidence.

“In 1954, the overexploitation was sufficiently serious that pumping was banned in the city center and wells were moved to the north and south of the basin,” Morgan says. Subsidence is now stabilized in the city center but is still a major problem in most parts of the metropolitan area.

“Water quality problems and water supply limitations in the Mexico City Metropolitan Area are principally driven by population growth and macroeconomic and regional economic trends,” according to a binational committee of U.S. and Mexican scientists organized by the National Academy of Sciences National Research Council Water Scientists Technology Board. They reported their work in Mexico City’s Water Supply: Improving the Outlook for Sustainability, published in 1995. The Committee is a consortium of Mexican and American scientists formed by a partnership between the National Research Council and the Mexican Academies of Science and Engineering.

Morgan and Leake agree that the way to prevent further subsidence is easy: Stop pumping. Although withdrawal has ceased in certain areas, it is not possible to stop relying on the aquifer entirely. Certain measures to solve the water shortage issue are already in place, such as bringing water in from other watersheds and constant monitoring of wells. As few fresh water supplies are available in the basin itself, Leake and Morgan advocate conserving and reusing water.

The Water Science and Technology Board committee has a few more suggestions. “More attention should be given to managing water demand through pricing mechanisms, education and conservation and reuse programs,” the committee recommended in its 1995 report. They add that public education campaigns must promote conservation.

“The challenge for decision-makers in Mexico City will be to balance the need for obtaining new sources of water with more careful management of existing sources,” the committee concluded.

Meg Rudolph
Geotimes contributing writer

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