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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easily compress when fluid is withdrawn.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
