The Metropolitan Area of the Valley of Mexico is built on what was once Lake Texcoco, home of the Aztec capital Tenochtitlan. Also called the Greater Mexico City, the crowded metropolis encompasses Mexico City and other surrounding areas. It obtains more than three-quarters of its drinking water from beneath the ground. But water extraction in the region dates back several centuries, beginning during colonial Mexico.
This long-term pumping pushed the groundwater further underground, drying the lake bed so much that clay sheets began to crack and compress. With the earth repacking itself more tightly, the ground started to shrink and subside.
Authorities first noticed that Greater Mexico City was sinking in 1900 when the metro was sliding at a rate of around 3.5 inches a year. When the government placed a cap on water extraction in the late 1950s, it was sinking 11 inches a year.
The cap slowed the rate of sinking back to 3.5 inches a year but failed to eradicate the problem. In the years since, land subsidence worsened as the metro’s population ballooned to 21 million and urbanization increased exponentially.
The study, which was published March in the Journal of Geophysical Research: Solid Earth, showed that parts of the city, including the historic downtown in Mexico City, are now sinking up to 16 inches a year. In the underdeveloped northeast, the ground is dipping 20 inches a year. If the northeast were to become more industrialized, land subsidence would get even worse.
The aquifer system beneath the metro was 17 percent compacted as of 2020. After analyzing more than a century of data, including recent satellite information, researchers predicted that the aquifer system would compact further to 30 percent, causing the metro to sink almost 100 feet in the next century.
The lost elevation is irreversible, leaving the region vulnerable to plenty of disasters. For one, subsidence makes the metro more prone to heavy flooding, which, in turn, can cause water shortages since floods can pollute the groundwater. Subsidence in the metro also fractures the ground, which previously damaged buildings, historical sites, sewers, and gas and water lines. This fracturing can also open up the earth to contaminated surface water, which can further restrict access to potable water.
“The stage is set for a dual water and subsidence crisis if no drastic water management actions are implemented,” the researchers concluded in their paper.
What is land subsidence?
According to the U.S. Geological Survey, land subsidence occurs when large amounts of groundwater are withdrawn from certain types of rocks, such as fine-grained sediments. Rocks compact because groundwater is partly responsible for holding the ground up. When sufficient amounts of groundwater is withdrawn, rocks fall in on itself, causing the ground to sink.
Land subsidence affects more than 17,000 square miles in the U.S., or roughly the size of New Hampshire and Vermont combined. More than 80 percent of these sank due to the exploitation of groundwater, as well as the increased development of land and water resources.
California experiences the greatest losses in elevation among 45 states affected by land subsidence. In the San Joaquin Valley, some areas sank by as much as 28 feet since record-keeping began. Water pumping is intensive in the region, which is one of the most productive agricultural lands in the country. Over time, over-pumping dramatically reduced groundwater levels, drying out the aquifer system and causing it to compact.
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