The ground beneath Mexico City is sinking rapidly, a phenomenon known as land subsidence. This geological crisis results from a complex interaction between the city’s unique foundation and its modern water demands. The sinking has been a documented issue for over a century. In recent decades, certain areas have experienced some of the highest rates of subsidence globally. This continuous sinking has profound implications for the infrastructure and future habitability of the massive urban area.
The Ancient Lakebed: A Susceptible Foundation
The city’s geological vulnerability stems from its location on the former Lake Texcoco basin. The Spanish drained this lake system, where the Aztec capital Tenochtitlán was built, and constructed the modern city over it. The subsoil is not solid bedrock but a thick sequence of highly saturated, compressible sediments.
These lakebed deposits are primarily soft, clay-rich materials interlayered with volcanic ash. The clay has an extremely high natural water content, making the soil highly plastic and elastic. This structure is easily compressed when its internal water is removed. The thickness of these deposits, which can be up to 100 meters deep, explains why the city is so susceptible to sinking.
The Driving Force: Groundwater Depletion
The active mechanism causing the sinking is the rapid and continuous extraction of groundwater from the underlying aquifer. Approximately 70% of the city’s water supply comes from this source, and the extraction rate is significantly faster than the aquifer’s natural recharge rate.
As water is pumped out, the pressure within the pore spaces of the clay layers (pore pressure) decreases. The weight of the overlying infrastructure then causes the clay layers to compact and consolidate under increased stress. This process is irreversible because the fine mineral grains permanently rearrange into a denser configuration. Studies show that the clay layers have compressed by up to 17 percent over the last century. This continuous dewatering means the land surface will continue to sink for decades, even if pumping were to immediately cease.
Measuring the Impact of Subsidence
Subsidence is measured using advanced satellite radar data, revealing highly variable rates across the metropolitan area. In the most severely affected zones, the ground is sinking up to 50 centimeters (nearly 20 inches) per year. Since the 20th century, the city has sunk by 9 to 10 meters in total.
Infrastructure Damage
The unequal sinking rates, known as differential subsidence, cause immense damage to critical infrastructure. Visible effects include visibly tilting buildings, fractured roads, and extensive ground cracking. This differential settling tears apart underground utility lines, leading to thousands of leaks in water and gas systems. A severe consequence is the disruption to the city’s large-scale drainage and sewage systems, which rely on gravity. As the ground drops, pipes flatten and reverse their slope, compromising wastewater removal and increasing the risk of recurrent flooding. Furthermore, the soil compaction alters the ground’s response to seismic waves, potentially increasing building vulnerability during an earthquake.
Efforts to Slow the Sinking
Addressing subsidence requires comprehensive water management strategies to reduce reliance on the aquifer. Regulatory efforts, such as capping wells in the city center, temporarily slowed sinking in those areas. However, the fastest subsidence rates have since shifted to the metropolitan surroundings where pumping continues.
A long-term strategy involves developing alternative water sources to ease the burden on the local aquifer. This includes expensive projects to import water from distant reservoir systems, such as the Cutzamala system. Another technique being explored is artificial aquifer recharge, which involves actively injecting water back into the underground reservoir to help maintain pore pressure.
Infrastructure adaptation is also underway, focusing on renovating the aging water distribution network to reduce significant water loss from leaks. Upgrading to more flexible and resilient piping materials is necessary to withstand the constant stress from the sinking ground. These changes are intended to manage the ongoing geological challenge.