Why is Mexico City sinking?

Almost the entire Mexico City was built on the site of the former Lake Texcoco. Many historical parts of Mexico City, especially the Historic Center, are based on the capital of the Aztec Empire, the city of Tenochtitlan.

Lake Texcoco was a natural body of water replenished primarily by rainfall and inflowing rivers and streams of the Valley of Mexico. The lake also had underground springs that contributed to its replenishment.

Evaporation and filtration through the ground played essential roles in regulating the water levels of the lake. Sunlight caused evaporation, while water also drained into underground aquifers, replenishing groundwater.

Lake Texcoco served as a freshwater source for the valley inhabitants.

With the arrival of the Aztecs, water management systems, including canals, dams, and artificial pathways, were developed to harness water, control flooding, and maintain the viability of cities on the lake’s islands.

Periodically, Lake Texcoco flooded coastal lands during seasonal rains. This natural fluctuation in water levels led the Aztecs to build dams and canals on the lake to prevent flooding and support their settlements.

The Aztecs’ construction of dams and embankments was crucial.

Dams enabled the creation of artificial pathways, ensuring the viability of their settlements on the islands, which became integral to their lifestyle and culture, allowing them to adapt successfully to their environment.

After the fall of Tenochtitlan, the Spaniards destroyed Aztec dams that prevented the islands from flooding. On the ruins of Tenochtitlan, Spaniards began to build a new city, the capital of New Spain – actual Mexico City.

Tenochtitlan was located on one of Lake Texcoco’s islands. During the rainy season, the lake overflowed its banks, flooding the islands. The Spaniards destroyed the Aztecs’ dams, causing the island to flood again.

To control the water level and reduce Lake Texcoco’s size, the Spanish built dams and canals. They redirected the water to other areas or outside the city. This allowed them to gradually reduce the size of Lake Texcoco.

Tenochtitlan had a peak population of about 200,000. The Aztecs expanded Tenochtitlan using floating gardens. However, the Spaniards found the island insufficient and began draining the lake to increase the city’s area.

To prevent the city and surrounding lands from flooding during seasonal rains, the Spanish built dams and embankments around the lake. This protected the newly built capital from flooding and reduced the lake’s volume.

Mexico City stands on unstable clay crust and lava rocks, a combination made worse by extensive paving. Groundwater overdrafts are depleting the aquifers beneath the city, causing it to sink.

Except for a few marshlands and canals in the south, Texcoco and the other lakes are gone. The former lake basins, with no natural water exit, and the deforested lands, which once acted as floodwater sponges, no longer serve as buffers between water and people.

The consequences of draining the lakes have been severe: the area turned semi-arid and now experiences water scarcity. The soft lake sediment makes the city vulnerable to soil liquefaction during earthquakes.

Mexico City faces the ongoing impact of building on an aquifer, a problem since the mid-19th century. Although the underground aquifers are replenished, the recharge rate is only about 50% of the groundwater extraction rate.

Mexico City suffered severe floods in 1555, 1580, 1604, and 1607, leading to a proposal to move the capital in 1630. However, after some reflection, the authorities decided against it, and flooding continued.

During the rainy season, Mexico City filled up like a cup. The city experienced serious floods in 1645, 1674, 1691, 1707, 1714, 1724, 1747, and 1763. One flood was so severe that the entire city remained submerged for 5 years.

By the 20th century, much of Lake Texcoco had been drained, and flooding was no longer a major concern. However, as the city expanded and migrants from other states flocked in, Mexico City’s demand for water surged.

As more water was extracted, subsidence began. The phenomenon was noticed in 1891 in the old part of the city but wasn’t studied until 1925. By 1948, it was proven that subsidence was a result of groundwater depletion.

In 1954, water pumping in the city center was prohibited, and wells were relocated to the north and south. While subsidence in the city center has since stabilized, it continues to pose a problem in most areas of Mexico City.

Mexico City is built on two different geological foundations.

Part of the land beneath Mexico City was fertile volcanic soil, which had exceptional water absorption properties, facilitating the seamless flow of moisture into underground aquifers without disrupting its structural composition.

When developers constructed buildings on the volcanic soil and covered the surface with concrete and asphalt, water was no longer able to penetrate the soil and percolate into the aquifers essential for the city’s water supply.

Other parts of the city are situated on clay, which does not absorb water, instead, it forms layers that trap water between them. When the water is removed, the clay layers crack and collapse, causing them to overlap each other.

With Lake Texcoco no longer available, Mexico City relied on groundwater for drinking water. This groundwater, then and now, is stored in the relatively shallow underground aquifers located beneath the former lake beds.

Groundwater overexploitation over the past century has caused land subsidence of up to almost 10 meters, resulting in damage to buildings, streets, sidewalks, sewers, stormwater drains, and other city infrastructure.

In its search for water, Mexico City turned to clay soil, overlooking the advantages of the beneficial volcanic soil. The city’s foundation, a combination of both geological types, led to uneven and unpredictable subsidence.

This sinking process leads to hazardous cracks and fissures, as well as uneven, undulating streets.

The impact of development

However, not all of Mexico City was built on the lakebed. Southwest of the city lies the region known as the Pedregal, which rests on hardened lava flows. Before the mid-1950s, this area was largely deemed uninhabitable.

The population of Mexico City tripled between 1950 and 1975.

Developers began to see potential in the region, despite its challenging terrain. The area was divided into luxury residential complexes, attracting hundreds of families who quickly built homes and established neighborhoods.

The swift expansion of the southwestern part of Mexico City had unforeseen repercussions. Between the mid-1950s and mid-1980s, the once extensive swath of volcanic rock of 8,000 ha, was engulfed by streets and buildings.

This rapid urbanization erased the unique ecosystem, paving over nearly all permeable surfaces. Consequently, despite months of regular flooding during the rainy season, very little of that water manages to seep underground.

The urban sprawl in Mexico City has effectively sealed off any natural drainage pathways with pavement. As a result, the city’s natural drainage system is hindered, exacerbating flooding challenges during the rainy season.

Modern Mexico City stands on a foundation of silt and groundwater, which are unstable materials. Therefore, when groundwater is extracted from underground reservoirs, ground surface subsidence may occur.

If the draining of Lake Texcoco had been carried out more systematically and completely, it could have created more stable conditions for the city’s construction. However, the incomplete drainage process and continued use of groundwater over time contributed to the soil’s instability and led to surface subsidence.

Today, Mexico City’s subsidence and upheaval problems are related to the use of groundwater resources, poor land and infrastructure management, and natural subsidence processes. These issues represent complex engineering challenges that are currently being addressed in the context of sustainable development and resource management.

Subsidence in Mexico City can cause buildings to lean or buckle, especially older or heavier structures, such as the cathedral and other historical buildings in the city center.

When the ground settles or subsides due to human activities, such as groundwater extraction, or natural processes, such as flooding, buildings can lose support and begin to shift. This can lead to uneven slopes and structural deformations.

To combat these problems, special engineering solutions are required, such as maintaining and restoring building foundations, implementing engineering measures to compensate for slope, and regularly maintaining and repairing buildings to ensure their safety and integrity.

Engineers and scientists are continually exploring ways to improve land and resource management to reduce the impact of subsidence on buildings and infrastructure in Mexico City.

A major issue in Mexico City is its water supply, which heavily relies on underground water resources. When water from these sources is extracted to meet the city’s needs, it can cause land subsidence.

Subsidence occurs because water in the soil layers helps maintain the soil’s structure.

When this water is withdrawn, the soil loses moisture and may begin to collapse or shrink, causing the ground surface to subside. This is especially true for large cities like Mexico City, where water supplies depend on deep groundwater reservoirs.

One of the main challenges in managing Mexico City’s water supply is to find ways to use and manage water resources more efficiently to minimize environmental and infrastructural impacts. This may include developing sustainable water extraction methods, controlling water flow, and employing engineering solutions to reduce subsidence.

Earthquakes may exacerbate Mexico City’s silt soil instability problem.

Soil composed of silt or clay is susceptible to compaction and deformation even due to small earthquakes. This can lead to additional settlement or deformation of buildings, especially those on unstable soil.

Earthquakes can also cause weak earth movements, increasing the tilt or uneven settlement of buildings. It is crucial to consider geological and engineering factors when designing and constructing buildings in earthquake-prone areas to minimize damage risks and ensure occupant safety.

Engineers and scientists are continuously researching methods to improve the earthquake resistance of buildings and infrastructure, including better construction standards, innovative technologies, and more effective methods for preventing earthquake damage.

Using water pipes to bring water from distant sources may be one way to manage water supply problems and prevent additional silt deposits from sinking into Mexico City. This is a practical solution that many cities use to secure their water supply.

Installing water pipelines from distant sources can allow a city to obtain water from other regions where groundwater levels are higher or where there are no problems with the subsidence of ground sediments.

This will reduce dependence on local water resources, which may be subject to scarcity due to lake drainage and land use. However, installing additional water pipes may require significant investment in infrastructure and maintenance.

Installing additional water pipes from distant sources could be an effective strategy for Mexico City. This can help reduce pressure on local water resources and prevent additional soil and infrastructure stability problems.

However, when assessing the costs and benefits of such projects, it is important to consider not only the initial costs of constructing water pipelines but also operating costs, maintenance, and environmental impact.

In Mexico City, ground conditions can vary greatly depending on depth.

At the surface and in the nearest layers of soil (several meters), unstable soils such as silt or clay can often be found, which can be subject to subsidence or deformation under the influence of water and earthquakes.

At greater depths, more stable soils can usually be found. For subway construction, deep tunnels pass through stable layers of rock or stronger types of soil, which ensures the safety of the subway infrastructure.

Subway depth in Mexico City varies in different parts of the city. In central Mexico City, it is about 20-30 m underground. In some areas, the depth can reach 40-50 m or even more, especially in peripheral parts of the city.

Tunnels pass through stable soils at sufficient depth to avoid problems with flooding or impacting former lake sites. All possible geological factors are taken into account to ensure the safety of the subway infrastructure.

Engineers and subway builders take precautions to minimize all possible risks. They use soil stabilization techniques and drainage systems to prevent unwanted impacts on the environment and underground structures.