How a city that floods is running out of water

One of the world’s largest cities, Mexico City is home to about 21 million people – rising to 27 million if you include the surrounding areas. About 20% of Mexico’s population lives there. By the year 2030, the authorities estimate that the population will grow to 30 million people.

Among other challenges, such a large population puts a costly strain on Mexico City’s water supplies. In fact, the parts of Mexico City’s infrastructure that supply water are crumbling. Its natural water reserves are also at risk; if trends continue, they are expected to dry out as soon as in 30 years. With so many people affected, this means one of the world’s largest water crises is in the making on the doorstep of the US.

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Located more than 2,000m above sea level, the city is subject to heavy rainfall. The wet season between June and September, in particular, brings frequent flash floods.

Flooding events are predicted to intensify as climate change has been linked to increasingly erratic weather patterns. This influx of water brings many problems. Traffic stops as the water floods roads. Inadequate infrastructure and flash floods can cause the sewage system to overflow. And sewage pipes can burst, filling houses – like this one – with dirty water.

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Burst pipes aren’t the only reason that Mexico City’s sewage overflows. Rubbish disposed of on the streets often clogs pipe drains, backing up the system. That can have serious consequences quickly, since Mexico City produces about 40,000 litres of sewage every second.

This means that workers frequently have to fix the water drainage system, (Emisor Central, pictured here). This 6m-wide tunnel has been overworked and corroded, reducing its capacity to drain sewage water efficiently.

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Despite flooding events and heavy rainfall, the city is facing a water shortage. Much of this is because of the inefficient and ageing infrastructure of Mexico City’s water networks: some 40% of the water is lost.

The poorest residents are the worst affected. Some are not connected to the city’s water network and must rely on buying it per litre. This is the most expensive way to access water, explains Arnoldo Matus Kramer, the city’s chief resilience officer. “Therefore, we need to revise [how people get water] and understand that access to water is a human right.”

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As a result, many of the city’s inhabitants have an interrupted water supply, perhaps only being able to turn on the tap and get water twice a week, says Kramer. While many houses store water tanks to overcome this lack of supply, these are expensive. And they add another burden: pollution, as lorries are needed to transport water across the city.

For instance, the Xochimilco community in the city’s southernmost district gets new water twice a week on a lorry. Donkeys then carry it the rest of the way to their homes.

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Even farmers no longer have adequate access to the water they need to grow and wash crops. Intense droughts have dried up the water holes and wells they once used. Farmers like those shown here wash their vegetables in treated water canals, that once flowed with natural spring water.

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Given Mexico City’s original geography, its lack of water may seem strange. The city was built on an island surrounded by a large natural lake basin. But when the Spanish colonised Mexico in the 1500s, they dried out the lake to build a bigger city.

This means that deep underground, Mexico City has fresh water reservoirs – which the city still depends on for about 40% of its water. In theory these natural aquifers should be replenished by rainfall. It can seep through water containment holes like these pictured here in this deforested patch of land.

But the shortage of water in the city means these natural water reserves are being emptied at a rate faster than they can be filled, especially during months of prolonged drought in the dry season.

“We are exploiting our local aquifers at a very high rate,” says Kramer. “At the same time, we haven’t invested enough resources to have a robust monitoring system. So there’s a lot of uncertainty how the local aquifers work.”

His team suggests that the aquifers could be depleted in 30 to 50 years, if current exploitation trends continue.

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The over-exploitation of its water reserve also risks increasing seismic activity and is causing subsidence within the city. Some parts of Mexico City are sinking by about 30cm (12inches) per year, the result of the aquifer not being ‘pumped up’ by water enough to support the ground above. This type of sinking has been linked to the earthquake in September 2017 that killed more than 200 people and caused many buildings to collapse.

The subsidence also affects infrastructure above and below ground – damaging the very pipes that bring water to people, as well as removing their waste.

Illegal buildings, such as these floating houses (chinampas) which can be found south of Mexico City, also worsen the water quality of the canals they occupy. The city’s water commission now has to frequently refill the canals with treated water or even recycled sewage water.

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From flooding to droughts and rising temperatures, much of Mexico’s extreme weather has been linked to climate change. But while more people are becoming aware of this connection, says Kramer, they are less aware of the diminishing water reserves in the aquifers and how this is causing the city to sink.

If the remaining water in the lake basin is lost, it will have another ripple effect. Research shows it will reduce humidity within the city.

Higher temperatures also will cause more water to disappear – while at the same time making the parched city ever thirstier.

It’s a vicious cycle with no easy solution in sight. As a first step, Kramer and his department are developing a “robust monitoring system” and identifying the key goals they need to overcome the city’s many problems. Better maintenance of water pipes for instance, is but one of many solutions that would ease the burden of Mexico City’s dwindling water supply.

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Another solution is being built in the form of a new underground waste-water tunnel, the Emission Oriente, one of the world's largest drainage tunnels. Although originally forecast to be completed in 2012, it is finally expected to be in use at some point in 2018. It spans almost 38 miles (62km) and is 200m deep below the ground.

Once it is in use, it will go some way towards easing the pressure from the decrepit Emisor Central (pictured in slide 3) – while also significantly increasing the amount of waste water the city can discharge.

(Photo credit: Carlos Cazalis/INSTITUTE)