1. Cutzamala: stress at the source

We set off before dawn to the Cutzamala reservoir system, the biggest single source of water for Mexico City. Located 120km from the Federal District, we have to drive through urban sprawl for several hours before the morning gloom starts to lift and we suddenly dip down into a verdant valley of pink and purple cosmos sunflowers, grassy slopes and fir forests.

This is the land of the Mazahua, one of Mexico’s 62 main indigenous groups. Since the arrival of the first Spanish, they have seen their territory and resources steadily eroded, initially in the name of “civilisation”, now “development”. Today, this means being forced to share their rivers, streams and springs with one of the world’s thirstiest cities.

Where Mexico City gets its water

The first dam – Villa Victoria – was constructed in 1962 as a hydropower plant, but its role changed a decade or so later, when urban planners realised water was more valuable than electricity. Today, the energy generated by the dam is used to pump a small lake each day from sea level up 1,100 metres, above the highest point in England.

It is both a military and an engineering operation. Underscoring the strategic importance, an army base sits next to Los Berros water treatment plant and pumping station at Cutzamala, which is surrounded by high walls, barbed wire and guard posts. The owners – the National Water Commission (better known as Conagua) – refuse us permission to visit, despite repeated requests in advance.

Instead, the Mazahua guide us around the perimeter, then drive us along the channel that takes the water from the Victoria reservoir to the purification plant. It is mostly covered with concrete slabs, but there are gaps. At one point, an unmarked water tanker fills up.

“We feel invaded,” says Manuel Araujo, a member of the Indigenous Frente Mazahua. “We used to live surrounded by nature, now we are surrounded by pylons and barbed wire.”

They have fought back. Most recently, several dozen Mazahua occupied the site of the chlorinisation plant for 15 days to demand potable water in every home in the community. Among them was Ofelia Lorenzo, who gets water just one day a week from a narrow hose that runs from below the ground to her garden. The rest of the time, she has to take a bucket down to a stream further down the valley, so she can bathe and wash her clothes. “It bothers me that they take water from here and I get nothing back. There is not even enough water in my home,” she says.

Lorenzo is a member of the Zapatista Army of Mazahua Women in Defence of Water, which is spearheading the public protests of their community. Dressing in traditional robes and posing with fake guns, their primary goal is to raise awareness in Mexico City about the problems faced by communities near the source.

“We noticed that the government didn’t pay much attention to the men, so we decided to join the struggle,” she says. “I joined the group in 2003 because many bad things were happening to our rivers. Our crops have been affected. There aren’t as many fish as before. Because they took the water from underground, the land is dry. It is all the fault of the Cutzamala system. Now, we’re asking the government to pay us back for what we’ve lost. We’re not fighting, we are just defending our rights.”

Water and land are at the centre of indigenous concerns in Mexico – and not just at Cutzamala. The Zapatista movement, which mounted an uprising against the state in the mid-1990s, has also joined in various “Defence of Water” campaigns, including water-related protests by the Xoxocotla in Morelos State, and the Yaqui and O’odham in Sonora.

They have won concessions. At Cutzamala, the government has built the Mazahua a community centre and fish farms, paved many roads and provided piped potable water to some homes. But leaders still feel they are being short-changed as more of their land is threatened.

Another demonstration is scheduled soon against the planned expansion of the Cutzamala system, which currently provides about 30% of Mexico City’s water. Officials want this share to increase so they can replenish the central city’s aquifers, which have been depleted to alarming levels. This will mean tapping more distant rivers – and quite possibly more tension with other indigenous groups.

2. Sacmex control centre: prime pumping

The stress surrounding our drop of water starts at the source. But there is more to come, not least the economic strain imposed by the long journey from Cutzamala to the Federal District.

From Los Berros potabilisation plant, it is pumped up to the highest point of the system – a 2,701-metre high oscillation tower. From there, it flows by gravity through one of three 235cm-diameter white pipes that take it more than 100km into the city. This is no easy task: the pipes and tunnels are 30 years-old and often need maintenance. Earthquakes, frequent in this region, can tilt them out of alignment – or wreck the system completely, as happened with the huge temblor in 1985.

Even when the ground is still, supply cuts are frequent and charges are high, particularly the electricity bill for lifting each litre of water out from the Cutzamala reservoir system. “Cutzamala water is probably the most expensive on the planet,” says Manuel Reyes, head of supply at the Mexico City Water Department, better known as Sacmex. “This is definitely the biggest hydro-engineering challenge in Latin America, maybe the world.”

Into the city

In the centre of the city, the immensity of the task becomes apparent at the water department’s control centre. From the top-floor windows, you can see distant volcanos and the grey expanse of one of the planet’s greatest and thirstiest urban sprawls. The water system is mostly hidden below the skyscrapers and shantytowns. But, with the help of maps and a bank of screens, engineers explain the route each drop takes once it enters the city.

First, it passes through a tunnel under the western Sierra de las Cruces; then it goes to holding tanks near the Cárcamo de Dolores – home to the once-underwater mural Agua, el Origen de la Vida (Water, Source of Life) by the artist Diego Rivera; then on to a second chlorinisation plant to remove toxins (each year, the city uses 20,500 tonnes of chlorine at a cost of 60m pesos); and then aqueducts and pipes – approximately 2,000km in the primary network of trunk pipes, and a further 12,000km of smaller pipes within each district.

The engineers tell me Mexico City has the greatest demand for water of any city in the world – 300 litres for each of the 8.8m inhabitants, plus millions of others who work here during the day. Quenching that thirst is a demanding job, not least because close to 40% of the water in the system is lost through leaks.

“There are huge problems,” says Ramón Aguirre, head of the Mexico City Water Department. “The city is overpopulated, the underground water quality is poor, and in some areas we are having to pump water up hills of 300 metres altitude inside the city. This is a huge challenge.”

About 70% of the city has fewer than 12 hours of running water per day. In the hardest-hit areas, 18% of the population have to wait several days for just an hour or two of supply. The situation gets worse in periods of drought. Without major intervention, the long-term trends inspire worry.

“Our best information suggests that in 40 to 50 years’ time, there will be very severe problems to take the water from underground. It will get worse each year until then, so we cannot afford to wait,” says Aguirre, adding that the key to solving the problem is to fix leaks and increase supply from Cutzamala and beyond.

Other engineers say the system is a source of constant tension. “It’s exhausting. There is a lot of stress. You see the numbers sometimes and know you can’t deal with them,” says Reyes, who is in charge of the 2.4bn pesos (£94m) annual budget to provide water to the city’s taps. His department is understaffed, underfunded and overworked. He and other workers often leave the office after 10pm: “It’s not good for the family – it’s not just me. I’m sure there are many divorces in this building because of the long hours and high stress.”

The question of how to fix the system is also causing political schisms. Mexico’s centre-right ruling coalition believes privatisation is the only way to finance the necessary upgrades. It has proposed a bill to amend the General Water Act that would allow private firms to take over the supply system.

Opponents say privatisation would raise costs without any guarantee of higher quality. They say the state must do more to meet the promise enshrined in the Constitution, of “safe, acceptable and affordable water”.

In March, protesters marched on the headquarters of Conagua to register their anger at privatisation plans. “¡El agua es nuestra, carajo!” (“The water is ours, dammit!”) exclaimed one poster they attached to the bars of the building. Faced by such public unease, the bill has been put on hold, but the battle is far from over.

While this dispute hots up, the system continues to deteriorate. After 29 years working in the government utility, Reyes says the immediate prospects are disturbing.

“If we don’t get more resources, we face a crisis. This isn’t pessimism, this is realism. Although we are now improving some parts of the system, other parts are getting worse because we don’t have the budget and staff to maintain the system,” he says. Asked if it is drought that he worries about, the engineer shakes his head: “No, what I fear is conflict between communities.”

4. Hidalgo: black water blues

Our drop of water is now unwanted. Once used – whether for drinking, cooking, diluting, cleansing, quenching thirst and fire, nourishing vegetation, or flushing human waste – it is suddenly unwelcome in the city. And so begins the journey out, which is almost as long and expensive as the route in.

Draining away across the concrete floor of Salgado’s home into a gutter, the drop enters a waste-pipe, then a district sewer, and from there flows into one of the three trunk channels that take Mexico City’s waste out into the surrounding states.

Where Mexico City’s sewage ends up

In most capitals, discharging waste is far easier and cheaper than providing clean water, but Mexico City seems determined to make life difficult for itself. Surrounded by mountains and having drained almost all of its original lakes and rivers, there is no natural exit. Instead, the city relies on a 11,000km system of pumping stations and concrete sewers, which are notoriously prone to blockages and floods.

Julio César Cu Cámara knows the dirty side of Mexico City’s water system more intimately than anyone. For the past 33 years, he has been been employed as a sewage diver whose principle task is to clear drains. This means swimming among human filth, as well as electronic waste, industrial pollution, car parts, dead horses and household furniture. Cámara guesses that he finds two or three human bodies a year. It must be one of the least glamorous jobs in the world, but the irrepressibly sanguine diver says the occupation has given him a privileged view of the strengths and weaknesses of his city. “There have been many improvements,” he says. “The pipes have been changed and modernised. We have upgraded our system to deal with the growing volume of garbage.”

But apart from the hardware investment, he sees a fundamental flaw in the way Mexico City deals with its waste. “We’re a poorly educated country. We don’t have the culture to separate our trash. We just discard stuff without thinking. We don’t recycle water. In the 33 years I have done this job, I haven’t seen much change in that sense. In this country, we don’t care about things until we lack them.”

Once flowing freely, these rivers of waste do not just run underground as in most cities. Instead, because the sediment has built up over such a long time, and the land around them has sunk so dramatically, they now flow more than five metres above houses and roads – a disaster waiting to happen each time there is a downpour. In 2010, the last time the canal’s banks broke, a flood of fetid sludge engulfed hundreds of homes in the northwest of the city. Residents were forced to flee to the second floor as their kitchens and living rooms were subsumed in noxious effluvia.

The government’s response was an even bigger engineering project than anything that came before. In the worst hit-region in the Valle de Chalco, Conagua is now building a new drainage system for aguas negras (black waters, or raw sewage) that includes four of the world’s most powerful submersible pumps (each requiring 122 tonnes of diesel a day), several vast lumbrera (intake pipes), and a new five-metre-diameter tunnel to divert floodwaters. This will eventually be linked to the Emission Oriente, a huge new discharge pipe that is still under construction but has already cost 22m pesos, more than twice its initially projected budget.

Even this, however is dwarfed by the final destination for our drop of water: a giant new waste treatment plant – said to be the world’s largest – which is almost completed and should soon process the entire output of sewage from Mexico City. The facility, built by a consortium involving Mexico’s richest man, Carlos Slim, fills a hillside in Atotonilco about 100km north of the city in the state of Hidalgo.

The plant, in the verdant Tula Valley, is an impressive sight, but – as at the start of our journey – this mega-project has stirred up anger among local indigenous communities: in this case the Hñähñü, who say their territory is being used as a dumping ground.

For more than a century, the Hñähñü have had to live with the filthy discharge that no one else would accept. Their land is home to the biggest cesspools and waste channels in Mexico. This is part of their life. Sewage flushed from millions of city homes has greened their valleys, irrigated and fertilised their crops, but also contaminated their rivers.

In the nearby town of Santiago de Acayutlán, children play in a crystalline spring – splashing and diving into the limpid waters, while their parents look on approvingly. On the road outside, cowboys trot by on their horses and a Mariachi band wander down the street towards their next performance. It is a bucolic scene, but Sabino Juarez – a poet, politician and political activist – takes me to the small finger of land where the local spring meets the murky waters from Mexico City. “When we were kids, this was crystal clear. You could fish and swim in this river. Now look,” he says, pointing to the stinking, yellowing flow. “All the fish are gone.”

Tracking the flow of sewage, we drive to the countryside village of Endhó, where the stench is nauseatingly strong. Here the effluent is churned by a weir, which makes the chemicals and waste bubble up into little icebergs of white foam. “Endhó is the toilet of Mexico City. It’s the most polluted place on the planet,” Juarez proclaims. “What you see here is a monument to corruption, cynicism, immorality and incompetence.”

His community has had to deal with the city’s black waters for more than 100 years – a sign, they say, of the prejudice towards indigenous communities. Nearby lands are also home to an oil refinery and other dirty industries that more prosperous states refuse to accept.

As the waters have become more polluted with heavy metals and other toxins, so the crops have been affected. Locals say it is no longer possible to grow tomatoes here, while yields of wheat and chillies have declined. The health impacts go far beyond the local community.

“This is water from Mexico City. It’s dirty. It’s poison,” Juarez says of the irrigation channels. “There is a vicious cycle of contamination. The city sends us pollution and we send it back in the food. It’s a big health problem.”

He claims that scientists at Chapingo University have identified toxins in the water that cause leukaemia, diabetes and cancer. According to researchers at the Universidad Nacional Autónoma, Mexico City has more cases of gastrointestinal infections from water consumption than any other major city. Cholera has also broken out from time to time, most recently in the 1990s.

Another activist, Karen Taxilaga Duarte, looks at the stinking streams and laughs bitterly. “You in Mexico City have this for lunch. It all goes on your lettuce.”

Despite the black humour, the residents are not resigned to their fate. They have tried to fight back, but the toll has been heavy. In 2008, Taxilaga and Juarez were among several hundred protesters demonstrating in La Cruz against the toxins in the water. They blocked the streets and chanted, “No queremos las aguas negras” (“We don’t want the sewage”) and “Traten sus aguas tóxicas en su casa” (“Treat your sewage at home”).

The riot police moved in. Taxilaga says one of them hit her in the face with a rifle butt, shattering the retina. To demonstrate, she removes her false eye. “We’re still fighting. I’m a mother and I don’t want my kids to have this foul water going through our land.”

Juarez claims he also suffered as a result of the protest. “I was taken away and beaten. They used electric shocks and a very Mexican form of torture called tehuacanazo – putting chillies in a can of soda, shaking it up, closing my mouth and then releasing it up my nose.”

Today, the focus of their fight is the waste-water facility. Sacmex officials say the plant will remove many of the toxins that now plague the area. Eventually, they plan to let the treated water filter down into local aquifers which they will then use to re-supply the city. But few details have been released about how the consortium will get a return on their investment, prompting anxiety among the Hñähñü.

Their main complaint is that they have not been consulted. While they have long wanted water that is free of contamination, they fear they could also lose the free, nutrient-rich sewage upon which they depend to water and nourish their crops.

“We’ve been told that Carlos Slim will give us some of the treated water, and sell the rest to Mexico City for irrigation of parks and gardens,” Juarez says. “We fear he’ll put a high price on it, as he does for his telephone service. So it means the small land-owners like us will struggle to pay and we’ll see more inequality. This is categorically not the solution to our problems.”

The authorities contest this. However, one thing is certain: the journey of our drop of water ends with as much controversy and stress as it started.

5. Valle de Chalco: Elemental returns

Having been diverted, treated, pumped, piped, tapped, trucked, flushed and discharged, our drop has followed a very man-made path. But there are other possible routes. Despite the efforts of Conagua’s best engineers, water has never completely stopped flowing naturally to where it historically belongs. Instead of resisting this, a growing number of conservationists and hydro-engineers believe it could be the solution to Mexico City’s problems.

The focus of their attention is the Valle de Chalco, the deepest part of the Mexico City watershed, which sits just outside the eastern boundary of the Federal District. Little more than 100 years ago, this was home to the last of the great lakes on the plain. Residents used to commute from here to the city by steamboat, until the waters were drained to create farmland and roads in the late 19th century.

Chalco, a suburban lake

But in the past few decades, the Lago de Chalco has started to return. Soon after the 1985 earthquake, the authorities drilled 13 wells nearby that emptied aquifers, caused subsidence and then – as the surface fell – created a giant puddle that just kept growing. After almost a century, the water had found its way back.

This is still not officially recognised on maps, but the lake now stretches over 554 hectares. Migrating cranes and storks rest on the wetlands here. Old men fish; young children play in the water. There are even sailboats. It is hard to believe that a short distance away from this body of water, on the other side of the Sierra Santa Catarina, more than a million people are suffering shortages in Iztapalapa.

“This should be the heart of the solution,” said Elena Burns, an activist with the Water for People, Water for Life campaign, as she looks out across the reeds and marshes. “Lakes are a really cheap way of dealing with this problem. If we made this lake eight metres deep, we’d have enough water for 1.5m people.”

Burns, a naturalised Mexican citizen, is co-founder of the Watershed Commission of the south-eastern Mexico Valley water basin, which is exploring the possibility of using natural catchment areas to solve the city’s water problems. It includes scientists, environmentalists and the two main government organisations dealing with water issues, Conagua and Sacmex.

The Watershed Commission’s management plan estimates that many problems in this area – groundwater overexploitation, subsidence, sewage flooding, lack of access and poor quality – could be solved by deepening the lake and building a waste treatment plant nearby. This would cost 7bn pesos (£275m), less than a third of the money the government has already spent on wastewater tunnels.

The president of the commission, Oscar Monroy, who is one of Mexico’s leading authorities on waste-water management, says this should be part of a city-wide scheme to collect rainwater rather than have it pumped from ever-more distant sources.

Monroy estimates that harvesting an area of 20sq km would be enough to provide for the city’s needs. As well as lakes and cleaned-up rivers, this could mean storing the rainwater collected by the roofs of shopping malls, schools and condominiums. “It’s not alien technology,” he says. “It can be done with existing technology. But it will only work if the government wants to, and right now they don’t.”

Elsewhere, several pilot projects are underway. The Metropolitan Autonomous University has pioneered a rainfall catchment project in schools in the Santa Catarina district of Iztapalapa.

The idea is simple: grated gutters around the playground channel rain through a filter into an underground storage tank. Half of the water is then injected underground to replenish aquifers, while half is pumped up to a tank on the roof of the school for use in flushing toilets and cleaning classrooms.

“There’s no secret to this. It doesn’t require much energy, the expense is modest and it works. For two years, the school hasn’t need to spend a single peso on water tankers,” says Juan José Santibañez, of the Metropolitan Autonomous University of Iztapalapa. “In Germany and Japan, there are systems for harvesting rainwater for agriculture, but we think we are the first to do so on this scale in schools. We have to be innovators because our crisis is so bad.”

With 25 schools already using the system, Santibañez is enthusiastic about its potential to ease shortages and replenish aquifers. But he says the city authorities baulk at the the set-up costs of 150,000 pesos (£5,850) per school.

The head of Sacmex is unimpressed: “This sounds intelligent, but it’s not. It’s cheaper to bring water from outside than to capture rainwater. There is not one city in the world that uses rainwater collection,” says Ramón Aguirre. “Where do we keep it and supply it? It needs to be huge so there is enough for months when there is no rain. It will need to be cleaned if it comes off the streets. Would you drink from a puddle? No, it is very polluted with hormones, drugs, vitamins and other emergent contaminants.”

Burns believes the authorities are too wedded to mega-projects to accept low-cost, sustainable solutions like turning Chalco Lake into a reservoir. “Ramón thinks this is a tiny project; he’s a wells and dams-kind-of guy. He likes spending money on expensive projects like long tunnels and deep wells that don’t get us anywhere near sustainability.”

She sounds idealistic – but there is logic to her argument that appeals not just to conservationists and academics. Marco Alfredo, president of the Mexican Association of Hydro-engineers, also advocates a return to the city’s lacustrine origins.

“Mexico City’s situation is chaotic and absurd. We could have natural pure water, but for hundreds of years we have been draining it away so we have created an artificial scarcity,” he says “This is not an engineering problem: we have the expertise and the experience. It is also not a problem of economics: we have the financial resources to do what needs to be done. It’s a problem of governance.”

His organisation – whose symbol is Tlaloc, the Aztec god of water, rain and lightning – plans to mark its 50th anniversary by drawing up proposals for reform in a “water declaration”, which they will submit to the Mexican president, Enrique Peña Nieto next year.

For Alfredo, the key is to work with, rather than against, the element that first attracted settlers to this high-altitude lakeland. “Water is not just remarkable, it’s miraculous,” he says. “It has a memory, an intelligence, it’s extremely strong. And it will always return. No matter whether it takes five, 50 or 500 years. It will come back.”