This fragile wetland is dying. Tour boats could be its unlikely savior.

Pollution is choking Xochimilco, Mexico City’s UNESCO World Heritage site. One scientist is harnessing the power of tiny bubbles to fix it.

Helmed by gondoliers, colorful tour boats known as trajineras, wend along a sun-dappled canal. They drift past serenading mariachi musicians in their own boats and vendors hawking snacks from wooden canoes.

This carnival-like scene plays out most weekends in Xochimilco (pronounced “zo-chee-MILK-oh”)—a UNESCO World Heritage site and a popular destination for tourists in southern Mexico City. Eleven percent of the country’s biodiversity can be found in this 6,400-acre wetland threaded by 105-miles of pre-Hispanic canals. It’s a fact few of the approximately two million tourists and chilangos (slang for Mexico City residents) who visit in a normal year know before boarding trajineras for an afternoon cruise.

But this fragile ecosystem faces an uncertain future, as pollution built up over decades squeezes life from these waterways, threatening a living heritage in the process.

In a surprising twist, Xochimilco’s trajinera tourists could be the wetland’s unlikely saviors, if a homegrown plan to use these wooden vessels to purify the canals’ murkiest depths, takes off.

An ecosystem in peril

Xochimilco’s wetland is regarded as one of the last living links to the Aztecs, thanks to the reserve’s remarkable floating farms known as chinampas. Humans built these islands—5,475 acres of them—from the nutrient-rich soil in the canal beds, making the chinampas one of the most productive types of agriculture in the world. In Mexico, they’ve been feeding the capital city for a millennium.

Today, some 55 tons of chinampa-grown vegetables—from beets to endemic crops such as talamayota squash—fill the trestle tables daily in Mexico City’s neighborhood markets and the sprawling wholesale supplier Central de Abastos de la Ciudad.

“Xochimilco has everything. It gives food and water, regulates the capital’s weather and mitigates flooding, provides work, and is rooted in tradition,” says Claudia Alejandra Ponce de León, professor of environmental sciences at the National Autonomous University of Mexico.

Yet the water used to grow these fruits and vegetables has been full of pathogens for years, says Refugio Rodriguez Vázquez, a clean water activist and biotechnology professor at Mexico’s National Polytechnic Institute, who began studying the wetland in 2016.

(This vibrant sanctuary show why wetlands are important—and the challenges they face.)

Vázquez says a toxic cocktail of agrochemicals from both land-based and floating farm runoff and wastewater is to blame. Wastewater is discharged from El Cerro de la Estrella and three smaller treatment plants within a 12-mile radius into the canals at an alarming rate of 2,000 liters per second, she says. It’s enough to fill an 8-foot deep pool measuring 2.5 feet by 2 feet every second.

Exacerbating the problem, nitrogen and phosphorus from these sources proliferate algae blooms made of lenteja de agua (water lentils) and lirio acuatico (water lily)—the latter, introduced in the 1980s by Mexico’s then president “for decoration,” has proven to be a problematic invasive species.

These blooms carpet the surface of the water, blocking out sun and oxygen. “When [the algae] dies, it settles at the bottom of the canal as sediment. This breeding ground for methane-producing bacteria then releases greenhouse gases back into the atmosphere,” Vázquez says.

“According to the Canadian government, our polluted canals are making migratory birds like the Mexican duck and the great blue heron really sick,” adds Armando Tovar Garza, a biologist with Humedalia, a local conservation organization. Ecological degradation has already driven endemic species like the axolotl salamander to the brink of extinction in Xochimilco. “The wetlands are important to preserve Mexico City’s quality of life as we know it,” says Garza.

(Can fame save Mexico City’s beloved axolotl salamander?)

Xochimilco’s problems began 40 years ago, when authorities bled its natural springs dry to provision Mexico City’s ballooning population, then refilled the canals with treated water. Compounding the toxic mix are illicit discharges of sewage from the borough’s informal settlements.

“This land has sunk six meters in the last two decades because of water extraction from aquifers to Mexico City,” estimates Luis Martínez, a third-generation chinampero, who cultivates 15 varieties of vegetables on his 2.5-acre plot in San Gregorio. “We desperately need to improve the quality of the water.”

A modern solution for an ancient waterway

If all goes well, an ambitious plan to harness the power of tiny bubbles—delivered by tourist boats—could restore the wetland’s canals and have implications beyond Xochimilco.

“Nanobubbles can penetrate this sludgy sediment,” Vázquez says. Approximately 2,500 times smaller than a grain of table salt, these microscopic air pockets literally breathe life into oxygen-starved waters, “staying up to six months … in the right conditions,” Vázquez says.

Since they were discovered in the 1990s, nanobubbles have been used to remove pollutants in many industries, including bio-pharma and food processing. Because nanobubbles have no natural buoyancy, they remain underwater, where each tiny, negatively charged bubble is attracted to positively charged pollutants and harmful toxins. This union causes the nanobubbles to release hydroxyl radicals, which can extinguish pathogens and slowly break down the cell walls of algae.

To deliver the bubbles, Vázquez has devised a guerrilla-style setup of pipes and solar panels that harnesses the wetland’s existing tourism infrastructure—the trajineras, all 1,103 of them.

These motorless vessels (accommodating up to 20 people) provide an essential income stream to an entire community of musicians, floating chefs, and remeros like José Gabriel Gonzales Franco. The fifth-generation gondolier is a descendant of the Xochimilcas who lived here long before Aztecs arrived. Remeros still make barge poles as their ancestors did, using wood from the endemic oyamel tree. “We are property of this land,” says Franco. “We [remeros] keep the tradition here.”

(Ecotourism could help the ‘Amazon of North America’ recover.)

Thus far, Vázquez has only tested the prototypes in her canal-side workshop and floating lab (an upcycled trajinera christened “little bird”). However, extensive computer models using a controlled study in a pool, plus tons of supporting research, show the application was successful in cleaning polluted water.

A stifling hot Saturday in October 2021 marked the first time the setup was installed on a trajinera with tourists aboard. Two 50-watt solar panels were tacked onto the curved roof of the boat owned by Carlos Díaz, whose family has been conveying visitors along Cuemanco and La Cruz canals from Flores Nativitas Pier, since 1960.

A tangle of wires connects the panels to a small control box, which creates photovoltaic energy. This powers a pump in the submerged tubes, which in turn is designed to extract water from the canal, redistributing it as thousands of bubbles the size of a COVID-19 particle. Vázquez says one aspect of her long-term plan is to communicate the project’s message in a series of infographics attached under the roof of each boat.

The following week, Vázquez and her team of doctoral students took the setup out into the field to ensure that all the moving parts work together. This time, they outfitted a fleet of tour boats carrying a group of academics curious to see her invention in action. The flotilla departed from Puenta de Urrutia Pier, gateway to some of Xochimilco’s famous floating farms.

Beyond Xochimilco

Thankfully, the setup worked—a small, but mighty victory in what’s set to be a challenging road ahead for Vázquez, who needs to patent her homespun technology and get the local government on board.

It hasn’t been possible to draw definitive data from the trial run yet, mainly because of its very small scale. Additionally, it will likely take several months for the benefits of the oxygenated waters to be tangible, due to the sheer volume of algae blooms. Still, hopes are high that the pump will succeed in achieving Vázquez’s goal of cleaning up a third of Xochimilco’s canal network.

Although there’s still a long way to go, the wider academic community in Mexico and beyond is already eyeing other possible applications for Vázquez’s invention. Jordi Morató Farreras, coordinator of the UNESCO Chair on Sustainability at Catalonia’s Polytechnic University, says he can imagine harnessing the technology in other protected destinations.

Oxygenating a lake in Colombia and decontaminating agricultural soil closer to home in the state of Puebla, are just a few of the ideas that many are floating.

That would be a point of pride for locals. “I first went to Xochimilco eight years ago and was amazed by the amount of trajineras, the color of the water, and how much it was polluted,” says local primary school teacher Gabriela Vianey aboard the trajinera’s trial run. “I had no knowledge of [Vázquez’s] project until today. I hope it [Xochimilco] will be around for my children’s children to see.”

Sarah Freeman is a United Kingdom-based journalist focusing on conservation-minded travel. Follow her on Instagram.

Read This Next

Why are these orcas killing sharks and removing their livers?
How to eat in 6 of the world’s most stunning places
Cliff art reveals the majesty of the Amazon’s aquatic realm

Go Further

Subscriber Exclusive Content

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet