As downpours intensified on 23 April on the outskirts of Bogotá, Colombia, toxic foam from an adjacent river started engulfing the Los Puentes neighbourhood.
Fluffy white froth emerged from the water and was whisked up by the wind until it reached a height of 5 metres and blanketed cars and roads, says restaurant manager MarĂa Morales. Eventually, it crept through the windows of her kitchen, forcing her to close the business.
Though the scenes may resemble a harmless giant bubble bath, the residents know better.
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“If it were normal foam, the kids would love it,” says Maria Chacue, who has had a hard time stopping her 22-month-old son from playing in the toxic chemicals, which include compounds from soaps and detergents. “But it’s not normal. It’s so polluted.”
Environmental officials blame the heavy rains driven by the La Niña weather phenomenon, which have flooded much of Colombia in recent weeks. As the Bojacá and Subachoque rivers converge in the lower river basin near Los Puentes at high speeds and volumes, they churn up the underlying contaminants.
“It’s like detergent in the sink,” says a government official overseeing environmental controls in the region. “If you don’t move it, nothing happens. But if you stir it, it foams up.”
Tests show that the responsible chemicals are largely cleaning products, he says, but a wide range of pollutants are present.
Oils from cooking and fertilisers from crops eventually find their way into the Bojacá and Subachoque rivers, says Sergio Valero, director of risk management for the region. Rubbish and even dead animals are dumped in the rivers, and though dumping waste is illegal, some businesses use the cover of night to discard toxic materials, he says.
Waste from communities upriver decomposes as it floats down to Los Puentes, mixes with organic compounds and eventually cascades into a lagoon in the lower river basin, foaming up and swallowing the community in a toxic cloud.
“It’s everyone’s fault, it’s a lack of environmental conscience,” says Valero.
A new waste-water treatment plant was installed in the small municipality in 2020 and the local council is educating businesses and residents on the importance of proper waste disposal.
Though residents say the amount of toxic foam is the highest it has ever been, the phenomenon isn’t new. Local people have grown accustomed to a pungent sulphuric smell that crops up whenever it rains.
Colombia’s poor waste-water treatment is killing its rivers, says at the University of The Andes in Bogotá. He says 60 to 70 per cent of domestic waste water goes straight into natural waterways untreated.
High levels of surfactants, such as soaps, shampoo and detergents, become toxic organic compounds if not treated, he says.
Camacho believes that high rates of diarrhoea in communities near Colombia’s principal rivers, such as the Bogotá and Magdalena, are due to high levels of viruses and bacteria in the untreated water.
Chacue’s son, Ian, has a cold, which residents say is a common occurrence – along with other illnesses such as diarrhoea – whenever it rains. She and other residents venture outside with hoodies and face masks to avoid getting an itchy, red rash from the foam.
Phosphates in Colombian detergents exacerbate the problem. These chemicals make cleaning products more effective, but are banned in much of Europe and the US as they are hard to break down through natural processes and can over-enrich plant life, causing toxic algal blooms.
In Los Puentes, phosphates have accelerated the aggressive growth of aquatic plants, such as water hyacinth, which trap contaminants.
The council for the surrounding municipality of Mosquera cleared the water hyacinths to help the river flow better, but it still contains the pollutants. This approach may reduce the foam, but it won’t solve the public health threat, says Camacho.
The best solution is better water treatment, he says. In the UK, for instance, London  began constructing waste-water plants in the 1900s and has today. Bogotá, home to a comparable population, has one. “Colombian politicians often discuss climate change and flooding, but we need to talk about water quality,” says Camacho.
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