Parisians swim in the Seine River. Paris invested about 1.4 billion euros ($1.61 billion) to improve water quality in the Seine before opening select stretches to the public, but experts warn that heavy rainfall can cause water quality to deteriorate sharply due to sewage and pollutant runoff. [Reuters]
Parisians swim in the Seine River. Paris invested about 1.4 billion euros ($1.61 billion) to improve water quality in the Seine before opening select stretches to the public, but experts warn that heavy rainfall can cause water quality to deteriorate sharply due to sewage and pollutant runoff. [Reuters]

As heat waves intensify, "wild swimming" — taking a dip in rivers, lakes and other natural bodies of water — is rapidly gaining popularity in the United Kingdom, the United States and Australia. Social media feeds overflow with videos of swimmers gliding through emerald rivers and forest lakes, but experts warn that the infection risks posed by sewage, agricultural runoff, rodent waste and toxic algae are far greater than most people realize.

According to a recent BBC report, wild swimming has taken root as a leisure pursuit in the UK, US and Australia over the past decade, driven by the spread of social media and urban "blue space" expansion policies. Research showing that swimming in nature and walking along riverbanks can reduce stress and improve mental health has also fueled its popularity.

Major European cities have been pushing projects to give their rivers back to residents.

Residents swim in the Seine River at the Bras Marie public swimming zone in Paris on July 18 (local time). The Seine near major Paris landmarks, including the Eiffel Tower, was opened to the public after a large-scale cleanup project, but experts warn that pollution risks can rise sharply immediately after heavy rain. [Reuters]
Residents swim in the Seine River at the Bras Marie public swimming zone in Paris on July 18 (local time). The Seine near major Paris landmarks, including the Eiffel Tower, was opened to the public after a large-scale cleanup project, but experts warn that pollution risks can rise sharply immediately after heavy rain. [Reuters]

Paris invested about 1.4 billion euros ($1.61 billion) in cleaning up the Seine, constructing a large underground reservoir near Austerlitz station capable of storing roughly 50,000 cubic meters of rainwater. The city accelerated its water quality efforts ahead of last year's Olympics and has since opened select stretches of the river to the general public.

London is also operating the Tideway, a massive sewage tunnel stretching about 25 kilometers beneath the Thames. The tunnel stores up to 1.6 million cubic meters of sewage and rainwater before routing it to treatment facilities, reducing direct discharge into the river.

Yet experts caution that even the most ambitious cleanup projects cannot fully protect swimmers immediately after a heavy downpour.

When heavy rain falls, sewage overflows drainage pipes and flows into rivers, while livestock manure and fertilizers wash in from farmland. Combined with pollutants from urban roads and disturbed riverbed sediment, these inputs can cause water quality to deteriorate sharply within a short period.

Isabel Douterelo Soler, an environmental microbiologist at the University of Sheffield, said that after heavy rain or storms, urban waterways "can be microbiologically unsuitable for swimming for a minimum of 24 to 72 hours, generally, due to sewage overflow, surface water runoff and disturbed sediments."

A resident jumps into the Canal Saint-Martin in Paris on July 11 (local time). [AFP]
A resident jumps into the Canal Saint-Martin in Paris on July 11 (local time). [AFP]

The problem extends even to sites that normally receive clean-water ratings. Because most water quality tests run on a fixed schedule, sudden contamination following heavy rain may not be reflected immediately. That is why experts advise swimmers not to rely solely on official water quality ratings but to also check whether it has rained recently.

Contaminated water can harbor not only bacteria such as E. coli and enterococci but also antibiotic-resistant organisms — so-called superbugs. Leptospirosis, which spreads through rodent urine, pathogens found in agricultural runoff, and toxins from cyanobacteria (blue-green algae) that bloom in summer are also among the leading hazards.

Infection can cause diarrhea, vomiting, fever, skin rashes, and ear and eye inflammation, with more severe symptoms in people with weakened immune systems. Leptospirosis, though rare, can lead to kidney or liver damage, while toxic cyanobacteria can irritate the skin and, in serious cases, affect the liver and nervous system.

Experts recommend that swimmers stay out of the water for at least 24 to 72 hours after heavy rain, avoid entering the water if they have open wounds or are feeling unwell, take care not to swallow water while swimming, and rinse off with clean water afterward to reduce the risk of infection.


sjy@heraldcorp.com