You know the feeling. It’s a scorching hot day. The temptation to dive into a stream of cool water is rising by the minute. It looks clean, clear and downright refreshing. It is, however, likely to be absolutely filthy.
Even in the cleanest rivers you can find contain bacteria that can be harmful – including faecal bacteria from livestock manure and human, um, waste.
Amidst announcements of new designated bathing sites in the UK – alongside holiday locations being deluged in sewage and rowers getting E. coli infections – we spoke to water and sanitation expert Prof David Werner to get the low down on wild swimming safely this summer.
Werner's research focuses on rivers, where treated sewage, 'stormwater' (untreated overflows of sewage following heavy rain), and sometimes illegal sewage dumping in dry weather impact human and ecological health. While human waste is released into the sea too, rivers are less diluted so concentrations of human waste, and the bad bacteria they may contain, tend to be higher – meaning you're more likely to pick up nasties there.
(In fact, Professor of microbiology William Gaze told BBC Science Focus that rivers have also only recently become subject to the same microbial testing as coastal spots. What's more, sewage released into the sea often undergoes "a higher standard of treatment" than in rivers, including UV disinfection.)
The following tips are river-focused, but you can still apply them to swimming in coastal spots.
The bottom line? Never, ever swim after it’s rained.
How do diseases enter rivers, and how risky is it to swim in them?
According to Werner, faecal pollution isn’t necessarily terrible as most people’s poo contains 'good' bacteria that keep us healthy. But sick people’s poo contain the bacteria that made them sick – and these can spread diseases.
The risks of catching a disease from the river are, Werner says, pretty low. But they're significantly higher if you end up ingesting the water (not advisable, but it happens).
If you do, there’s a chance you could pick up infections or uncomfortable symptoms like diarrhoea. The worst-case scenario, Werner says, is getting infected with an antibiotic-resistant bacteria – but this is very unlikely.
So should you forget wild swimming? Ultimately, it comes down to whether you’re willing to accept this (generally low) risk for the sake of the mental health benefits, says Werner.
“There is no such thing as 100 per cent safe or zero risk,” he says. “Every time we get into a car, we make a judgement that the benefits of the trip outweigh the risks of having an accident. [Similarly,] there are substantial mental health benefits from recreation in nature, especially for urban populations.”
How to stay safe from sewage when swimming
Werner and his team are currently developing a device to rapidly test water for the presence of sewage, “similar to a lateral flow Covid Test. But there is still a lot of research to be done to deliver this.”
Until that becomes available, there are a few simple ways to spot signs of sewage while swimming in rivers this summer. Here’s what Werner recommends.
1. Choose your spot wisely
Whatever you do, never swim downstream of a sewage treatment work, Werner says. Untreated sewage overflows may, understandably, be what people are more worried about, but treating sewage doesn’t remove all the risks – it just dilutes them.
“Even if you treat sewage, you’re going to reduce the level of hazardous bacteria substantially, say by a factor of 1,000, but not to zero,” says Werner.
“So if that location is, say, just downstream of a discharge point for a treatment work, I would expect that level of faecal bacteria to be there all the time because the discharge is continuous.”
You can check 'poo maps' to follow your local river upstream and see if there is a treatment plant along its route.
2. Check the poo maps
UK charities The Rivers Trust and Surfers Against Sewage have produced interactive maps where you can see where treated sewage is regularly discharged, as well as how much untreated sewage overflowed in 2023 in locations around the country.
To see where sewage treatment works are on The Rivers Trust's map, click on the yellow squares in your area – or tap the brown circle icons and scroll through the cards. If you see the words 'Sewage discharges – Final/treated effluent – Water Company', that's a sewage treatment work.
This year, the charities started to incorporate available near-real-time data into the maps. “This is huge progress for people that want to be informed”, says Werner. “I would definitely advise that people look at these online sources of real-time information.”
Now, you can see whether there has been a sewage discharge in the last 24 hours – though the only inland data comes from designated bathing sites and rivers in the Thames Water region. (Northumbrian Water also has its own county-specific map). The rest are coastal sites.
3. Watch the weather
While you’re likely to want to swim when it’s sunny, make sure it’s been dry for a few days. Swimming after rain significantly increases the risks of swimming in polluted water, because heavy rainfall causes sewer systems to overflow and discharge the excess.
To make things difficult, it may seem like you’ve got a good dry spell for a safe dip when, actually, something nasty is lurking upstream. That’s because thundery weather conditions can cause localised rains – meaning it might be sunny where you are, but heavy rain may have fallen upstream.
“That’s when the risk is highest,” says Werner. Look out for changes in the river’s levels or a turbid flow – these are signs that there have been storms upstream.
4. Avoid small rivers
In addition to turbid waters, smaller rivers tend to be worse as any sewage – treated or untreated – will be less diluted.
Werner’s measure of safety is that raw sewage should be diluted 1,000 times to splash in, and over 10,000 times to swim in. While we’re not suggesting you work out these dilution levels for yourself, there’s a simple conclusion. “If you have a big river with a big flow, naturally there will be more dilution,” Werner says.
'Flashy' urban rivers – those with large changes to their flow that happen rapidly in response to rainfall – are also bad.
You don’t need to worry too much about the different parts of a river, though; one side may look more stagnant compared to the middle but it all mixes together.
That said, in faster flow zones you need to be careful of other hazards, like strong currents and shock from entering cold water, Werner warns.
5. Follow the paper trail
As your common sense will tell you, don’t swim if you can see toilet paper! But even worse omens than toilet paper are wet wipes and sanitary pads: things made of plastic that “people shouldn’t flush down the toilet but they do.”
Werner says these don’t disintegrate in fast-flowing water like toilet paper and faeces.
“To me, this is the clearest indication that you have sewage in the river: sometimes you see wet wipes along the high flow line, in the vegetation. Maybe you’ve never realised, but once you see them, you see them everywhere.”
What’s not always clear is how long they have been there. So, if you see them, the next step is to confirm via the maps...
6. Keep your head up
If you’ve followed these steps and do decide to go for a dip, a few simple measures will help to keep you from getting sick.
Ideally, you should avoid putting your head under entirely – and keep your mouth closed, Werner says. Sadly, that probably means foregoing the summer snapshot of you and your friends taking the plunge.
Wearing goggles to prevent eye infections and covering any open wounds will also minimise the chances of bad bacteria entering your body.
After you’ve enjoyed your swim, Werner says it’s vital to wash or disinfect your hands – especially before devouring that riverside picnic – and make sure to shower when you get home.
“Once you’ve played in the river, you should do whatever helps to get you clean again. Just standard good personal hygiene.”
About our experts
David Werner is a Professor of Environmental Systems Modelling at Newcastle University. Previously, his research focused on sanitation and water quality in low-income countries – conducting DNA sequencing in rivers and testing for microbial risks. He turned to UK rivers when the pandemic restricted travel, and found that they had similar levels of faecal bacteria after rain. His research has been published in journals Water Science & Technology, Science of the Total Environment, and Environmental Pollution.
William Gaze is a professor of microbiology at The European Centre for Environment and Human Health at the University of Exeter's Medical School. His research has been published in the International Journal of Antimicrobial Agents, Food and Environmental Virology, and Water Research.
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