An investigation into six council-run splash parks and paddling pools in London and the home counties has found bacteria levels at two sites more than 100 times above the threshold regulators consider unsatisfactory, raising fresh concerns about water safety for children this summer.
Will Stoddart, a reporter for Good Health, collected water samples from the six popular facilities on August 11, when temperatures reached the mid-to-high 20s Celsius, and August 12, when they hit around 30C. Samples were taken both while children were playing in the pools and afterwards, between 2pm and 8pm.
The samples were tested at Biodet, a laboratory at the University of Hertfordshire, for E. coli and coliform bacteria, which can signal faecal contamination, for Pseudomonas aeruginosa, and for total viable count (TVC), a broad measure of how well a pool's filtration and disinfection are working.
Bacteria levels at each site
According to the Pool Water Treatment Advisory Group (PWTAG), an independent body whose standards are used by the Health and Safety Executive and local councils, a well-run pool should normally have a TVC below 10 colony-forming units per millilitre. Anything above 100 is classed as "unsatisfactory."
Gadebridge Splash Park in Hemel Hempstead and Bruce Castle Park in north London both recorded TVCs below five. Howard Park in Letchworth came in at 20, twice the normal level, and Lordship Recreation Ground in north London, tested on the hotter of the two days, recorded 30.
But Runnymede Pleasure Grounds in Egham and California Country Park in Berkshire both returned TVCs above 10,000, more than 100 times the unsatisfactory threshold, even though the water at Runnymede looked clean and inviting. E. coli and coliforms were not detected in any of the six samples.
Dr Richard Smith, director of Biodet, said the two "notably high" readings should trigger an investigation into chlorine levels and filtration. Professor Lucy Robertson, a parasitologist at the Norwegian University of Life Sciences, said results that high raise the suspicion that chlorine and water checks weren't being carried out often enough, and should prompt corrective action by the council or operator.
How the contamination builds up
Sweat, urine, faeces and skin cells washed from swimmers' bodies react with and use up chlorine, according to the PWTAG. Sunlight can break the chemical down further, meaning levels can fall quickly in busy outdoor pools on hot days, said Professor Kelly Reynolds, an environmental microbiologist at the University of Arizona.
If chlorine drops too low, bacteria can multiply rapidly, with some species doubling every 20 minutes, according to Dr Primrose Freestone, an associate professor in clinical microbiology at the University of Leicester. The PWTAG recommends chlorine be checked every two hours at splash parks and lidos while they are open, but says some operators skip these checks and have "good evidence" of failing to run required monthly laboratory tests for bacteria.
Reynolds said children have poorer hygiene habits and are twice as likely to swallow pool water as adults. She has seen children sit over water jets and drink from them, and has seen parents change nappies poolside and rinse their children's bottoms in the water, wrongly believing chlorine kills germs instantly. Depending on the bacteria, that can take minutes; for cryptosporidium it can take up to ten days.
Martin Wood, an environmental microbiologist and co-founder of Pool Sentry, said chemical dosing needs regular checks and maintenance, but financial pressure often leaves operators without the resources to do enough of them. Runnymede and California Country Park both use automatic chlorine dosing systems, yet still recorded high bacteria counts. In Haringey, the paddling pools at Bruce Castle and Lordship are emptied, cleaned and refilled three times a week during summer.
The findings come as a report this year from the All-Party Parliamentary Group for Swimming found more than 1,200 public pools in Britain are over 40 years old and past their expected lifespan, with 56 per cent of pools in Scotland over 30 years old.
The threat from cryptosporidium
None of the six pools tested was checked for cryptosporidium, a microscopic parasite spread in faeces that causes severe watery diarrhoea, cramps, vomiting and dehydration. The UK Health Security Agency said it is the pathogen most commonly linked to outbreaks from recreational water use because it resists chlorine.
The PWTAG says a single millilitre of infected diarrhoea can contain around a million of the parasites, and Reynolds said as few as one may be enough to cause illness, compared with the thousands of E. coli bacteria typically needed. There were 14 outbreaks linked to swimming pools or water parks in England between 2023 and 2025, affecting at least 86 people, some of whom were hospitalised.
Professor Rachel Chalmers, head of the Cryptosporidium Reference Unit at Public Health Wales, said current UK practice is to test during outbreaks or suspected filtration failures rather than as routine prevention. But Dr Seak Lin Ly, a water quality analyst and author of a study in The Journal of Infectious Diseases, argued high-risk pools, particularly children's pools in summer, should be tested proactively. Her study found crypto can circulate in a pool for an average of 27 days before it is detected and the pool closed for treatment.
Killing the parasite requires chlorine levels many times higher than normal, which can irritate eyes and skin and harm airways, forcing pools to close until levels return to normal. James Ebdon, a professor of environmental microbiology at the University of Brighton, said a more cost-effective long-term fix would be added treatment steps such as physical filtration, ultraviolet light or ozone, which could remove crypto along with other bacteria, viruses and moulds. Professor Joan Rose, a water microbiologist at Michigan State University, said even the best treatment cannot protect swimmers instantly, since a swimmer after an infected person can be exposed before the water passes through the system. Reynolds added that infected people can shed the parasite without realising it, and many return to swimming too soon after diarrhoea stops.
Advice for parents
The PWTAG advises keeping children out of pools for at least 48 hours after diarrhoea or vomiting symptoms stop. It recommends looking for its PoolMark accreditation or asking operators how often chlorine is checked and whether water is filtered and recirculated, and trying to stop children swallowing water or drinking from splash jets.
Councils respond
A Runnymede Council spokesman said water testing is carried out routinely during opening hours, and that the council had become aware of fluctuating water quality during the heatwave and commissioned its own independent tests. The pool has been closed in the meantime.
A Wokingham Borough Council spokesperson for California Country Park said water there is tested every two hours during opening hours, and that a review of records for the two weeks around August 11 showed pH and chlorine levels remained within PWTAG-recommended levels during operating hours. The council said it is carrying out a shock chlorine treatment of the system and further microbial testing to confirm there is no ongoing issue.
Despite the findings, experts stressed that parents should not be put off taking children to paddling pools and splash parks.
