More testing needed at Edmonton's natural pool to track contamination risk, germ experts say

Years after public health researchers urged the City of Edmonton to allow no more than 45 swimmers per day in Canada's only natural swimming pool, municipal officials say the water is just fine.

But microbiologists are concerned that allowing hundreds of people each day to take a dip in Borden Natural Swimming Pool is putting its fragile filtration system under unnecessary strain.

The city should adopt more stringent daily testing, said Jason Tetro, an Edmonton-based microbiologist, science columnist and author of The Germ Code.

"We really should be testing daily, particularly when we have higher bather loads and higher temperatures that could potentially lead to more microbial growth in those waters," Tetro said. 

"I probably wouldn't swim in the pool right now because I know there is that added risk factor." 

Opened to fanfare in 2018, the $14.4-million pool relies on plants, zooplankton, granite and sunlight — rather than the conventional chlorine or salt — to filter the unheated water.

The kids' pool was recently closed down due to unsafe levels of contamination — the second time it has been shut by Alberta Health Services this season due to unacceptable levels of contamination.

The main pool, overlooked by lily pads that are part of the filtration system, has remained open all summer despite algae blooms that are clouding the water. 

The city allows up to 980 people per day in the pool, a capacity limit that far exceeds what researchers deemed necessary to keep the waters sufficiently clear of harmful bugs and bacteria.

A study, published in 2021 in the Journal of Water Science, found that the pool was slow to clear common contaminants. It recommended that capacity be constrained to no more than 45 swimmers daily to maintain an "acceptable level of risk" to swimmers and daily testing for fecal contamination.

Nicholas Ashbolt, a co-author on the study, said all pools can harbour harmful germs, but it is especially critical that crowded pools that don't use chemical disinfectants can effectively filter out pathogens.

"It's always a risk to swim where other people are swimming, no matter what sort of pool," said  Ashbolt, who was a professor with the School of Public Health at the University of Alberta when the research was conducted in co-ordination with the city.

"The more people in the pool, that risk goes up."  

Ashbolt, now the Peter Teasdale Chair in Environmental Health Risk Assessment at the University of South Australia, said he was surprised to learn from CBC that the daily capacity had been set at 980.