Explained: Is your favourite swimming hole safe for a dip?

Most New Zealand swim spots are suitable for swimming most of the time - but there is always a risk. Photo / Michael Cunningham

• As Kiwis dig out their togs this summer, long-term monitoring shows two thirds of our monitored river sites rate poorly for E.coli and ecological health.
• A similar proportion of New Zealand’s lakes are in poor or very poor health: but water quality at beaches is generally safe for swimming.
• A scientist says water quality in rivers can vary daily - and encourages swimmers to check LAWA’s website, or take the “toe test” before taking a dip.

Warmer weather is sending Kiwis flocking to their favourite swimming holes. But when is it best to skip taking a dip? Jamie Morton explains.

How are our lakes and rivers faring?

On the whole? Poorly.

About two-thirds of council-monitored sites grade badly for harmful E. coli and ecological health, while a third have issues with water clarity.

Trends aren’t much better for our lakes, with two thirds in poor or very poor health, although nearly 70% of upland lakes are still in fair to very good condition.

The worst-graded spots for E. coli levels are usually surrounded by pastoral countryside such as dairy farms or urban land, while those near native and exotic forest are comparably much cleaner.

Data collected by the collaborative Land, Air, Water, Atmosphere (LAWA) project shows dozens of river sites that rank poorly for the nasty bacteria over the long-term.

Many of those sites – like Kaiate Falls in the Bay of Plenty, the Rere rockslide into the Wharekōpae River near Gisborne and Selwyn River at Coes Ford in Canterbury - also happen to be popular spots for recreation.

On the other hand, monitored coastal sites around New Zealand are suitable for swimming almost all the time – with the exception of seaside lagoons.

Rere Rockslide, south of Gisborne, into the Wharekōpae River.

“While lagoons may seem like a safer option, especially for young child, as they are warmer, without fast flowing water or waves, they tend to have poorer water quality,” says LAWA’s science lead, Dr Elaine Moriarty.

“This can be due to an abundance of birdlife and their poo, all sloshing around in a nice, warm slow-moving waterbody - ripe for bacterial growth.”

Do poor long-term grades mean those spots are always unswimmable?

Not quite: the long-term grade is risk-based and doesn’t necessarily reflect the conditions on a given day.

LAWA’s “Can I swim here” interactive webpage carries regularly updated water quality results for more than 850 sites.

“We know from monitoring results that most New Zealand swim spots are suitable for swimming most of the time, but there is always a risk.”

Those sites graded poor over the past five summers represent a wide range of water quality results.

“Poor sites include those that may have been suitable for swimming most of the time but have routinely exceeded the threshold for swimming suitability once or twice per season, through to sites that frequently have failed swimming water quality guidelines,” she says.

Many popular swim spots are also known to exceed the guidelines after rainy weather, which can send pollutants like wastewater or farm run-off into rivers - yet otherwise have suitable water quality.

Around two-thirds of council-monitored river sites grade poorly for harmful E. coli. Photo / Andrew Warner

“That’s why we advise swimmers to avoid the water for two to three days following heavy rain, regardless of the most recent monitoring result or the long-term grade.”

Will I get sick if I swim in a dirty river?

It’s not guaranteed, but swimming in water with high levels of faecal bacteria like E. coli means you run the risk of a range of unpleasant symptoms.

That includes vomiting and diarrhoea, respiratory disease, or eye, ear, nose and throat infections.

LAWA’s site uses a traffic light system to show the health risk at each swimming spot, with green indicating all clear, amber signaling caution for people such as the very young and old, and red meaning steer clear.

Moriarty says people can apply a simple test themselves to avoid getting sick at their local swimming spot, by standing in calf-deep water and looking down.

“Can’t see your toes? It’s a no-go.”

What are some other risks to watch for?

In general, people shouldn’t get in the water if it’s murky, smells unpleasant, is flowing fast, or if there’s signage advising against swimming.

Another big hazard is toxic algae, which thrives in waterways when there’s low rainfall, warm temperatures, the right level of nutrients, and plenty of sunlight.

“These conditions often come together in a Kiwi summer,” Moriarty says.

“The brown-black mats of toxic algae are a common sight in some rivers and a number of lakes are prone to being affected by pea-green or reddish-brown toxic algae blooms.”

Phormidium, a common toxic algae, is toxic to humans and dogs.

While naturally occurring, even a tiny bit of toxic algae - think the size of a 50c coin - can cause serious harm if eaten.

“In humans, contact with toxic algae can cause vomiting, diarrhoea and skin irritation, and there is the potential for serious poisoning to small children if they swallow it,” she says.

“Sadly, numerous dogs have died in New Zealand from toxic algae. If your dog ingests toxic algae, you should take it to the vet immediately.”

Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.