Lake Wiritoa near Whanganui declared unsafe for swimmers following water testing

It is unclear when the lake will be safe for swimming again.

Lake Wiritoa near Whanganui is off-limits to swimmers due to high levels of E. coli and toxic algae, with a reopening date yet to be determined.

In a statement, Whanganui District Council said the presence of E. coli suggested there were other pathogens in the lake that were harmful to humans.

“The algae can lead to gastroenteritis, respiratory illness and ear and skin infections,” the council said.

Warning signs have been in place since December 6.

“Please avoid activities that involve drinking the water or skin contact until it’s safe to enter the lake again.”

E. coli is a bacteria that normally lives in the intestines of humans and other warm-blooded animals. It can indicate other disease-causing bacteria – such as campylobacter, viruses and protozoa – are also present.

Horizons Regional Council environmental monitoring scientist Ian Hurst said the lake was tested on December 3 and again on December 6, with results of the latter yet to come in. Swimmability testing was usually done once a week, he said.

“The high E. coli volume we got is incredibly unusual for that lake.

“Even if E. coli comes back as green [safe] – and, touch wood, it does – it would still be closed due to the cyanobacteria [blue-green algae] value.

“That would need to reduce before we could call it swimmable again.”

Hurst said he could not guarantee the lake would be open for swimming before the end of summer but “there is a pretty good chance”.

In November 2022, Whanganui District Council sprayed the lake with herbicide to try to tackle the ongoing problem of “pest plant” hornwort. According to Land, Air, Water Aotearoa (Lawa), it is the third time the 22ha lake has been unsafe for swimming since December 2021.

Of the 130 tests undertaken by Horizons since the start of 2019, 92% said it was safe for swimming and 5% said caution was advised.

Lawa has given Lake Wiritoa a long-term (five-year) rating of “fair” for water quality, meaning it is generally safe to swim in.

Rangitīkei‘s Dudding Lake – located southeast of Lake Wiritoa off State Highway 3 – is also out of bounds for swimming and has a long-term rating of “poor”, according to Lawa.

A Horizons statement on Monday, December 9 said high counts of potentially toxic blue-green algae meant the lake was unsuitable for recreational use.

It said the algae could produce toxins harmful to humans and animals if swallowed or through contact with skin, which could happen when swimming, water skiing or kayaking.

Dudding Lake has been tested 131 times since the start of 2019, with 79% saying it was safe for swimming, 12% saying caution was advised and 9% saying it was unsuitable for swimming.

Hurst said the rule of thumb for freshwater swimming was the higher up a catchment is, the better the water quality.

“Of course, that comes with a caveat – checking for debris, not swimming after rainfall, all that kind of thing,” he said. “Rivers can change a lot quicker than lakes.”

• This story has been updated to reflect that spraying Lake Wiritoa for hornwort is done by the Whanganui District Council.

Mike Tweed is a multimedia journalist at the Whanganui Chronicle. Since starting in March 2020, he has dabbled in everything from sport to music. At present, his focus is local government, primarily Whanganui District Council.