Just a quarter of Sydney’s largest drinking water catchment, the Warragamba Dam, is safe for consumption, prompting fears residents will need to conserve water.
Ben Blayney, the head of water supply and production at Sydney Water, said the past year’s floods had placed extreme pressure on the city’s water filtration system.
The Warragamba Dam is the largest of Sydney’s five drinking water catchments, catering to more than five million people in Sydney and the lower Blue Mountains.
During the peak of the floods, just 5% to 10% of the water in the dam was suitable to be treated and cleaned, despite the dam being full. It had since risen to 25%.
“The problem we have is the pressure our systems are under to filter the raw water,” Blayney said.
“We currently only have a narrow section of about 15 metres from the available 60 metres in Warragamba Dam which is suitable to treat and clean, and while it’s been improving since July people are using more water.”
Water usage has increased from 64 litres a day from April to June to 69 from July to September, Sydney Water research shows.
Usage typically spikes in the warmer summer months, putting further pressure on the system.
Blayney said there was often a misconception that high rainfall and full catchments meant an abundance of water.
In the past year, the Warragamba Dam has been lashed with 1,581mm of rain, including 54mm in the past week alone. The 2,064,680ML catchment was currently sitting at 100% capacity, a 2.4% increase on the same time last year.
He said in the instance of another major flooding event, requesting residents conserve water would be “one of the steps you’d take in an emergency”.
“It’s something that would be considered with NSW Health, it’s quite serious,” he said.
“Sydney Water in the last few months has installed processes at plants to get us through summer but what we’re dealing with now is still a remainder of what happened in July.
“It’s a risk we manage every day … we have pressure on our treatment plants and it’s a good time to remind customers water is still precious.”
Prof Stuart Khan, director of the graduate school of engineering at the University of New South Wales, said the shortage “wasn’t at all unexpected” but a consequence of compounding extreme weather events.
“After the Black Summer bushfires a lot of ash went through the Warragamba catchment, 60% was burnt out,” he said. “The catchment became prone to erosion. And after floods or intense rain, you get a lot more. Here we are nearly two years later and soil is not held in a stable way anymore – it’s mud.”
Khan said there was now a “very narrow slither” of water that could be targeted due to the high sediment levels and algae presence.
“In the first floods we relied on drawing water from other sources – flexibility and diversity of supply is the secret,” he said.
For areas that relied on the Warragamba Dam, though – like the Orchard Hills water filtration plant in Western Sydney – Khan expected restrictions would be imminent, with the rest of Sydney soon to follow.
“Western Sydney doesn’t take water from other sources, it’s hit hardest and the first told they need to start conserving,” he said. “There’s also a high likelihood of a big algae bloom which could affect the surface. It’s likely we’ll be asked to conserve water.”
The managing director of Sydney Water, Roch Cheroux, said New South Wales had experienced “unprecedented” rainfall in the past year, placing great pressure on water filtration plants while the state’s dams sat at 98.4% capacity.
“Our water filtration plants are working much harder because of the amount of debris and dirt in the raw water.”
Three dams in NSW – the Avon, Blowering and Rydal – remain closed to the public due to flood and rainfall damage, while a further three catchments have been issued with a red alert for blue-green algae.
Water NSW warned last week Copeton Dam near Inverell, the Chaffey Dam near Tamworth and Peel River had high levels of the potentially toxic algae, advising residents to seek alternate water supplies and avoid recreational activities.
Potentially toxic blue-green algae can cause gastroenteritis, skin and eye irritations as well as liver damage and other health problems if consumed.
In Victoria, all 36 of the beaches in Port Phillip Bay were listed as “poor” by the Environmental Protection Authority and considered unsuitable for swimming due to stormwater pollution from recent and forecast rain.
The four catchments along the Yarra River including Healesville, Warrandyte, Kew and Launching Place were also listed as “poor”.
To investigate the risks of pathogens and sewerage levels in the water after widespread and ongoing flooding, the Environment Protection Authority has launched a “high quality” sample analysis program at 19 Victorian sites, including nine waterways.
Victoria’s chief environmental scientist, Mark Taylor, said it was the “single most comprehensive” assessment of water quality and the pathogen E coli that had been undertaken in Australia’s history.
Early indications show E coli levels, a key indicator of water health, varied greatly across Victoria but were better than expected – with some exceptions, including the Maribyrnong and Barwon rivers.
“Bacteria levels subside quickly but our testing shows at some locations on the river, E coli levels were many times above the guidelines, at least for a period of time,” Taylor said of the Maribyrnong, particularly around Footscray in Melbourne.
On Friday, the testing program found continued high enterococci contamination in the Barwon River in Geelong and foaming waters due to flooding. The EPA urged against contact with the water including adjacent surf beaches.
