Sydney's drinking water could be polluted by bushfire ash in Warragamba Dam catchment, expert says

Sydney’s drinking water supply is at risk of the same “worst case scenario” facing some New South Wales regional communities, where large amounts of bushfire ash has been swept into dams by heavy rainfall, a water expert has warned.

Stuart Khan, a water quality security expert and environmental engineer from the University of NSW, said recent bushfires had left large deposits of ash in the Warragamba Dam catchment, which supplies 80% of the city’s drinking water.

“The situation in Sydney is really serious,” Khan said. “Having a reservoir full of soil and sediment and ash is in itself a real problem, because it makes water treatment processes more difficult.

“There will be a huge amount of ash sitting in the catchment now. The best case is we get gentle rainfall for weeks and months that allows some gentle regrowth. If we have a big storm or a big wet weather event in Sydney’s drinking water catchment, we will see some of that ash running into the dam.”

WaterNSW has said fires in the catchment areas posed no risk to water quality “at this time” and that the authority had been working to minimise potential effects.

“The potential for water quality impacts on dam storages will depend on the extent and intensity of the fires and timing of the next significant rainfall,” it said last week.

Precautionary planning was under way, and it could use sophisticated measures, including taking water from variable places or storage depths, or deploying booms and curtains to isolate flows that posed a water quality risk.

Khan said some regional communities, such as Tenterfield in the northern New England region of NSW, had already had their drinking water supply polluted by ash. Tenterfield residents have been boiling their water for more than 70 days.

When ash – which contains large amounts of organic carbon – enters the water supply it can take oxygen from the water and change the water chemistry. Khan said fish kills were possible, as was blue-green algae.

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The more sediment in the water, the more difficult to treat. Khan said that while Sydney had backup systems to deal with a similar situation, the threat was much greater in many regional communities where treatment infrastructure was old or outdated.

“Across the east coast of NSW, almost all of our water supply is surface water,” he said. “The way we protect catchments is having forested areas around them. If there is a bushfire in the catchment, it affects that protection.

“We’ve got lots of places where supplies draw from a single reservoir, a single treatment plant, with relatively simple treatment in place.

“It’s time to have a good hard look at drinking water plants around NSW and ask the question: are they up to scratch on a day-to-day basis? Do they have the resilience … [to] withstand these hazardous events and continue to reliably provide safe drinking water quality?”

Patrick Lane, a forest hydrologist from Melbourne University told the ABC that steep, dry-forested areas in particular were prone to mud slides after bushfires.

“I think water authorities in the past probably thought this was something that would come along once every now and again, but I think we need to be thinking much more about this could be something that happens every few years,” Lane said.