Forecast heavy rains across bushfire-hit areas of Victoria and New South Wales may cause mass fish deaths as ash and sediment is washed into rivers, the Murray-Darling Basin Authority has warned.
Central parts of Victoria and and NSW are forecast to see falls up to 80mm between Thursday and Sunday, some of it in areas recovering from bushfires. Fire grounds in the Snowy Mountains and south coast regions of NSW may get strong falls.
Ash washed into rivers may lead to algal blooms that could cause fish kills. Thousands of fish have already died in a northern NSW river when ash from firegrounds washed into the waterway.
Experts say the ash is not likely to affect drinking water, except in some smaller catchments.
The Murray-Darling Basin Authority described the forecast falls as a “double-edged sword”.
The authority’s executive director of river operations, Andrew Reynolds, told Guardian Australia: “We would not be surprised to see fish deaths in streams that are picking up ash and sediments from the rainfall.”
He said the ash and sediments could reduce the amount of dissolved oxygen in the water, but the impacts would depend on where the rain fell, and the intensity of downpours.
Bushfires had burned “approaching half” of the catchment of the Hume Dam, he said, which is a main water storage for the Murray-Darling system.
“The fire map shows us the extent, but it doesn’t mean everything has burned. But it’s a large area and almost certainly we will see impacts on water quality following the rain forecast in coming days.”
He said while the rain was welcome in helping firefighters, having ash and sediment running into waterways was “the last thing our fire- and drought-ravaged communities need right now”.
Water running off the fire grounds could also have picked up contaminants from burned buildings.
Reynolds said: “These catchments are very dry. Some of the rain will run off but we won’t expect to see large inflows given how dry things have been.”
Anyone concerned about local water supplies should contact their water authority, he said.
Dry, hot and windy conditions had meant water quality had continued to worsen, with nine “red alerts” for blue-green algae raised in the past two weeks, bringing the total across NSW and Victoria to 20.
Weather Update: Heavy rain and the potential dangerous impacts to fire grounds in NSW and East Victoria.— Bureau of Meteorology, Australia (@BOM_au) January 14, 2020
Video is current at 12pm AEDT 14 Jan 2020.
Info: https://t.co/t2CaglH2RW and follow advice from emergency services.#nswfires #vicfires #ausfires @NSWRFS @VicEmergency pic.twitter.com/zQrk6RAa4s
Prof Stuart Khan, an environmental engineer and water expert at the University of New South Wales, said the main Sydney water catchment was a concern, although he did not expect immediate problems.
He said: “We are in a vulnerable position with all that ash sitting on a catchment that’s unstable and prone to erosion that could include landslides and trees being dislodged.
“We’re not expecting extreme rainfall, but if any places do and they’re areas that have been burned, then we’d expect ash and soil running into waterways.”
He said adding nutrients to the water would increase the risk of toxic blue-green algal blooms.
Increased nutrients and ash could also cause problems for treatment plants, especially smaller ones, which would need to work harder, slowing down productivity.
But he said Sydney should not expect impacts “to the water coming out of customers’ taps”.
WaterNSW said about 320,000 hectares had burned in the catchment for Sydney’s main water supply, the Warragamba Dam, where inflows are at record-low levels.
Devices to limit ash entering the dam had already been installed, but the WaterNSW CEO, David Harris, said the rainfall was not expected to be high enough to cause problems.
“Therefore we do not anticipate a water quality problem over the coming week, however we will need to carefully manage the catchment over the coming years in order to protect our water supply from these sorts of impacts.
“We are taking a range of precautionary measures on site, including the deployment of silt curtains and floating booms at key water inflow points to the dam storage, additional monitoring and modelling, and planning with other agencies.”
Since November and December, the NSW Department of Primary Industries has been receiving reports of thousands of fish dying in the upper and middle reaches of the Macleay River near Kempsey in northern NSW.
There are reports of a fish kill in the Macleay River west of Kempsey (NSW). The cause is reported to be the "result of heavy rain upstream washing ash from recent bushfires into the river, which is having an effect on oxygen levels". https://t.co/R2F2pXlAyc pic.twitter.com/HPrdNPGumn— Stuart Khan (@stukhan) January 15, 2020
Australian bass, freshwater mullet and eel-tailed catfish have been among the species killed.
A spokesperson for the NSW Department of Primary Industries said the fisheries department was still receiving reports of fish deaths in the river.
“Current rainfall events are adding ash from the extensive bushfires throughout the region into local catchments, as well as other organic matter and sediment. This can cause rapid drops to oxygen levels in the water.
“DPI Fisheries holds concerns around the potential impact on fish and other aquatic organisms over the coming weeks, as predicted rainfall is likely to mobilise sediment and ash in coastal waterways.”
Residents were encouraged to report any fish deaths or observations to the Fishers Watch hotline on 1800 043 536.