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Bushfire ash contamination in Sydney's main water source has forced utilities to use alternatives for the city's drinking water after last week's deluge in the catchment area.
The rain, doubled water levels in the dam last weekend but brought with it significant amounts of sediment, ash and debris.
The ash, sediment and debris is "clearly visible" on the surface of Lake Burragorang at Warragamba Dam, which normally supplies the majority of Sydney's water, WaterNSW said on Friday. © Jenny EvansAsh from bushfires has been washed into the Warragamba Dam, prompting authorities to use alternative water sources.
The unusually high levels of debris were the result of "unprecedented" levels of bushfire ash and soil erosion due to reduced ground cover, after more than 320,000 hectares within the Warragamba Catchment were affected by bushfires, WaterNSW said.
Water has instead been drawn from alternative sources - the Prospect Resevoir and the Upper Canal.
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Though it is not uncommon to use the Upper Canal to supplement Sydney's water, it is limited in its ability to supply a sufficient amount of water on its own, Stuart Khan, professor at UNSW's School of Civil and Environmental Engineering said.
The use of the "historic" Prospect Resevoir is significant, since it is rarely used as a main source of Sydney's drinking water, Mr Khan said.
The supply of safe drinking water to Sydney's metropolitan region is continuing at a normal rate.
The sediment is "behaving exactly as [WaterNSW] staff predicted" earlier in the season, sitting mostly on the top of the water, David Harris, WaterNSW CEO said.
"This means we can safely draw good quality water from much further down in the water column if we need to supply from Warragamba Dam - which we are not doing at the moment."
A statement on Saturday evening from WaterNSW said the supply was being transitioned back to Warragamba Dam this weekend, with the water quality continuing to be monitored.
On Friday WaterNSW said that while quality of the raw water at Warragamba was improving, "more inflows may cause further deterioration in water quality at the dam wall".
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