As the bushfires continue to burn around the state there is an increasing realisation that another very likely impact of these widespread fires is contamination of urban water supplies. This happened to Canberra’s water supply following its devastating 2003 bushfires and has happened in other places where dam catchments have been severely burnt.
It happened recently to Tenterfield’s water supply. Before the bushfires its water supply had been under stress because the dam level had dropped in October to about 18% and residents had been advised to boil their water. A storm late in November topped up the dam but damaged silt traps designed to prevent sediment entering the dam. Massive amounts of ash and debris from the recent bushfires were swept into the dam.
According to Stuart Khan, a water security expert from the University of NSW, a combination of events have created Tenterfield’s problem.
“First of all you’ve got a drought which means the catchment is very dry,” he said. “It also means the reservoir level is very low and there’s no opportunity to dilute new flows that come in.”
“Fire followed by heavy rain will wash ash into waterways. There’s a lot more erosion because you don’t have the trees and roots holding the ground together. Having a reservoir full of soil and sediment and ash is in itself a real problem because it makes water treatment processes more difficult.”
Water quality impacts can include deoxygenation and the growth of cynobacteria, which can be toxic.
He also pointed out that a lot of towns in NSW “don’t have the resilience in their drinking water supplies to get through these sorts of scenarios.”
There are now concerns for Sydney’s water supply because of fires burning in the Lake Burragorang catchment. This lake sits behind Warragamba Dam and accounts for 80% of Sydney’s water supplies.
In relation to Sydney’s situation Stuart Khan is concerned about the impact of heavy rain in the catchment . He said, “The best case is we get gentle rain for weeks and months that allows some gentle regrowth.”
his article was originally published in the VOICES FOR THE EARTH column in The Daily Examiner on December 23, 2019