COLORADO—Recent rainstorms have brought some relief to fighting the megafires that have been burning in Southwestern Australia since September 2019, but now erosion threatens drinking water supplies. Erosion becomes a concern after brushfires when rains hit and flow across bare soils in forest floors. These soils are baked from fires and become water repellent, causing the surface runoff to carry loads of sediment and ash. Scientists at the Rocky Mountain Research Station are helping Australian municipalities predict and contend with post-fire water quality problems.
“Sydney’s watersheds burned extensively and now they have major concerns about water quality,” says Pete Robichaud, a Rocky Mountain Research Station research engineer based in Moscow, Idaho. “Once the fire’s out the trouble is not over, longer term impacts may last for years.”
The New South Wales Water authority, which manages Sydney’s water supply, reached out to Robichaud to help with Australia’s latest round of large fires. He had previously supported efforts in Australia to help mitigate the impacts of sediment and erosion following fires in 2009.
“Wildfire is no longer only a domestic issue, it’s a global concern,” Robichaud says. “In the last five years, every continent except Antarctica has had problems with fire and post-fire water quality.”
Robichaud helped to develop the USDA Forest Service Watershed Erosion Prediction Project, a suite of models designed to estimate erosion processes on hillslopes and in small watersheds taking into account climate, topography, land use, disturbances (such as fire), vegetation and soil properties. Robichaud is using the models to help specialists in Australia estimate how much sediment is expected to flow into the streams and reservoirs that support municipal water supplies. The models will help managers prioritize their post-fire responses by identifying areas on the landscape that are most vulnerable to erosion and identifying where to place erosion controls such as wood mulch from burned trees to slow surface runoff.
Big pulses of nutrients like nitrogen and phosphorous carried by the sediment and ash can create algal blooms in municipal reservoirs. These algal blooms can cause significant taste and odor problems and increase treatment expenses. Wildfire ash often contains high amounts of nutrients, heavy metals and contaminants. These issues not only adversely affect water quality but can also clog treatment systems. It may take years for ash to settle out depending on rain events.
“When water treatment plants were built, these kinds of large fires were not a consideration,” Robichaud says. “Dealing with large amounts of ash is new.”
Robichaud is part of an international team of scientists who are developing the first ash transport model. It’s designed to predict how much ash is present after fire, how much will be transported into streams, how much stays suspended once it reaches a reservoir and how it will affect water quality.
Researchers at the Rocky Mountain Research Station have a long history of collaborating with scientists throughout the western United States and applying knowledge globally to support fire recovery.
“We are excited to work with universities here in the U.S. and globally,” Robichaud says, “because now, more than ever, tackling these bigger problems requires a diverse team.”