Bushfires and water quality fact sheet

Department of Sustainability, Environment, Water, Population and Communities, 2012

Bushfires have the potential to degrade water quality and alter the dynamics of stream ecosystems in many complex ways.

Most critical effects occur if there is heavy rain soon after fire as loss of vegetation and altered soil structure can make fire-affected soils more erodible. Runoff can carry sediments and pollutants that affect aquatic environments, drinking water quality and agricultural industries.

Myrtleford, Victoria.
Irene Dowdy, MDBA.

How do bushfires affect water quality?

The extent to which water quality is affected by fire depends on factors such as: the size and extent of the fire; the type of surrounding vegetation, soil and erosion; the geographical features and size of the catchment; and the time period between the last fire and a significant rainfall event.

High intensity fires can cause enormous damage to water catchments by destroying ground-cover and changing hydrology, as well as altering the structure, behaviour and erosion of soil. Furthermore, the chemical reactions triggered by fire can release nutrients, metals and other toxicants stored in vegetation and soil. Post-fire rainfall has significant impacts on water quality as it often washes these contaminants into waterways and reservoirs. When this occurs, water may be unsafe for agriculture or human consumption without additional treatment or alternative sources of water. Poor water quality and loss of amenity can therefore have substantial financial implications.

The entire local food chain may be adversely affected by the loss of riparian vegetation after fire as it can lead to increased light availability, higher water temperatures, loss of habitat, and reduced protection from predators for in-stream biota. When combined with increased contaminant loading, the increased water temperature can trigger greater breakdown of organic matter by bacteria, which may deplete oxygen levels in the water. Fish suffocation is a common after-effect of fires as a result of this sudden depletion of dissolved oxygen.

Picture showing how organic material releases dissolved carbon, causing bacteria to multiply and deplete dissolved oxygen.

Possible post-fire changes to a water catchment

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Possible post-fire changes to a water catchment:

  • decreased transpiration
  • burnt vegetation, ash and debris
  • decreased soil cover
  • increased soil evaporation
  • increased rainwater run off and infiltration
  • debris flow and total discharge increase
  • increased stream volume and power
  • increased sediment

How can the effects of bushfires be managed?

State, territory and local governments are responsible for planning for bushfire situations, including appropriate risk management, early detection systems and public warnings. Effective and consistent communication strategies are essential for protecting catchments and water supply and treatment infrastructure.

Local water authorities are primarily responsible for ensuring the health of water catchments and are in charge of measures such as fuel reduction burns and creating fire breaks. Controlled burning prior to predicted fire conditions can reduce the intensity and extent of bushfires by decreasing vegetation fuel loads.

Actions to protect water catchments are very important in the immediate aftermath of a fire and before the onset of rain. Such actions include rehabilitation of control lines and access tracks; establishment of water quality monitoring programs; and sediment and erosion control to prevent debris being washed into water bodies. Soil erosion can be minimised after fire by pushing back top soil with heavy machinery, erecting silt fences and planting shrubs and trees to stabilise the soil structure.

Depending on the severity of the fire, freshwater catchments are usually naturally regenerated to pre-fire conditions within five to twenty years. Aquatic ecosystems are remarkably resilient and often recover quickly if there is connectivity between affected and unaffected habitats.

What is the Australian Government doing about the impacts of bushfires on water quality?

Managing and controlling bushfires is primarily the responsibility of state, territory and local governments. Australian Government agencies may act as central coordination points and provide policy advice and funding assistance. A recent literature review of bushfire impacts provides a range of recommendations for rehabilitation of water supplies and can be found at: www.environment.gov.au/water/policyprograms/nwqms/index.html.

The Australian and New Zealand Guidelines for Fresh and Marine Water Quality and Australian Drinking Water Guidelines specify safe levels of contaminants in water and are often adopted by state and local governments in regulations. Both of these documents are part of the National Water Quality Management Strategy, a framework for setting water quality objectives and implementing preventative and rehabilitation actions which can be tailored to individual environments.

Glossary

Biodiversity:
the range of interrelated plant and animal taxa and the habitat in which they live.
Blackwater:
a natural feature of lowland river systems during flooding when organic material from floodplains is consumed by bacteria, which leads to depletion of dissolved oxygen in water.
Ecosystem:
a specific composition of animals, plants and micro-organisms which interact with one another and their environment.
Hydrology:
the physical make-up of water and seasonal patterns of climate, temperature, rainfall, and flow.
Phosphorus and nitrogen:
chemical nutrients essential for growth and emitted by bushfire smoke and burnt organic matter. These nutrients can lead to blue-green algae outbreaks.
Sediment:
sand, clay, silt, pebbles and organic material deposited in water.