Bushfires and water quality

​​​​​​​​​​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.

Use this guidance to help you understand and manage the effect of bushfires on water quality in Australia.

Effects of bushfires on water quality

Degree to which water quality is affected by fire depends on factors such as:

  • geographical features and size of the catchment
  • size and extent of the fire
  • time period between the last fire and a significant rainfall event
  • type of surrounding vegetation, soil and erosion.
Waterway in Myrtleford, Victoria, following a bushfire. Depicts loss of vegetation and increased debris clogging the waterway as a result of the fire.
Myrtleford, Victoria. Irene Dowdy

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. The loss of riparian vegetation may result in high volumes of sediment (measured as turbidity) entering the stream and may also increase stream temperatures due to a lack of shade.

Chemical reactions triggered by fire can release nutrients, metals and other toxicants stored in vegetation and soil. Rainfall after a fire washes these contaminants into waterways and reservoirs, which can have substantial implications for agriculture, human safety and amenity.

Use of affected water may be unsafe for agriculture or human consumption without additional treatment or alternative water sources may have to be found.

Local food chain can also be affected by loss of riparian vegetation after a fire, which leads to:

  • higher water temperatures
  • increased light availability
  • loss of habitat
  • reduced protection from predators for instream biota.

Combined with increased contaminant loading, 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 result of this sudden depletion of dissolved oxygen.

Possible post-fire changes to a water catchment
Fire results in burnt vegetation, ash and debris, decreased transpiration, decreased soil cover and increased soil evaporation. The loss of vegetation may result in increased rainwater run off and infiltration, increased debris flow and total discharge in waterways, also increasing stream volume and power and sediment transfer.

Managing the effects of bushfires on water quality

Effective, consistent communication strategies are essential for protecting catchments and water supply and treatment infrastructure from the effects of bushfires.

After a bushfire and before the onset of rain, immediate actions to protect water catchments should include:

  • establishment of water quality monitoring programs
  • rehabilitation of control lines and access tracks
  • 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.

National guidance and strategy

Coordination, policy advice and funding assistance is sometimes provided by Australian Government agencies during and after bushfires.​

The National Water Quality Management Strategy (NWQMS) is a framework for setting water quality objectives and implementing preventative and rehabilitation actions which can be tailored to individual environments.

Safe levels of contaminants in water are specified in national guidance including:

These guidelines are often adopted by state and local governments in their regulations and management approach.

Land and water managers can also use eWater guidance on managing bushfires and water catchments. eWater is an Australian Government not-for-profit organisation.

Recommendations for rehabilitation of water supplies after bushfires were made as part of a ​literature review of the effects of bushfires on water quality, conducted by University of Melbourne.

Regional approach

State, territory and local governments are responsible for planning for bushfire situations, including appropriate risk management, early detection systems and public warnings.

Controlled burning prior to predicted fire conditions can reduce the intensity and extent of bushfires by decreasing vegetation fuel loads.

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.

Resources for regional bushfire management:

Glossary

Biodiversity: the range of interrelated plant and animal taxa and the habitat in which they live.
Blackwater: 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: specific composition of animals, plants and micro-organisms which interact with one another and their environment.
Hydrology: 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.

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