Impact of Bushfires On Water Quality

Key findings and conclusions

This review critically examines the Australian literature on the impact of bushfires on water quality. A core focus is on the uses or values of water and how fire-related changes to water quality may impact upon these values. Overall, it finds that water quality values are threatened especially in respect to suspended sediment, nutrients and metals. The key issues identified are:

•  Drinking water: Elevated levels of suspended sediment, nutrients and metals, as well as possible cyanobacteria blooms are of most concern. For other constituents, notably sulfate, chloride, organic carbon, cyanide, PAHs, PCDDs/Fs and PCBs, post-fire maximums generally do not exceed guideline values. However, with a limited number of water quality studies available, there remains considerable uncertainty regarding post-fire levels.
•  Aquatic ecosystems: Extremely high levels of turbidity and low dissolved oxygen concentrations resulting from export of fine sediment and organic matter present the greatest threat. Other adverse aquatic ecosystem impacts may be attributed to increased water temperatures and increased inputs of nutrients.
•  Recreation and aesthetics: Suspended sediment and cyanobacteria are probably of most concern.
•  Agriculture: High loads of suspended sediment and, to a lesser extent, nutrients, metals and cyanobacteria may have potential impacts on water quality values. Chloride and sulfate levels appear to remain well within the guideline value limits.
•  Cultural and spiritual values: While there are no specified guideline values, the perceived concerns are likely to be similar to those for recreation and aesthetics.
•  Industrial water: There are no guideline values for industrial water but sediment, nutrients and metals are of potential concern for food processing.
•  Water resource infrastructure: High loads of suspended sediment, metals, chloride and algal blooms are a concern for water resource infrastructure, particularly for irrigation systems and water treatment infrastructure. A greater threat is probably that of increased flooding damage.

These findings highlight the importance of high magnitude and high intensity post-fire rainfall events. Numerous studies have shown that high intensity rainfall events may result in the largest water quality impacts after fire. Various factors following fire contribute to water quality impacts (such as the loss of vegetative cover, low infiltration rates, very intense rain and steep terrain). Suspended sediment, nutrients and metals are of particular concern to water quality. These impacts can occur large distances downstream of the source and last for weeks or up to several years.Finally, the report considers management actions that can be taken before, during and after a fire to minimise the impacts on water quality, including:

•  Before the fire: preparations to reduce the risk of fire in the water supply catchments (such as fuel-reduction burning and the construction of fuel breaks) as well as risk assessments to identify townships whose water supply is most vulnerable were a bushfire to occur.
•  During the fire: adherence to best practice and local standards in relation to the construction of mineral earth breaks and the application of fire retardants.
•  After the fire: immediate deployment of resources to make a rapid assessment of the burnt area to identify priorities such as the rehabilitation of control lines and establishment of sediment control measures, erosion mitigation works and water quality monitoring.