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Landscape scale fires in forested south-eastern Australia have critically impacted a range of catchment values, including water quality, and heightened community concern about fire management (Ellis et al., 2004; Parliament of Victoria, 2008; Victorian Bushfires Royal Commission, 2009). While extended drought has increased the frequency and scale of fire, severe follow-up storms have resulted in large quantities of sediment, nutrients, organic matter, ash and metal contaminants entering streams and reservoirs. This change in water quality may result in water supplies that are unfit for use (White et al., 2006; Smith et al., 2011). The objective of this report is to review literature on the impact of bushfires on water quality, including recommendations for management actions immediately before, during and after a bushfire. A major focus of the report is on how various uses and values of water may be impacted by fire-related changes to water quality.
Several factors may contribute to water quality impacts following fire. Rates of runoff and erosion often increase as a result of landscape disturbance, particularly on the soil surface – i.e. increased soil water repellence, loss of surface vegetation and canopy cover, and ash sealing of soil pores (Shakesby and Doerr, 2006; Sheridan et al., 2007b). Combustion of organic matter, soil heating and the production of ash and charcoal contribute to the release of numerous nutrients, metals and toxins that might otherwise be unavailable for transport into waterways. For example, ash contains particulate carbon, various nutrients, trace metals and other contaminants (Amiro et al., 1996; Goforth et al., 2005; Johansen et al., 2003). The loss of riparian vegetation reduces the buffer effect that traps sediment before it enters streams and means that there is less shade to prevent increases in stream temperatures (BAER, 2009). Fire suppression activities may also contribute to water quality impacts, particularly the construction of control lines with bulldozers and possibly the use of fire retardants and fire suppressant foams (Boulton et al., 2003; BAER, 2009).
The impact of bushfires on water quality can be highly variable for many of the individual water quality constituents (Smith et al., 2011). This variability is caused by a number of landscape influences and climatic factors, most notably rainfall. High magnitude and intensity rainfall events soon after fire generate the largest impacts on water quality and sometimes trigger extreme erosion events (e.g. localised flash floods, large floods and debris flows). For example:
two large storm events eroded most of the annual sediment yield in two small headwater catchments of the East Kiewa River in the first year after fire in north-eastern Victoria (Lane et al., 2006)
a very high intensity, short duration storm event in the burnt Upper Buckland River catchment (north-east Victoria) generated debris flows resulting in very high sediment concentrations (59,000 mg L-1 or 129,000 NTU – Nephelometric Turbidity Units) (Leak et al., 2003) and dissolved oxygen concentrations near zero levels (EPA 2003)
a large rainfall event in the burnt catchment area of the Gippsland Lakes in eastern Victoria caused flooding which resulted in elevated nutrient concentrations and a prolonged blue-green algae (cyanobacteria) bloom (Cook et al., 2008), and
an intense summer storm in the catchment area for the Ovens River in north-eastern Victoria resulted in concentrations of iron, copper, zinc, chromium, arsenic and lead that were 47, 32, >50, 40, 4 and 33 times the pre-event concentrations, respectively (North East Water, 2003).
Debris flows in burnt forest environments are an emerging area of research in Australia and have only recently been recognised as a major contributor to water quality impacts following fire (Nyman et al., 2011). Occurring in steep, upland terrain they are a fast moving mass of unconsolidated saturated debris, which cause large amounts of channel scour and deliver large quantities of sediment downstream.
For some of the water quality parameters there is very little information available, which makes it difficult to draw conclusions about bushfire impacts. For example, there is little Australian literature on the effects of fire on bulk water chemistry, such as for levels of chloride and sulfate. Limited research from North America on these anions suggests that only small impacts from fire may occur (Gallaher et al., 2002; Malmer, 2004; Mast and Clow, 2008). However, it is difficult to extrapolate these findings to the Australian context given the differences between North American and Australian forest environments. Similarly, there is very little information on stream temperatures or concentrations of fire pyrolysis products such as cyanides, polycyclic aromatic hydrocarbons (PAHs), polychlorinated dibenzo-ρ-dioxins and dibenzofurans (PCDD/Fs), and polychlorinated biphenyls (PCBs).
In relation to water quality impacts within reservoirs, the extent of post-fire water quality changes will reflect the magnitude of pollutant loads entering the reservoir relative to its capacity to attenuate impacts (reflecting the reservoir size, storage levels after the fire, and the extent of stratification). Additionally, benthic sediments within storages receiving large amounts of organic matter can release much larger than normal amounts of methane, ammonia, phosphorus, sulfide, arsenic, iron and manganese. Numerous reservoirs have experienced substantial water quality impacts as a result of fire. Examples include the Bendora Reservoir in the ACT (White et al., 2006), Lake Glenmaggie in eastern Victoria (Goulburn Murray Water, 2008) and Lake Buffalo in north-eastern Victoria (Goulburn Murray Water, 2008). Yet other reservoirs have shown little to no water quality changes despite high constituent loads in tributary streams (e.g. Dartmouth reservoir in north-eastern Victoria (Alexander et al., 2004) and Mount Bold reservoir in South Australia (Morris and Calliss, 2009)).
The constituents of most concern following fire from a drinking water perspective are elevated levels of suspended sediment, nutrients and metals, while possible blooms of cyanobacteria after fire also present a threat. For example, in the Bendora Reservoir (supplying Canberra) after the 2003 ACT fires, turbidity was 3000 NTU at the bottom and iron and manganese concentration levels exceeded all previous peaks by factors of three and four, respectively. This necessitated the introduction of water restrictions in Canberra (White et al., 2006). For other constituents (e.g. sulfate and chloride, organic carbon, cyanide, PAHs, PCDDs/Fs and PCBs), post-fire levels generally do not exceed guideline values. However, with such a limited number of studies available for these constituents, it is difficult to draw conclusions about their impacts on drinking water quality following fire.
Extremely high turbidities and low dissolved oxygen concentrations resulting from large post-fire inputs of sediment and ash to streams pose the greatest threat to aquatic ecosystems following fire. For example, there was a large decline in the abundance of fish populations following a post-fire debris flow in north-eastern Victoria (Lyon and O’Connor, 2008). Other threats may result from increased water temperatures and increased inputs of nutrients. Despite the immediate impacts, aquatic ecosystems in Australia are quite resilient to major disturbance events such as fire, and often populations of aquatic fauna recover quickly provided there is connectivity between affected and unaffected habitats (Lyon and O’Connor, 2008). Those species that are more vulnerable tend to have smaller, more isolated populations or are not as well-adapted to survive periods with elevated suspended sediment concentrations.
For recreation and aesthetic values of water, elevated levels of suspended sediment and blooms of cyanobacteria are of most concern after fire. Suspended sediment affects the visual clarity of the water and although this constituent has no guideline value, acceptability thresholds would have been crossed during some events described in this review. Cyanobacteria are a potential health risk and therefore if a bloom is triggered following a fire, recreational use of the water may be affected.
The major water quality concerns for agriculture following fire relate to high loads of suspended sediment and, to a lesser extent, nutrients, metals, cyanobacteria, chloride and sulfate. The primary concern with suspended sediment is the potential for increased sedimentation of dams. Guideline values for agriculture in relation to phosphorus, nitrogen and various metals were not exceeded except following an extreme erosion event after fire in the Upper Buckland River (north-eastern Victoria). Similarly, in the few studies that showed some elevation of chloride and sulfate levels, ions that can affect some crops such as spray irrigated citrus, levels remained well below the guideline values.
There are numerous management actions that can be taken prior to, during and after a fire to minimise the impacts on water quality. Before the fire there can be preparations to reduce the risk of fire in water supply catchments (such as fuel reduction burning, the construction of fuel breaks and a greater preparedness for early fire suppression), as well as risk assessments to identify townships whose water supply is most vulnerable were a bushfire to occur. Operations during a fire should adhere to the best practices and local standards in relation to the construction of mineral earth breaks and the application of fire retardants. Following fire, managers need to act quickly to prepare an area before it rains and particularly before intense rain, although in many landscapes this may not be feasible or may conflict with other values ascribed to the catchment. The first task is to undertake a rapid assessment of the burnt area to identify priorities. Then various works such as rehabilitating control lines, sediment control measures, erosion mitigation works and water quality monitoring may commence. Development of integrated fire and water quality risk management plans would enable better coordination of all management actions designed to mitigate post-fire water quality impacts.
There are substantial knowledge gaps in our understanding of water quality impacts following bushfires in Australia. Greater understanding of these impacts from regions outside south-eastern Australia is required, while further research in south-eastern Australia is also needed, given that the steep, forested highland areas in central Victoria and NSW are particularly prone to large, severe bushfires. Expanded stream and reservoir monitoring before and after fires would provide important information on concentrations of constituents (particularly metals, organic carbon and nutrients) that are less frequently measured. Furthermore, greater knowledge of post-fire erosion processes and catchment sources of key constituents would contribute to development of models to quantify post-fire risks to water quality, as well as guiding post-fire management actions aimed at reducing water quality impacts.
1 Introduction 5
1.1 Preamble 5
1.2 Report objectives 6
1.3 Scope and limitations 6
1.4 Uses and values of water and associated guidelines for water quality 6
2 Processes that may cause changes to water quality following fire 8
2.1 Runoff and erosion 8
2.2 Release of nutrients, metals and toxins 9
2.3 Riparian disturbance 9
2.4 Fire suppression and post-fire salvage harvesting 10
3 Potential changes to water quality following fires 10
3.1 Suspended sediment 10
3.2 Ash 15
3.3 Nitrogen and phosphorus 15
3.4 Metals 19
3.5 Other constituents 19
3.6 Dissolved oxygen 21
3.7 Cyanobacteria 22
3.8 Temperature 22
3.9 Water availability and flow 22
4 How changes to water quality may affect the major uses and values of water 23