Background

Impacts of Wildfire on Water Quality

Water quality may be directly affected as a result of changed water balance, replacement of the forest litter by an ash/charcoal layer, and enhanced washoff of materials into streams. Washoff carries solids (charcoal, soil particles and clay) and dissolved materials (nutrients, dissolved organic matter) into streams and reservoirs. The extent to which washoff occurs in different catchments depends on their hydrological characteristics, and on the sequences of rainfalls that occur after the fire. Consequently, transfer of conclusions from one catchment to another is difficult, even though the same principles apply.

Washoff from burnt forests contains mineral clay particles that result in turbidity (cloudiness) and a host of other constituents that alter the chemistry of water. Both independently affect water quality, and interact with aquatic biota in complex ways that can have highly variable effects on stream health. The significant changes to be expected in water quality are described below.

The impacts on wildfire on water quality in streams can be of shorter duration and quite different than in major water-supply reservoirs. If aquatic habitat in streams is changed as a result of fire (e.g., by massive sediment deposits), then stream water can also be affected in the long term. Therefore, we first outline some important impacts of fire on the quality of water in streams and then how impoundments and other sources for water supply are affected. Aquatic habitat and environmental flows are considered separately.

The Influence of Fire Intensity on Stream and Reservoir Water Quality.

Low-intensity fires which do not burn the crown of the forest lead to leaf fall shortly after the fire. The first post-fire rains leach organic material out of these fallen leaves and deliver relatively large concentrations of dissolved organic matter (DOC) to the storage. In addition, large amounts of leaf litter may be delivered. Much of this organic matter is readily degraded microbially, with the concomitant consumption of oxygen. The resultant anoxia leads to elevated manganese concentrations, formation of reduced sulphur compounds with associated taste and odour problems, and water discolouration (so called "black water"). All these factors require additional treatment measures. The high DOC increases the chlorine demand in the water and, if chlorine is used as the disinfectant, much higher concentrations of trihalomethanes (THM) in the water, with consequent health risk. It is also possible that the water will contain relatively high concentrations of nitrate.

High intensity fires which burn most of the above-ground organic matter, or where fires occur on pasture land, lead to a different suite of water quality consequences. Most of the organic matter is removed in the fire through volatilisation. Much of the inorganic nutrients originally contained within the leaves or grass are leached out in the first post-fire rainfalls, and pass into the soil. Washoff of ash and fine soil particles delivers higher concentrations of phosphorus (but low concentrations of nitrogen) to the receiving waters. As a consequence, there will be a higher risk of blue-green algal blooms if the water enters the surface layer of a water storage reservoir. In agricultural areas, or where the degree of land disturbance is high, there can be severe local erosion and debris transport into streams exacerbating the problems noted above.

Key reference

• Chessman, 1986

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Influence of Town Water Source on Quality

Water supplies for towns are commonly withdrawn from weirs on streams with little if any significant storage, from sandbeds or aquifers adjacent to streams, or from reservoirs capable of holding a year or more's water supply. These may or may not incorporate filtration treatments before the water is reticulated. The impact of fire on water quality for these town water supplies will therefore depend on the degree of water treatment that is available, and the characteristics of water impoundments.

Quality of water from "Run of river" sources will suffer only in the short term, until the first flushes of contaminated streamflows pass by, although these impacts can be severe locally, particularly where upstream disturbance is considerable, such as in agricultural areas. Little can be done to improve the quality of this water, except to remove large organic debris and increase the dosage of disinfectant (chlorination) to counteract the presence of higher levels of turbidity and organic matter. Water withdrawn from sandbed or other aquifers will probably suffer an undetectable decline in water quality.

Reservoirs fed directly by streams from burnt catchments, such as the Burrinjuck reservoir in NSW or the Corin and Bendora reservoirs in the ACT, may experience more severe water quality problems. Most storages deeper than about 10 metres are stably stratified. Consequently, moderate inflows of water enter the water column of the storage at a depth where their density is the same as the surrounding waters. Thus colder flows go to the bottom and warmer flows will skate across the surface. Flows which have a high DOC load which enter at intermediate depths are essentially cut off from supply of oxygen, so the resulting anoxia will be more pronounced than if the contaminated water enters at the surface. For nutrient laden waters from high intensity fire sites, the risk of algal blooms is diminished if the water enters deep within the storage. If it enters the surface layer the algal bloom risk is enhanced. Very large flows will cause complete overturn of water in the reservoir and mixing of the "new" and "old" waters.

The position of the layer of contaminated water within a storage should be established, and, if it is possible, the position of the water offtake changed to avoid taking the lower quality water into the drinking system. Storage managers need to be alert to the possibility of seiching within the storage leading to oscillation of the depth of the contaminated layer at the offtake. This leads to fluctuations in quality of the water being withdrawn, and makes smooth operation of treatment plants more difficult.

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