Wildfire effects on water quality in forest catchments: A review with implications for water supply

Abstract

Summary Wildfires burn extensive forest areas around the world each year. In many locations, fire-prone forest catchments are utilised for the supply of potable water to small communities up to large cities. Following wildfire, increased erosion rates and changes to runoff generation and pollutant sources may greatly increase fluxes of sediment, nutrients and other water quality constituents, potentially contaminating water supplies. Most research to date has focused on suspended sediment exports and concentrations after wildfire. Reported first year post-fire suspended sediment exports varied from 0.017 to 50 t ha −1 year −1 across a large range of catchment sizes (0.021–1655 km 2 ). This represented an estimated increase of 1–1459 times unburned exports. Maximum reported concentrations of total suspended solids in streams for the first year after fire ranged from 11 to ∼500,000 mg L −1 . Similarly, there was a large range in first year post-fire stream exports of total N (1.1–27 kg ha −1 year −1 ) and total P (0.03–3.2 kg ha −1 year −1 ), representing a multiple change of 0.3–431 times unburned, while NO 3 - exports of 0.04–13.0 kg ha −1 year −1 (3–250 times unburned) have been reported. NO 3 - , NO 2 - , and NH 3 / NH 4 + concentrations in streams and lakes or reservoirs may increase after wildfire but appear to present a generally low risk of exceeding drinking water guidelines. Few studies have examined post-fire exports of trace elements. The limited observations of trace element concentrations in streams after wildfire found high levels (well over guidelines) of Fe, Mn, As, Cr, Al, Ba, and Pb, which were associated with highly elevated sediment concentrations. In contrast, Cu, Zn, and Hg were below or only slightly above guideline values. Elevated Na + , Cl − and SO 4 2 - solute yields have been recorded soon after fire, while reports of concentrations of these constituents were mostly confined to coniferous forest areas in North America, where maximum sampled values were well below recommended limits. Likewise, reported wildfire effects on dissolved organic carbon were generally minor and elevated concentrations largely reflected background conditions. Available cyanide concentrations in small streams may approach levels of concern soon after fire, but increases are likely to be of short duration. Post-fire concentrations of polycyclic aromatic hydrocarbons in streams have been found to increase but remained within the recommended limit. Large increases in exports and concentrations of some constituents after wildfire indicate the potential for impacts on water supply from forest catchments. In response, various water treatment measures may be required and in the absence of adequate treatment facilities or alternative sources, water supplies may be vulnerable to disruption.