Quantity, composition and water contamination potential of ash produced under different wildfire severities

Abstract

Wildfires frequently threaten water quality through the transfer of eroded ash and soil into rivers and reservoirs. The ability to anticipate risks for water resources from wildfires is fundamental for implementing effective fire preparedness plans and post-fire mitigation measures. Here we present a new approach that allows quantifying the amount and characteristics of ash generated under different wildfire severities and its respective water contamination potential. This approach is applied to a wildfire in an Australian dry sclerophyll eucalypt forest, but can be adapted for use in other environments. The Balmoral fire of October 2013 affected 12,694 ha of Sydney's forested water supply catchment. It produced substantial ash loads that increased with fire severity, with 6, 16 and 34 Mg ha(-1) found in areas affected by low, high and extreme fire severity, respectively. Ash bulk density was also positively related to fire severity. The increase with fire severity in the total load and bulk density of the ash generated is mainly attributed to a combination of associated increases in (i) total amount of fuel affected by fire and (ii) contribution of charred mineral soil to the ash layer. Total concentrations of pollutants and nutrients in ash were mostly unrelated to fire severity and relatively low compared to values reported for wildfire ash in other environments (e.g. 4.0-7.3mg As kg(-1); 2.3-4.1 B mg kg(-1); 136-154 P mg kg(-1)). Solubility of the elements analysed was also low, less than 10% of the total concentration for all elements except for B (6-14%) and Na (30-50%). This could be related to a partial loss of soluble components by leaching and/or wind erosion before the ash sampling (10 weeks after the fire and before major ash mobilisation by water erosion). Even with their relatively low concentrations of potential pollutants, the substantial total ash loads found here represent a water contamination risk if transported into the hydrological network during severe erosion events. For example, up to 4 Mg of ash-derived P could be delivered into a single water supply reservoir.

At the nexus of fire, water and society

“…During post-fire floods, streams can deliver bedload (generally coarse sediment), suspended sediment (turbidity) and debris to reservoirs. Post-fire inputs of chemical constituents derived from ash and the underlying soil can lead to long-term effects on water-storage reservoirs [62]. It has been estimated that there are 16.7 million reservoirs globally [63].…”

Using Thermogravimetry as a Simple Tool for Nutrient Assessment in Fire Affected Soils

“…De la Rosa et al () have suggested that WLs at the refractory pool correspond mainly to charred organic residues, whereas Santín et al () reported that the production of pyrogenic OM has its maximum at temperatures over 600 °C. Several studies have previously suggested a direct impact of ash on soil nutrient content, basing this hypothesis on laboratory or field leaching experiments (Marion et al, ; Molina et al, ; Escudey et al, ) or studies of ash composition (Khanna et al, ; Pereira et al, ; Santín et al, ) and post‐fire soil nutrient content (Khanna & Raison, ; Andreu et al, ; Dorta Almenar et al, ). However, unlike these previous studies, we demonstrate a direct quantitative link between the content of charred organic residues in the soil (represented by the SOM refractory pool) and soil nutrient values using a field approach and samples affected by a wildfire.…”

Laboratory simulation of postfire effects on conventional drinking water treatment and disinfection byproduct formation

“…The materials were air dried (Bodí, Mataix-Solera, Doerr, & Cerdà, 2011) at room temperature (20 C), mixed, and weighed daily until constant mass was reached. The sampling approach was similar to the method used by Santín, Doerr, Otero, and Chafer (2015) in a wildfire ash and laboratory heating study. Soils were sifted through 6.25-mm mesh to remove rocks and large detritus and then passed through a #20 sieve (0.841 mm).…”