Effects of wildfire on mercury mobilisation in eucalypt and pine forests

Abstract

Wildfires and subsequent rainfall play an important role in the redistribution of major, minor and trace chemical elements, which could be mobilised from burnt soils and ashes. In particular, the fire-induced release of mercury (Hg) into the environment is relevant to study due to its volatilisation and toxicity. However, the impact of wildfire followed by rainfall on mercury redistribution is poorly documented. Hence, eucalypt-and pine-covered hillslopes (1-3) were surveyed in two burnt areas (Ermida and S. Pedro do Sul, Portugal), as well as in a nonburnt eucalypt slope (Ermida). Top-soils (0-2 cm) and ashes were sampled 4 and 14 weeks after the fire, the latter following an episode of heavy precipitation. In order to clarify the effect of temperature on Hg release from the soil, a heating experiment was also conducted. Results revealed that 30% of the Hg retained in eucalypt soils was released by the fire, corresponding to a loss of 1.0-1.1 g Hg ha-1 of burnt soil. Levels in burnt eucalypt soils were twice the values registered in burnt pine soils for both fire severities. Mercury in ashes pointed to similar trend between the two types of vegetation. Results from the heating experiment with soil suggest that temperature reached in the wildfire was insufficient to destroy the most stable bonds of Hg-ligands. Mercury concentrations were not correlated with organic matter content, both in soils and ashes. It was estimated that rainfall caused a loss of 1.0 g Hg ha-1 from ashes and an uptake of 0.5 g Hg ha-1 of soil washed out. The difference between the previous estimations, 0.5 g Hg ha-1 , corresponds to the quantity of Hg transported to the surrounding area or eventually introduced into aquatic systems. Hence, the results obtained in the present study highlight the importance of wildfire and subsequent rainfall in the mobilisation of Hg in the environment.

Assessing the adequacy of SWAT model to simulate postfire effects on the watershed hydrological regime and water quality

“…Fire severity is a measure of the physical changes caused by fires and expresses the immediate effects on vegetation and soil (Keeley, ), being usually categorized as ‘low', ‘moderate', and ‘high'. One of the most detrimental off‐site effects from wildfires is the transport of ash and sediment into streams and reservoirs, which become contaminated with several toxic compounds attached to the eroded particles (Campos et al, ; Campos, Vale, Abrantes, Keizer, & Pereira, ; Nunes, Naranjo Quintanilla, et al, ) or susceptible to the formation of algae blooms due to an increase in the nutrient load, particularly nitrogen and phosphorus (Smith et al, ). Nutrient load increase due to the impact of wildfires can vary markedly among sites, ranging from a slight increase (0.3–2‐times unburned) to a substantial increase (20–431‐times unburned), depending on vegetation and soil type, burned area, and fire severity (Smith et al, ).…”

Fire Promotes Arsenic Mobilization and Rapid Arsenic(III) Formation in Soil via Thermal Alteration of Arsenic-Bearing Iron Oxides

“…Importantly, fires can also enhance trace metal/metalloid mobilization from burnt soils, altering soil properties and potentially degrading surface water quality in burnt catchments (Bladon et al, 2014;Abraham et al, 2017;Burton et al, 2019). An increasing body of field-based research provides examples of elevated trace metal/metalloid concentrations in soils and surface waters after wild-fires (e.g., Stein et al, 2012;Burke et al, 2013;Odigie and Flegal, 2014;Campos et al, 2015Campos et al, , 2016Burton et al, 2016;Abraham et al, 2018). However, in general there remains a distinct lack of mechanistic studies exploring fire-induced geochemical changes to metal/metalloid speciation or to the mineralogy of their host-phase(s) and how this may be contributing to observations of enhanced trace metal/metalloid mobility (Cerrato et al, 2016).…”

Origin, Reactivity, and Bioavailability of Mercury in Wildfire Ash

“…On the basis of color and percent loss-on-ignition (LOI), 11,13 wildfire ash can be operationally divided into two major classes: black ash (BA; low intensity fire; 200−500°C) and white ash (WA; high intensity fire; > 500°C). 13 However, it should be noted that within each class, ash may consist of a mixture of materials with contrasting mineral and organic matter contents.…”