Wildfires, both natural and man-made, release and mobilize hazardous substances such as heavy metal(loids) (HM), which are known carcinogens. Following intense rainfall events, HM bound to soil organic matter are transported from the soil to surface water, resulting in water quality degradation. This study reviews the pollution status of HM in wildfire-affected soil and surface water, as well as their toxic effects on aquatic organisms and humans. The rate of HM release during wildfires depends on factors such as the type of tree burned and fire severity. The mobility of HM from soil to surface water is influenced by soil pH, organic matter content, rainfall intensity, and duration. The risk priority number (RPN) analysis indicates that both wildfire-affected soil and surface water require remediation to address HM contamination. HM concentrations in both soil and surface water decrease over time due to soil erosion, wind, storm events, and the depletion of burnt residues. The greatest percentage changes in HM concentrations in burned soils compared to unburned soils were observed for vanadium (340%), nickel (260%), and arsenic (110%). In surface water, the highest increases were seen for iron (740%), vanadium (530%), and aluminium (510%). Wildfire-affected water has been shown to cause toxic effects in aquatic organisms, including DNA damage, oxidative stress, and lipid peroxidation. The consumption of HM-contaminated water and fish poses significant health risks to humans. Therefore, post-fire monitoring of wildfire-affected areas is essential for designing treatment plants, assessing risks, and establishing maximum allowable HM concentrations in water.

Understanding the processes of mobility and availability of potentially toxic elements in soil is crucial for informed decision-making in the development of public policies aimed at minimizing environmental impacts. Monitoring, in combination with the determination of natural concentrations, can provide effective tools for controlling pollution sources. In this study, enrichment, pollution, and ecological risk indices were used for some potentially toxic elements in an anthropogenically influenced watershed in southwestern Bahia, Brazil. The study involved 63 composite surface soil samples collected from areas with natural forest, crops, pastures, and urbanization. The samples were analyzed for fertility and particle size. Metal extraction followed the EPA 3051A method, and element determination was carried out via ICP-OES. The soils in the Verruga River watershed exhibit a high variability in fertility and granulometric attributes. The Kruskal-Wallis test at a 5% significance level was employed to assess the impact of land management on the availability of elements (As, Co and Pb), while Spearman's correlation, along with hierarchical clustering analysis, was used to comprehend element dynamics. Geostatistics were applied to identify pollution hotspots. Consequently, it became evident that potentially toxic elements can accumulate in the soil depending on land use and management practices (As, Co, and Pb), as well as the weathering process linked to the type of source material, such as diamictite deposits (Ni and Co). Soils in the Verruga River watershed qualify as having minimal enrichment, low pollution levels, and individual ecological risk concerning Cd. The percentage of samples enriched with Cu, As, Zn, and Cd exceeded 67%, with agricultural activities being the primary source of pollution. Meanwhile, in pasture and urban areas, Co and Pb were notably prominent, respectively.

Soils play an essential role in supporting and sustaining life on this planet. In fire-impacted environments, fire causes considerable changes to the soil, especially in the various elements. The present work provides a comprehensive and up-to-date review of the effect of fire on soil geochemistry, and its impact on the cycling of different biogenic, major, minor, and trace elements in the soil. Results from both natural and experimental fires (field-scale and lab-scale) are considered in this review. The temperature at which mineral transformation occurs in the soil during fires is summarised. The review suggests that fires can significantly alter mobility and hence, the cycling of many elements in fire-affected regions. Change in speciation of elements following fires risks formation and/or increased availability of the toxic forms of elements in the soil. The unique physical, chemical, and biological conditions observed during fires make many unlikely reactions more likely. However, the information available in the literature is often fire, vegetation, and element specific. More studies on this topic by changing these three variables will improve our understanding of changes in the soil caused by fire. Hence, with fires being touted to increase global presence in the coming years, more studies on understanding their effects on soils are recommended.

The presence of toxic substances is one of the major causes of degradation of soil quality. Wildfires, besides affecting various chemical, physical, and biological soil properties, produce a mixture of potentially toxic substances which can reach the soil and water bodies and cause harm to these media. This review intends to summarise the current knowledge on the generation by wildfires of potentially toxic substances, their effects on soil organisms, and other associated risks, addressing the effects of fire on metal mobilisation, the pyrolytic production of potentially toxic compounds, and the detoxifying effect of charcoal. Numerous studies ascertained inhibitory effects of ash on seed germination and seedling growth as well as its toxicity to soil and aquatic organisms. Abundant publications addressed the mobilisation of heavy metals and trace elements by fire, including analyses of total concentrations, speciation, availability, and risk of exportation to water bodies. Many publications studied the presence of polycyclic aromatic hydrocarbons (PAH) and other organic pollutants in soils after fire, their composition, decline over time, the risk of contamination of surface and ground waters, and their toxicity to plants, soil, and water organisms. Finally, the review addresses the possible detoxifying role of charcoal in soils affected by fire.

The surroundings of mines and smelters may be exposed to wildfires, especially in semi-arid areas. The temperature-dependent releases of metal(loid)s (As, Cd, Cu, Pb, Zn) from biomass-rich savanna soils collected near a Cu smelter in Namibia have been studied under simulated wildfire conditions. Laboratory single-step combustion experiments (250-850 °C) and experiments with a continuous temperature increase (25-750 °C) were coupled with mineralogical investigations of the soils, ashes, and aerosols. Metals (Cd, Cu, Pb, Zn) were released at >550-600 °C, mostly at the highest temperatures, where complex aerosol particles, predominantly composed of slag-like aggregates, formed. In contrast, As exhibited several emission peaks at ~275 °C, ~370-410 °C, and ~580 °C, reflecting its complex speciation in the solid phase and indicating its remobilization, even during wildfires with moderate soil heating. At <500 °C, As was successively released via the transformation of As-bearing hydrous ferric oxides, arsenolite (As₂O₃) grains attached to the organic matter fragments, metal arsenates, and/or As-bearing apatite, followed by the thermal decomposition of enargite (Cu₃AsS₄) at >500 °C. The results indicate that the active and abandoned mining and smelting sites, especially those highly enriched in As, should be protected against wildfires, which can be responsible for substantial As re-emissions.

The increasing frequency and severity of wildfires poses human health risks, especially for those within burnt regions. The potential health effects of fire ash on farmworkers in orchards via inhalation exposure after fire is rarely studied. After the 2017 Thomas Fire, in Ventura County (California, USA), fire ash and corresponding soil samples were collected from several impacted orchards and analyzed for eight trace elements (TEs) and 16 polycyclic aromatic hydrocarbons (PAHs). Results indicate that except for mercury (Hg), the concentrations of TEs and PAHs were higher in ash samples compared with the corresponding soil samples. In general, ash samples showed greater potential to expose farmworkers to health risks than the corresponding soil samples. One site had particularly high concentrations of As (778 mg kg^-1), Cr (629 mg kg^-1), and Cu (499 mg kg^-1) in the ash. This location corresponds to a house which was burned during the Thomas Fire, which might have contained chromated copper arsenate as a wood preservative. Therefore, the existence of construction materials in orchards could add hazardous materials to ash deposited on soil. Furthermore, a monitored dust generation experiment was designed to obtain the particle emission factors (PEF) of soil and ash, which is an essential parameter for the calculation of inhalation health risks. A two-fold difference in the PEFs was found between ash and the corresponding soil sample. Hence, health risks through inhalation exposure from fire ash may be underestimated if the default PEF suggested by the US Environmental Protection Agency is used.