Contaminants such as trace elements, radionuclides, asbestos and other contaminants occur naturally in soils due to geological and pedological processes without anthropogenic influence (ISO, 2015; Tian et al., 2017). In this instance, their concentration in the soil, at a given place and time, is called the natural background concentration (Reimann and Garrett, 2005). Natural soil background concentrations are variable, depending on the mineralogical composition of the soil parent material and on the pedogenetic (soil-forming) processes (Kabata-Pendias and Pendias, 2001; Wilson et al., 2008). Organic contaminants such as polycyclic aromatic hydrocarbons can also occur in the environment as a result of natural forest fires (Kim, Choi and Chang, 2011).
Some elements are essential micronutrients to soil microorganisms, plants, and animals, such as iron (Fe), copper (Cu), zinc (Zn), manganese (Mn), nickel (Ni), boron (B), selenium (Se) and molybdenum (Mo) (Viets, 1962), while other elements have no known metabolic function, such as lead (Pb), cadmium (Cd) and mercury (Hg) (Eisler, 2006; Flora, Gupta and Tiwari, 2012). When there is no anthropogenic influence, trace elements are normally present in low concentrations and do not pose problems for the environment or human health (Alloway, 2013a). However, certain types of rocks have particularly high concentrations of trace elements (Table 1) and are highly toxic to humans and other species. Weathering and pedogenetic processes may lead to increased concentration in soils (do Nascimento et al., 2018; Shacklette and Boerngen, 1984). For example, the occurrence of geogenic arsenic (As) occurs as a result of weathering of arsenic-containing minerals and ores (Díez et al., 2009), and from naturally occurring mineralized zones of arsenopyrite (gossans) that are formed by the weathering of sulphide-bearing rock (Scott, Ashley and Lawie, 2001) or concentrated through geothermal activity. Soils in close proximity to ore deposits for a variety of trace elements may have much higher concentrations of those elements as compared to background levels. Alloway (2013) has published a detail review of the presence of trace elements in soils and their bioavailability.
Volcanoes are major sources of elements such as aluminium (Al), copper, fluorine (F), nickel, lead, manganese, mercury and zinc, spreading them from high crust depths or the mantle to both atmosphere and soil as a result of lava, ash and gas emissions (Nagajyoti, Lee and Sreekanth, 2010; Seaward and Richardson, 1989; Vigneri et al., 2017). Basaltic volcanic rocks of Réunion island are well-documented parent material with naturally high content of trace elements. The soils developed from these volcanic rocks have high natural background levels of mercury, chromium (Cr), copper, nickel and zinc (Dœlsch, Saint Macary and Van de Kerchove, 2006; Dœlsch, Van de Kerchove and Saint Macary, 2006). Indonesian volcanic soils and Japanese volcanic soils also have high concentrations of chromium and nickel (Anda, 2012; Takeda, Kimura and Yamasaki, 2004).
There is a wide range of radioactive elements that occur naturally and cause human health and environmental concerns (Cygan et al., 2007), but radon (Rn) represents the largest natural radiation dose to humans (Appleton, 2007). Radon is a radioactive, colourless, odourless, tasteless noble gas that is produced by the natural decay of radium (Ra), thorium (Th) and uranium (U). The isotope 222Rn has a half-life of 3.8 days, time enough to move from bedrock to soil through soil pores, and to accumulate inside buildings, causing severe health problems (WHO, 2009). Radon undergoes radioactive decay through several intermediate species with different half-lives until it becomes relative stable 206Pb (US EPA, 2015b). Radon has a global distribution and can be found in all bedrock types (Cinelli et al., 2015; IARC Working Group on the Evaluation of Carcinogenic Risk to Humans, 1988). Soil parent materials with high natural Rn include acidic igneous rocks such as granite (Blanco-Rodríguez et al., 2017), feldspar and illite-rich rocks (Blume et al., 2016), and sea and lake deposits (Gregorič et al., 2013). Soil texture, porosity and moisture regulate Rn diffusion and convection fluxes from deeper layers to the soil surface (Gates and Gundersen, 1992; Hafez and Awad, 2016). Several attempts to estimate radon fluxes from soils and determine its possible impacts on human health have been made worldwide. The European Atlas of Natural Radiation (EANR) is a set of maps of radiation from different sources that has been developed by the Joint Research Center of the European Commission in cooperation with national research institutions (Cinelli et al., 2019; Gruber et al., 2013). The map of geogenic radon is under preparation (JRC, 2016). In China, Zhuo et al. (2008) developed a model to estimate radon seasonal and annual fluxes from soil surfaces. A model that considers soil properties and climatic variations has been developed to estimate radon fluxes in Australia (Griffiths et al., 2010). Other naturally occurring radioactive elements include uranium (238U), potassium (40K) and thorium (232Th), which are found in natural soils developed from dolomite and limestone rocks (Babić et al., 2020).
Naturally occurring asbestos is comprised of fibrous silicate minerals and occurs in soils formed from ultramafic rocks such as serpentine and amphibole (Rodríguez Eugenio, McLaughlin and Pennock, 2018). These minerals have been extensively used in industrial and commercial applications for centuries due to their physicochemical properties such as heat stability, thermal and electrical insulation, flexibility, tensile strength, and resistance to chemical and biological degradation (Barlow et al., 2017; International Agency for Research on Cancer, 2012). Small fibres of asbestos can be easily mobilized from rocks, soils, and asbestos-containing products, and be transported long distances entrained on dust (Bloise et al., 2016; International Agency for Research on Cancer, 2012). Asbestos is a known carcinogen causing mesothelioma and its mining and industrial use have been regulated or even banned in many countries (Lee et al., 2008; U.S. Department of Health and Human Services, 2016). Due to its relevance to human health, many national institutions and research groups have made efforts to identify areas rich in naturally occurring asbestos (Figure 1) (Bloise et al., 2016; Cagnard and Lahondère, 2019; Hart, 1988; Hendrickx, 2009; Lucci et al., 2018; Ministry of Environment of the Republic of Korea, undated; U.S. Geological Survey, 2016; Virta, 2002).
Aside from the weathering of soil parent material, organic soil contaminants may result from other natural processes such as atmospheric deposition after forest fires. Soil organic matter strongly retains many soil contaminants through adsorption and absorption with different persistence times and availabilities (Conte et al., 2001; Gunasekara and Xing, 2003) (see Chapter 2). Wildfires can lead to the release of contaminants into the atmosphere as particulate or ash-bound material. In the atmosphere, these contaminants can be transported over short or long distances, or mobilized within the soil profile (Campos et al., 2016). This is especially concerning for peatlands and other organic carbon-rich soils, which are exposed to an increasing prevalence and severity of fires due to climate change and anthropogenic management (Evangeliou et al., 2014; Tsibart et al., 2014; Turetsky et al., 2006). Trace elements, and polycyclic aromatic hydrocarbons (PAHs) are the main contaminants associated with wildfires (Chen et al., 2018; Fernández et al., 2018; Meneses et al., 2019a; Young, Balluz and Malilay, 2004). Gribble (2003) noted that more than 3 800 organo-halogen compounds are produced by living organisms and abiogenic processes such as forest fires, volcanoes and other geothermal processes.
Polycyclic aromatic hydrocarbons (PAHs) comprise a large group of organic contaminants that are formed as a result of incomplete combustion of organic material (Rodríguez Eugenio, McLaughlin and Pennock, 2018). Even though the major concerns regarding PAHs focus on their emission from fossil fuel combustion and industrial activities, PAHs are also generated in forest fires, with a predominance of low molecular weight PAHs (Chen et al., 2018; García-Falcón, Soto-González and Simal-Gándara, 2006; Kim, Choi and Chang, 2011; Vergnoux et al., 2011). It has been observed that post-fire rainfall events contribute to diffuse soil pollution and the mobilization of soil contaminants in the environment (Abraham, Dowling and Florentine, 2017; Campos et al., 2016; Meneses et al., 2019b).