Most inorganic contaminants have a natural origin, and therefore they are found in soils all over the world, although with an irregular distribution depending on the type of parent material. Trace elements, naturally-occurring asbestos and radionuclides are the major soil contaminants of natural origin.
Many trace elements, rare earth elements and other raw materials are extracted from specific locations and used to produce everyday goods that are used and ultimately discarded all over the world, such as electrical and electronic devices, paints, and building materials. This leads to a redistribution of these contaminants in the environment and, therefore, to the alteration of the natural distribution of these elements (natural background values, see Glossary). Higher concentrations than the natural values are found especially in soils surrounding the main sources of contaminants, such as mining, industrial, and urban areas, proximities of transport networks, landfills and informal recycling areas (Fujimori and Takigami, 2014; Guagliardi, Cicchella and De Rosa, 2012; Reimann et al., 2014). In agricultural soils there is also a redistribution of inorganic contaminants due to the use of contaminated water and wastewater for irrigation, as well as inputs of agrochemicals and trace element-containing feed. Agricultural soils in the vicinity of mining, industrial and urban areas also present higher concentrations of inorganic contaminants (Reimann et al., 2014). Biomass burning is also a significant source of inorganic contaminants redistribution (Prasad, El-Askary and Kafatos, 2010; Sundseth et al., 2017).
Point-source and diffuse soil pollution by inorganic contaminants is therefore a global problem. Inorganic contaminants have been extensively studied due to their toxicity and ubiquitous distribution. There are plenty of local studies on the spatial distribution of inorganic contaminants associated with concrete specific sources (see Chapter 3 and Regional Chapters). Local, national and regional studies to determine the natural background values to discern the natural origin of anthropogenic emissions are distributed throughout the world (Dragović, Mihailović and Gajić, 2008). Some regional studies have also been carried out on specific contaminants associated with accidents, such as radiocesium-137 on the European continent after the Chernobyl catastrophe (De Cort et al., 1998). However, to date and to the best of our knowledge, there is no worldwide assessment of the distribution of these contaminants in the environment.
Trace elements such as copper, iron, gold, lead, mercury, silver or zinc have been mined and used by humans since ancient civilizations (Healy, 1979; Vaxevanopoulos et al., 2018). Even today’s natural and protected areas may have been exploited in past and, therefore, may present elevated concentrations of trace elements (Camizuli et al., 2018). These trace elements mobilized from the metallic ores to the topsoil remain sorbed or trapped in the soil matrix since they do not degrade. If changes occur in the soil system due to management practices, changes in precipitation and temperature patterns, changes in land use and vegetation that previously sequestered these trace elements may become available to organisms (Sun et al., 2006). The harmful effects of trace elements on the environment and human health have long been reported (see Chapter 4).
Industrial activities and transport release important amounts of trace element-rich aerosols that are subjected to both local and LRT. The dominant direction of local winds and water flows, paired with terrain characteristics, determine the distribution of trace elements on a local scale (Ding et al., 2017). Soils neighbouring mining and industrial areas show higher levels of pollution by trace elements, and these levels decrease with increasing distance from the source. Atmospheric transport and deposition is a major source of trace elements in rural areas (Hou et al., 2017). The influence of emitting sources on the concentration of contaminants has been observed up to several tens of kilometres away, even in forest and agricultural soils (Adedeji, Olayinka and Tope-Ajayi, 2019; Maas et al., 2010). Several long-term biomonitoring surveys exist to determine trace element LRT and atmospheric deposition but further data collection and systematic monitoring is needed for the majority of trace elements (Schröder et al., 2016; Wright et al., 2018).
Mercury is the trace element with the highest potential for LRT in its gaseous elemental form (Wright et al., 2018). Mercury has been found to be transported between hemispheres, so that its distribution is global, with evidence of anthropogenic emissions dating back to ancient civilizations (Sun et al., 2006). Anthropogenic mercury emissions are mainly due to artisanal and small-scale gold mining, fossil fuel and biomass combustion (especially coal burning), non-ferrous metal production, and cement production. Anthropogenic emissions have increased atmospheric mercury by up to 450 percent above natural levels (UNEP, 2019a). It is estimated that there are about one million tonnes of mercury stored in mineral and organic soils, 15 percent of which is due to anthropogenic activities (UNEP, 2019a). Atmospheric mercury deposits in soils and water bodies and can be remobilized and enter other environmental compartments. Soils are emitters of inorganic mercury to the air and it can also be mobilized to water bodies by erosion and runoff. In the aquatic environment, mercury can be methylated by biotic processes and transformed into methylmercury, a potent neurotoxin (see Chapter 4). Methylmercury is readily bioavailable and bio-accumulative, so it enters and accumulates in food chains (Chételat et al., 2020). Mercury is also released from the oceans to the atmosphere in the form of gaseous elemental mercury, transported in atmospheric currents, even thousands of kilometres, and eventually deposited back on land and in inland water bodies and oceans (Figure 7). Therefore, the biogeochemical cycle of mercury, exacerbated by anthropogenic emissions, represents and will continue to represent a global source of this contaminant of concern for human and environmental health (UNEP, 2020). Although anthropogenic emissions are to be reduced in compliance with the Minamata Convention, mercury stocks sequestered in soils, vegetation and oceans may re-enter the atmosphere and recirculate between environmental compartments for a long time. Therefore mercury pollution will continue to pose a risk to human health and the environment even centuries from now (Selin, 2018).
Given the multiple applications of trace elements, their extraction, use and global diffusion, it is expected that their concentration in soils will continue to increase. However, advances in mining and industrial technologies that control emissions and manage waste in a more sustainable manner could contribute positively to a slowdown in trace element emissions into the environment (IISD, 2019; Masson, Walter and Priester, 2013).
In addition to in situ mobilisation and long-range transport of trace elements by anthropogenic activities, the weathering of certain rock types can be an important source of elevated trace element concentrations in soils, which can vary by more than 3 orders of magnitude (Hamon et al., 2004). For several decades, international efforts have been made to harmonise methodologies and to develop comparable geochemical maps to determine more accurately natural background values of trace elements in soils and thus estimate concentrations due to anthropogenic contaminants (Hamon et al., 2004). The Global Geochemical Baselines working group, operating under the auspices of the International Union of Geological Sciences (IUGS) and the International Association of Geochemistry (IAGC), has coordinated a global action and is supporting countries technically for the development of national maps (Smith et al., 2012). Countries like China, India, and the United States have already produced their maps. Reimann and Caritat recently developed the geochemical background values of trace elements for Australia (Reimann and de Caritat, 2017). At regional level, great progress has been done in Europe through the project FOREGS/EuroGeoSurveys with the publication of the ‘Geochemical Atlas of Europe’, which mainly considered forest soils (Tarvainen, Salminen and Vos, 2005). The GEMAS project complemented the Geochemical Atlas of Europe with information for agricultural and grazing land soils (Reimann et al., 2014). International efforts to determine natural background concentrations of trace elements in soils should continue and further studies are needed to determine the specificity of these elements at each site to determine whether current concentrations of inorganic contaminants are of anthropogenic origin or not. This will allow more appropriate land-use decisions to be made at sites or prevention measures to be taken to avoid the mobilisation and bioavailability of these contaminants.
The policy context in the promotion and adoption of best available techniques to control industrial emissions varies widely among countries, although in general there have been important advances in recent years (OECD, 2018). International agreements such as the phase-out of lead in paints and gasoline (IPEN, 2017; UNEP, 2019b) or the implementation of the Minamata Convention on Mercury with the support of the UNEP’s Global Mercury Partnership (UNEP, 2020), have proven to be effective in mobilising governments and reducing contaminant emissions. For example, lead deposition in Norwegian soils from transboundary pollution decreased by approximately 95 percent between 1977 and 2010 (Steinnes, 2013), which could be explained by the phased-out of leaded gasoline, which was attributed up to 74 percent of global lead emissions in 1995 (MSC-E, 2014). However, in developing countries the sources of lead differ from those in developed countries and lead emissions still pose a great risk, especially to children, where the most common sources are recycling and informal manufacturing of lead-acid batteries, mining and metal processing, and electronic waste. These emission sources are less regulated nationally and globally and therefore continue to pose a risk (Ericson et al., 2021). It is therefore clear that greater efforts must be made at the legislative level to control all possible sources of contaminant emissions, taking into account regional and socio-economic differences in order to ensure that international agreements benefit the world’s population and the environment as a whole.
In addition, no national, regional or international legal framework can effectively contribute to the reduction of emissions and the adoption of best solutions without accurate data and information on emissions, LRT, deposition and remobilisation of contaminants on a global scale (Selin, 2018). The Minamata Convention, being aware of this constraint, includes in its rules the establishment of inventories of mercury production, use and emissions (Minamata Convention on Mercury, 2019). The scientific community is working on the development of simple indices that will allow policymakers to accurately and easily report the changes that have occurred and thus be able to quantify the effectiveness of this international mechanism (Hou et al., 2017; Kowalska et al., 2018). The same approach should be adopted for other toxic trace elements with a global diffuse distribution.
Strengthening legal frameworks for the control of industrial and mining emissions and their effective enforcement, the environmentally sound management of industrial, municipal and hazardous waste and mine tailings, and “right to repair” policies or initiatives that incentivise or penalise manufacturers to encourage the manufacture of products that can be easily repaired to reduce planned obsolescence, coupled with increased recycling of household products to reduce the demand for new mining operations, are key elements in reversing the growing threat of global trace element pollution of soils.
Emissions of nitrogen (NOx) and sulphur oxides (SOx), mostly originating from agriculture and fossil fuel combustion, have been major contributors to air pollution and wet and dry acid deposition leading to soil acidification in many regions of the world. Soil acidification plays an important role in the mobilization and bioavailability of contaminants (see Chapter 4) and is therefore worth discussing in this report.
Agriculture is responsible for about 72 percent of global nitrogen dioxide (NO2) emissions, mainly associated with the use of nitrogen fertilizers, with significant differences between regions (FAOSTAT, 2020). While in the Pacific and South America, agriculture is responsible for about 90 percent of NO2 emissions, this share drops to 61 percent in North America and 43 percent in Central America, where the main emitters are the energy and industrial sectors, respectively (FAOSTAT, 2020). Global NO2 emissions have gradually increased since the 1990s (Figure 8) (Ehhalt et al., 2001). In Europe, there was a significant reduction in the 1990s, but have remained relatively stable since the beginning of the 21st century. China, the United States and India are the countries with the highest emission rates, followed by Brazil, Mexico, Indonesia, the Russian Federation, Australia and Pakistan (FAOSTAT, 2020). The energy, waste and international aviation and maritime transport sectors have experienced the greatest growth in recent decades. China leads in emissions in these three sectors, while Mexico ranks first in industrial emissions, which are three times higher than the United States or the Russian Federation. However, there is a softening of the upward curve that coincides with the first period of implementation of the Kyoto Protocol (2008-2012), which could indicate a slight positive effect of this international agreement (Aichele and Felbermayr, 2013).
Emissions of sulphur oxides have decreased in recent years (Fioletov et al., 2016) thanks to the strengthening of controls and regulations on industrial and transport emissions (EEA, 2014; Ji, 2020) and the gradual substitution of energy from coal by other sources (Dahiya et al., 2020). However, SOx emissions remain high, with about 29 million tonnes emitted in 2019 attributable to anthropogenic sources, namely coal and oil-fired power plants, smelters, and sources related to oil and gas industry (Figure 9), of which more than 7 million tonnes of SOx are from coal combustion (NASA, 2020). Particularly important are the emissions from coal combustion from India and Turkey (NASA, 2020). Major sulphur deposition occurs in East Asia, Central Europe, North Africa, and Eastern China (Gao et al., 2018). Although the major emitters are point-source industries, terrestrial, aerial and marine traffic also contribute significantly to diffuse emissions of these contaminants (Ji, 2020; Manisalidis et al., 2020).
These contaminants are transboundary transported by atmospheric currents and are deposited on land and in water, where they cause acidification and eutrophication, affecting large areas (Driscoll et al., 2001). Acid deposition increases the amount of protons (H+) and strong acid anions in the soil solution. The buffering capacity of soils neutralises excess anions by mobilising cations (e.g. Mg2+, Ca2+, Na+, K+) from the surface of soil particles, which results in leaching of base cations and reduction of soil base binding. But this capacity is limited, and if acid deposition exceeds the natural neutralizing capacity of the soil, other cations, such as Al3+ or Fe2+, can be mobilised from clay structures and organo-mineral complexes, entering the soil solution (Driscoll et al., 2001). These cations can be toxic to plants and soil organisms, reducing soil health and biodiversity. Excess acid deposition ultimately results in excess anionic charges and thus a reduction in soil pH (Johnson et al., 2018). Reduced fertility and nutrient deficiencies are also observed in soils affected by acidification process (Likens and Butler, 2020).
As described in Chapter 2 (section 2.2.4), soil pH plays a key role in the fate and bioavailability of many contaminants. Under more acidic conditions, many trace elements are mobilized into the soil solution and become available for plant uptake (Alloway, 2013). Thus, acid deposition involves two soil health degrading processes: soil acidification and increased availability of contaminants, exacerbating the risks arising from point-source pollution (Driscoll et al., 2001; Saha et al., 2017). In addition, acid deposition affects soil organisms, reducing soil biodiversity and activity, as well as plant cover (Johnson et al., 2018; Wright et al., 2018).
Nuclear weapons testing and major nuclear accidents have resulted in the release of great quantities of radionuclides into the environment, especially into the atmosphere, causing a fallout that has affected virtually all regions of the world. Soil pollution from dry and wet deposition of radionuclides, especially long-lived radionuclides such as strontium-90 (90Sr) and radiocesium-137 (137Cs), after nuclear accidents and weapons test have significantly contributed to external dose exposure of global populations (UNSCEAR, 2000). The chemical behaviour of radionuclides in soils is analogous to trace elements although the environmental impact is a result of emitted radiation.
The United States of America, the Union of Soviet Socialist Republics (USSR), the United Kingdom of Great Britain and Northern Ireland, France and China developed their nuclear potential after World War II and, between 1945 and 1996, more than 2 000 nuclear weapons tests were conducted worldwide concentrated in around 60 sites (Figure 10), three quarters of which occurred underground (CTBTO, 2012). However, atmospheric detonations are the most significant in terms of radionuclides release (Bennett, 2002). The United Nations Scientific Committee on the Effects of Atomic Radiation has estimated that some 160 megatonnes (Mt) of radioactive debris from atmospheric detonations were deposited worldwide, resulting in an external exposure dose that peaked at 150 µSv in 1963, and has gradually decreased since then (UNSCEAR, 2000).
Although different in terms of causes and radiation emitted, accidents of commercial nuclear power plants contribute to the overall radiation exposure dose. Three Mile Island in 1979 (Middletown, Pennsylvania, United States), Chernobyl in 1986 (Ukraine, former Soviet Union), Fukushima Daiichi in 2011 (Fukushima, Japan) have been the largest nuclear accidents in history (IAEA, 2016), while other minor accidents have also occurred in Kyshtym in 1957 (Ozyorsk, the Russian Federation), Windscale in 1957 (Cumbria, United Kingdom of Great Britain and Northern Ireland), Sodium Reactor Experiment in 1959 (Los Angeles, California, United States of America), and Ventanilla in 2014 (Ventanilla, Peru).
The emitted radiation and radioactive debris affected mostly the northern hemisphere, where nuclear accidents and most of the detonations took place (Figure 10). However, there is also a circulation and diffusion of radionuclides throughout the rest of the planet due to atmospheric circulation (Figure 11). At higher altitudes, radioactive aerosols in the atmosphere descend by gravity and are transported with the general air movements at lower levels. In the troposphere, there is an irregular migration of air masses in the form of Hadley cells at low latitudes (UNSCEAR, 2000). These cells increase or decrease in size and move latitudinally with the season. Due to the exchange of air between the stratosphere and the troposphere in the mid-latitudes and the air cells circulation in the troposphere, a greater deposition occurs in the temperate regions, approximately doubling the deposition in the equatorial and polar regions (Aoyama, Hirose and Igarashi, 2006; CEA, 2021).
After deposition on the soil surface, radionuclides migrate within the soil profile and are strongly adsorbed by clay minerals and soil organic matter (Strebl et al., 2007). Low soil pH and low concentrations of organic matter and low clay content lead to increased mobility of radionuclides in soils (section 5.3.2) that can be taken up by plants, thus entering the food chain (Zhu and Shaw, 2000), although normally in sub-trace quantities (Strebl et al., 2007). Soils are the main reservoirs of radionuclides of anthropogenic origin, and therefore the sustainable management of soils and appropriate farming practices to avoid acidification of soils and mobilization and transfer to the food chain is essential to ensure low exposure of humans and the environment to these contaminants.