Increasing pressures on the environment due to population growth and the resulting growing demands of agri-food, industrial, energy and transport systems, as well as their emissions and waste, are leading to an acceleration of environmental degradation on a global scale. Along with climate change, environmental pollution, and especially soil pollution, is one of the main global challenge facing humanity today. Soil pollution is one of the major threats to world’s soils and jeopardizes the provision of key soil ecosystem services, including the provision of safe and nutritious food, the availability of clean water, and the existence and conservation of soil biodiversity.
The main anthropogenic activities that are sources of soil contaminants, in order of global importance, are industrial activities, mining, waste treatment, agriculture, extraction and processing of fossil fuels, and emissions from transportation. There are significant differences in the impact of these sources between regions, although there are no specific and comparable data on the actual emissions of each sector in each region.
Since ancient times, human activities have released thousands of different synthetic chemical compounds and potentially toxic natural elements into the environment. These contaminants have different residence times in the environment, from hundreds to thousands or tens of thousands of years, and are distributed around the planet. In addition, natural sources of potentially harmful contaminants also contribute to the environmental load of contaminants.
Soil is the main recipient of environmental contaminants. At the same time, soil has the natural capacity to filter, buffer, retain and degrade contaminants. Soil mineralogy, organic carbon content, content of clays, minerals and other organo-metallic complexes, and soil organisms are the main soil properties that determine the filtering and buffering capacity, and thus the fate of contaminants in soil. The physicochemical characteristics of the contaminants, as well as the mixture of existing contaminants, will also condition the fate of contaminants in soils, and their possible leaching to groundwater, evaporation to the atmosphere or uptake by plants and organisms.
When soil contaminant attenuation capacity exceeded, soil pollution becomes a critical concern for the health of the soil, the environment and people. Unsustainable soil management increases the risk of soil pollution from misuse/overuse of agrochemicals, irrigation with low quality water and other practices, and increases the risk of remobilization of contaminants in the environment by increasing other soil degradation processes. Wind and water erosion and runoff, loss of soil organic carbon, alterations in soil pH and redox status, and changes in moisture and temperature regimes all contribute to the remobilization of soil contaminants. Soil thus can become a source of contaminants for other environmental compartments, including food, water, air, and terrestrial and aquatic organisms and humans. Ecosystem health and human health are therefore interconnected, as the “Planetary Health” and “One Health” initiatives underline. Soil, as the foundation of terrestrial life and provider of most of our food and water, should be at the heart of both initiatives, as neither can be effectively addressed without tackling soil pollution.
Soil pollution affects above and below ground biodiversity, both by reducing the number of organisms due to the toxicity caused by the contaminants, and by producing changes in communities, due to the replacement of more sensitive species by more pollution-tolerant ones. Although contaminants are generally found at low concentrations in the soil and result in adaptive strategies of organisms through changes in physiology and feeding behaviours, cases of severe pollution can have irreparable consequences in the medium and long term, resulting in the loss of ecosystem services, productivity and resilience. Soil pollution causes reduced crop yields and food wastage due to high levels of contaminants, loss of biodiversity and increased incidence of pests, decreased water quality and eutrophication of the marine environment.
Cases of natural attenuation of soil pollution and gradual recovery of ecosystems have been reported. But this requires several decades and a full recovery of communities, diversity and activities of biogeochemical cycles has not yet been observed. Overall, severe soil pollution leads to land degradation and the inability to use the land for productive, residential and recreational uses, which ultimately leads to land abandonment and a depreciation of the price of adjacent land.
Particularly relevant for humans is the transfer of contaminants to the food chain, since, along with occupational exposure, this represents the major route of exposure to contaminants for humans. The intake of low concentrations of multiple contaminants throughout our lives can have important effects on our health and on the development of future generations, causing a multitude of acute and chronic diseases. The economic cost of soil pollution-related illnesses, many chronic and with long-term effects, and the loss of human productivity are often overlooked. These cause recurrent temporary sick leave leading to a reduction or effective loss of workforce and productive capacity, and may be worth billions of USD per year.
Soil pollution has the greatest impact on the health of the most vulnerable population groups, such as foetuses, children and pregnant women. Pollution also has significant effects on the health and well-being of the poorest and most marginalized groups. Low- and middle-income countries accumulate the highest rates of environmentally attributable mortality and burden of diseases, but there are also differences in health status between different social groups within a country.
With a large part of the world’s population moving from rural areas to urban centers, especially in less developed countries, large cities are expanding and occupying former peri-urban agricultural and industrial land that could present different concentrations and mixtures of contaminants. Industries are also being relocated to smaller communities, where environmental control measures are less strict. The most disadvantaged groups, ethnic minorities and lowest-income families are being relegated to these areas and could be exposed to higher environmental risks with less access to clean and green areas. Apart from differences in exposure to soil pollution and access to clean green spaces and healthy food, other socio-economic aspects play a role in the unequal distribution of the burden of disease attributed to the environment, such as access to public health services or health insurance, environmental and urban development policies, and access to clean technologies.
The impact of soil pollution is very evident in the poverty-food-water-air-health axis, but there are other aspects of the 2030 Agenda that are also directly influenced by soil pollution. Failure to address all aspects that cause and aggravate soil pollution will hinder the achievement of the 2030 Agenda for Sustainable Development.
Soil pollution is a global problem that has impacts at local and national levels and has transboundary effects. Many contaminants are subject to large-scale transport directed by atmospheric and sea currents, and are transported outside the borders of the emitting country by dust storms, runoff and by the commercial movement of chemicals, foodstuffs and wastes. Soil contaminants are therefore found on all continents, even in the most remote areas, and are easily transported from one country to another. Although most soil contaminants are ubiquitous, their distribution and major sources vary from region to region, from country to country and even between areas within the same country. There are also significant differences in the regulatory frameworks that attempt to address soil pollution.
Since the beginning of the 21th century, the global annual production of industrial chemicals has doubled to approximately 2.3 billion tonnes and is projected to increase by 85 percent by 2030. Industrial contaminants are released into the environment throughout their life cycle, from manufacturing to the production of the contaminant-containing materials, their transport, use and disposal. However, most releases of contaminants to soil are not easy to quantify and in most cases are not controlled or monitored, therefore, they remain highly uncertain. Agrochemical inputs may represent an exception to this fact, as there are international and national efforts to quantify the production, marketing and land application of these products (e.g. FAOSTAT indicators). However, regional assessments show that there is still unregulated trade and use of many agrochemicals in many parts of the world.
Decades of research, inventory and monitoring of point-source polluted soils in several countries have greatly contributed to the understanding of the distribution of contaminants in soil, and their fate and interaction with living organisms. However, there are still significant knowledge gaps and uncertainty about the number and extent of affected areas. Given the large number of contaminants, the variety of their physicochemical characteristics and their multiple interactions with the soil (which determine the fate of the contaminants) the estimation of the contaminant load is very complex.
There are also important knowledge gaps regarding interactions between the multiple contaminants found in soils and their combined effect on organisms. This is aggravated by the emergence of new contaminants for which the available information is very limited. The proliferation of organic contaminants and emerging contaminants such as pharmaceuticals, antimicrobials leading to resistant bacteria, industrial chemicals and plastic waste is a growing societal concern. In the current global pandemic situation caused by COVID-19, the pressure on the environment has increased due to intensified release of wastes. Scientific knowledge on the fate of emerging contaminants is still scarce. This makes the establishment of global distribution models very difficult in the absence of regular systematic analyses in soil laboratories (which focus more on the agronomic part of soils) and monitoring systems in many countries of the world.
The knowledge gap on soils affected by diffuse pollution, the extent of long-range transport of contaminants on a global scale, and the impact of soil pollution on other environmental compartments is even greater than for point pollution.
This report highlights the multitude sources of contaminants affecting the soil, and a generalized upward trend in the use of polluting chemical compounds and elements. As such, soil and environmental pollution is expected to increase unless there is a rapid shift in production and consumption patterns toward less harmful products and lower production rates and greater recycling and reuse options for raw materials and products. It is also essential to reach strong political commitment to a truly sustainable management of natural resources that discourages overexploitation and prioritizes respect and care for nature, and thus human well-being, over purely economic ends.
Identification and assessment of potentially polluted sites is the essential first step in the management of soil pollution. If contamination at a given site is at levels1 that can cause harm to organisms including humans, information about that site should be collected at the appropriate governmental level and made available to the public. Remediation or risk minimization actions should be taken accordingly, especially if the site is used for food production or as a water reservoir for human consumption. The identification of the site also allows the tracing of ownership of the site, which is fundamental to the “polluter pays” principle. Although many countries have effective processes in place to identify and assess polluted sites, this fundamental step of identifying the liable party (polluter) is still lacking in most countries.
The management and remediation of polluted sites is necessary to protect human and environmental health. Remediation of soil pollution is a technically complex, medium to long-term and costly undertaking, ranging from tens of thousands to hundreds of millions of dollars per year. The cost of remediation varies from site to site depending on the characteristics of the site, the type of contaminants and their concentration, the environmental compartments affected (e.g., topsoil, groundwater, surface water), the protective measures to be taken to protect the population during remediation work and the land use after remediation, as well as the technology chosen.
The regional chapters show that there are examples of successful approaches to the management and remediation of polluted sites in all regions. Those using nature-based solutions are of particular interest. Sharing experience regionally and internationally would greatly facilitate progress in combating soil pollution.
Technological and industrial innovation should be focused on obtaining less harmful and environmentally persistent alternatives. Their commercialization should be subject to reliable studies on their fate in soils, interaction with other contaminants and impact on a wider range of living organisms than those considered in models to date.
To ensure the identification, management and remediation of contaminated soils, as well as the adoption of preventive measures, it is necessary to strengthen communication channels between academia, policy makers and society. This will ensure that all stakeholders have timely and science-based information on the potential threats posed by these contaminants and can take informed decisions.
In the current scenario of a worsening global trend in soil pollution, greater political, business, and social commitment is needed to seek alternative solutions to the use of highly toxic contaminants. Increased investment in research, prevention and remediation is also required. Enhanced cooperation and partnership are essential to ensure the availability of knowledge, the sharing of successful experiences, and universal access to clean and sustainable technologies, leaving no one behind.