Lead authors: Bernd M. Bussian (FAO-GSP), Natalia Rodríguez Eugenio (FAO-GSP) and Hugo Bourhis (FAO-GSP)
Soil pollution is caused by both naturally occurring and synthetic human-made contaminants. Naturally occurring contaminants have always existed and have circulated between environmental compartments (air, water, soil, organisms) throughout earth’s history. However, over the past three centuries, the environment has increasingly been exposed to both natural occurring chemicals and synthetic hazardous compounds due to industrialization, large-scale production of synthetic chemicals, rapid urban development and agricultural intensification (Birnbaum, 2008; Kabir et al., 2012; Yun et al., 2018).
The environment, including soil, is exposed to chemical substances during all stages of their life cycle (i.e., from production through their use and ultimate disposal). The life cycle of anthropogenic contaminants starts with the mobilization of elements from the earth’s crust through mining and extraction of raw materials followed by the industrial production of synthetic compounds. Production includes the industrial processing of raw materials, as well as the further processing and formulation (i.e., mixing of substances into preparations) of the marketed product, its transport and its storage. Use occurs at all scales from large-scale (industry) use through to small-scale private or consumer use, and is followed by disposal, including waste treatment (e.g. incineration, landfilling or recycling). During each of these life cycle steps, contaminants can be intentionally or accidentally released into the environment.
The most common entry routes of contaminants into the soil are direct application onto soil, atmospheric deposition, application with irrigation water, rainwater, runoff water, or river and lake sediments. Many of these contaminants remain in the vicinity or point of their emission (thereby causing point-source soil pollution), but others, due to their ability to release as a vapour from a liquid (or fugacity) and their subsequent mobility are spread all over the world and hence can show high environmental persistence (Mackay and Arnot, 2011). Atmospheric and ocean currents and climatic event, such as, storms, floods or tsunamis, can also contribute to the remobilization and regional or global redistribution of contaminants, thus contributing significantly to diffuse soil pollution and causing transboundary pollution (EEA, 2005; Steinnes, 2013).
Soil can act to store contaminants (i.e., act as a sink) or as a source of contaminant release into air or water. Many soil components such as organic matter and clay are highly reactive and can store long-lived chemicals for lengthy periods of time. Ultimately, however, the chemistry or physical structure of the contaminant may be altered during storage and the contaminant may be remobilized into the environment; in this case the soil is acting as a secondary source of environmental pollution.
As the sources of point and diffuse pollution at the local level have already been described in general terms in Chapter 3, and their distribution in different regions and countries is presented in Chapters 6 to 12, this chapter focuses on describing the dispersion mechanisms of soil contaminants at global scale and presents some examples of diffuse soil pollution affecting nearly all soils of the world. To understand the extent of global soil pollution this chapter also tries to answer two key questions: