UN Enviroment Programme

Chapter 5. Global status of soil pollution

Transport and mobilization mechanisms of contaminants on a global scale

There are several mechanisms of transport and remobilization of soil contaminants that act on a global scale contributing to their dispersion and the diffuse pollution of the world’s soils. The following is a simplified description of these mechanisms, which are of great relevance for understanding the global state of soil pollution and allow inducing possible trends independently of point-source emissions.

5.2.1. Long-range transport

Emissions on one continent may cause pollution on another continent, and even movement between the hemispheres is possible; these transfers are called long range transport (LRT) (Figure 1). It has been estimated that LRT and atmospheric deposition could contribute up to 80 percent of the concentration of some trace elements in topsoil worldwide (Nicholson et al., 2003; Steinnes, 2013). Atmospheric deposition of air contaminants is therefore a major global source of diffuse soil pollution (Wright et al., 2018).

Figure 1. Long-range transport of environmental contaminants.

Source: adapted from Climate Change Science Program and Subcommittee on Global Change Research, 2003.

Industrial and traffic emissions release considerable amounts of trace elements and organic contaminants into the atmosphere. Although stringent legislation and controls on industrial emissions have been adopted in many regions, and filtering technologies have evolved for both industries and transportation, large amounts of contaminants are still being released worldwide (Gidden et al., 2019) and growing demands of certain services, such as electric and electronic devices, will contribute to further emissions in the next decades (Belkhir and Elmeligi, 2018).

Once released into the environment, volatile contaminants are transported by the atmospheric circulation and undergo a cycle of successive evaporation and deposition (Figure 2). They are deposited by rain on the surface of the ocean (i.e., the global fractionation process) and are transported by water and evaporated when they reach areas of warmer conditions. The corresponding process is called the “grasshopper effect”, which is an interaction of evaporation under warmer ambient temperature conditions and condensation under colder conditions (Wania and Mackay, 1996). Over time, at higher latitudes and in colder climate regions, contaminants condense more easily (i.e., the cold trap effect). The different forms of compounds such as PAHs or PCBs have different behaviours. The lighter and more volatile a congener or homologue (see Chapter 2) is, the more it moves to higher latitudes. Eventually, the most common contaminants at the poles are dominated by the more volatile compounds (Bhardwaj et al., 2018).

Figure 2. Global transport processes that contribute to global soil pollution.

Source: adapted from Bhardwaj et al., 2018.

The pathway to the soil for a given compound depends on several parameters that describe the complete life cycle of the compound, namely compound’s inherent physical-chemical properties such as fugacity, water solubility, half-life, as well as parameters that describe the interaction of the compound with the environment such as partition coefficients, metabolisation rates, but also production and use patterns (Mackay et al., 2009). LRT should therefore be calculated considering multiple media interactions and processes (Kawai et al., 2014). On the other hand, a “fractionation” of contaminants can be observed because soils act as a retardant to LRT (Ockenden et al., 2003). Soils with high biological activity, such as boreal forest soils, are playing an important role in ‘protecting’ the poles from the adventitious supply of persistent organic contaminants.

5.2.2. Soil dust mobilization and atmospheric deposition

Global mobilization of soil dust is another major contributor to soil pollution in areas affected by these plumes. Soil dust plumes from arid regions, particularly from North Africa (the Sahara Desert and the Sahel) (Figure 3) and from West Asia (the Tibetan Plateau, the Gobi and Taklamakan Deserts), account globally for 3 000 Tg of mineral dust per year (Fitzgerald et al., 2015), and can reach distant areas of South America, the Caribbean (Figure 4) and North America, Europe (Earth Observatory, 2021), Near East and North Africa (Figure 5), East Asia (Figure 6), and the Pacific (Prasad, El-Askary and Kafatos, 2010; Zhao et al., 2018). Particulate matter (PM) transported in these soil dust plumes contains soil minerals and nutrients that contribute to enriching the soils where they are deposited (Prospero, 1999) but also contain many contaminants bound to clay particles and aluminium and iron oxide (hydroxides) (Garrison et al., 2003; Middleton, 2017; Prospero, 1999).

Figure 3. Sahara dust plume over the Atlantic Ocean.

Source: SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE, 2000.

Figure 4. Saharan dust over the Caribbean.

Source: MODIS Rapid Response, 2007.

Figure 5. Dust above the Red Sea.

Source: Schmaltz and LANCE/EOSDIS Rapid Response, 2017.

Figure 6. A large blanket of dust covers eastern China.

Source: SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE, 1999.