Soils play an important role in the global cycling of organic contaminants, as soils can act as a sink but can also be a source of these contaminants to other environmental compartments. Multiple organic contaminants are found in significant concentrations in soils and may eventually affect water, food and feed quality despite having been banned recently or in recent decades (e.g. ECHA, 2020; Stockholm Convention, 2019a; US EPA, 2020). Persistent organic pollutants (POPs) have been listed and regulated under international agreements due to their persistence, toxicity and abundance.
In many regions legacy POPs are no longer produced and have not been used since decades, but atmospheric levels of the most well studied legacy POPs, especially PCBs, HCB, or DDTs are only declining slowly in the northern hemisphere including the Arctic (Braune, Gaston and Mallory, 2019; Meijer et al., 2003). This gradual decline is considered to be due to simultaneous processes of new releases (both intentionally and unintentionally), such as volatilization from old stockpiles or waste dumps (see the example reported for Eurasia in Chapter 7), and releases from secondary sources. Such secondary sources are re-emissions to the atmosphere from environmental reservoirs. Soils have a high sorption potential through the binding of contaminants to soil organic carbon and some minerals and become active re-emitters when organic carbon is lost due to management practices and land use change. Even in remote areas, secondary sources contribute significantly to the total emissions inventory. Temperature fluctuation between summer and winter, as well as the effects of climate change, trigger remobilization of POPs.
Nizzetto and co-workers stated that the current balance between primary and secondary sources when determining global pollution of POPs is not easy to assess because rates of both types of emissions remain highly uncertain (Nizzetto et al., 2010). There are several estimates of POPs inventories (Breivik et al., 2002a, 2004, 2007; Wegmann et al., 2004), however for an accurate estimate of the emissions and eventually existing burden for the environment, systematic global mapping and monitoring is still needed.
Environmental contaminants are normally detected at the sites of their production and in the region of their specific use. For example, halogenated hydrocarbons such as PCBs, are used mainly in industrialized countries and are therefore found to a greater extent in the northern hemisphere. DDT is produced predominantly in industrialized countries but is used in areas near the equator and subtropics as a malaria vector control. PAHs are generated in industrialized regions but also by biomass combustion (wood, crop residues) in rural areas. However, since most POPs are volatile and lipophilic, they are subject to LRT by air along with intermediate sorption by air and seawater, which means that these compounds are also found in the Arctic and Antarctic.
Global soil pollution by persistent organic contaminants is conditioned by natural factors, such as partitioning between the environmental media (air, water and soil) and retention processes in soils (sorption to organic carbon, physical occlusion, biodegradation), and by anthropogenic factors, such as changes in land use, agricultural practices, or climate change (Dachs, Eisenreich and Hoff, 2000; Monteith et al., 2007; Nizzetto et al., 2008) (Dachs, Eisenreich and Hoff, 2000; Monteith et al., 2007; Nizzetto et al., 2008). Legacy contaminants DDT, PCBs (polychlorinated biphenyls), and PAHs (polyaromatic hydrocarbons) are introduced as reference cases since their key properties and the resulting soil pollution differ significantly. Table 1 illustrates that each of the selected compound class is characteristic in terms of the driving forces for pollution, namely emission scenarios, use, partitioning in the environment and secondary sources, waste handling and legacy inventories.
The following sections therefore present some examples of legacy organic contaminants that due to their ubiquitous distribution and toxicity are of particular concern to human health and the environment.
Polycyclic aromatic hydrocarbon (PAHs) emissions are continuous, as they are unintentionally formed as by-products of fossil fuel use and biomass burning, making them distinct from many other POPs such those listed in the Stockholm Convention (Ravindra, Sokhi and Van Grieken, 2008). PAHs represent a group of several hundred homologues. Generally, PAHs formation decreases with increasing combustion temperature but also the formation of the more highly condensed PAHs will increase. The extent of PAH formation during combustion of organic matter and the fingerprint of PAHs homologues formed depends on local temperature and mixing conditions in the flame (Wilcke, 2000). Low molecular weight PAH homologues are more volatile and are affected by long-range transport in the atmosphere, whereas high molecular weight PAH homologues are more persistent in the environment and show a greater potential to be taken up by soil organic matter or fatty tissue of organisms. In addition, toxicity, teratogenicity, and carcinogenicity increase with molecular weight. In combination with the ability to condense at low temperatures, low molecular weight PAHs tend to accumulate in Polar Regions and may be re-volatilized during the polar summer (cold trap effect). Climate change and increasing average temperatures at the poles may contribute to a re-volatilization of environmental PAHs.
In addition to PAHs released to the atmosphere from combustion processes, PAHs found in Arctic marine waters and sediments originate predominantly from natural submarine seeps (Balmer et al., 2019). PAHs therefore exhibit a widespread occurrence. PAHs can be transported globally by air and water and subsequently deposited in surface waters, soils, and sediments, thus potentially contaminating remote regions, including the Arctic. Human exposure to PAHs occurs through multiple pathways depending on environmental and living conditions, where the main route for non-smokers is the consumption of contaminated food, from soil, industrialised processed food or some domestic cooking practices (Zelinkova and Wenzl, 2015).
Unlike intentionally produced and emitted contaminants, such as most POPs, whose emissions can be quantified with reasonable accuracy from overall production rates, the emission of PAHs into the atmosphere – which is the dominant environmental exposure pathway – can only be estimated by quantifying the emissions caused by burning of fossil fuels. Despite the uncertainties inherent in these modelling scenarios, trends and major sources of contaminants can be identified. The estimate of global atmospheric emissions for the period 1960 to 2030 considering fuel consumption from 69 major sources adds up to 500 Gg of the 16 priority PAHs. Residential/commercial biomass burning contributes 60.5 percent to total emissions, open-field biomass burning, agricultural residue burning, deforestation, and forest fires account for 13.6 percent of emissions, and petroleum consumption by on-road motor vehicles accounts for 12.8 percent (Shen et al., 2013). The spatial distribution shows that half of the world’s total PAHs emissions are contributed by the South (87 Gg), the East (111 Gg) and Southeast Asia (52 Gg), which are the regions with the highest PAHs emission densities. According to Shen and co-workers, global total PAHs emissions have declined gradually from 592 Gg in 1995 to 499 Gg in 2008 mainly due to a decline in the developed countries from 122 Gg in the early 1970s to 38 Gg in 2008 (Shen et al., 2013). The authors predict a decrease by 46−71 percent and 48−64 percent in PAHs emissions for developed and developing countries, respectively.
Despite the estimated reduction in PAHs emissions, PAHs continue to have adverse effects on ecosystems and human health. Terrestrial environment and soils are the main sink for PAHs, which are strongly sorbed to SOM and plant tissues. However, soils also act as a source of PAHs due to leaching, evaporation and migration into other environmental compartments (Sweetman et al., 2005; Wilcke, 2000).
Numerous in-depth studies on the occurrence of PAHs in soils have been published over the last three decades. A summary of selected papers is shown in Table 2 and correspond to PAHs pollution in industrial and urban environments, in agricultural and grazing lands, and in remote areas including the poles.
In a worldwide study, Nam and co-workers found concentration levels for 15 PAHs homologues in soils ranging over 5 orders of magnitude from <1 to 7 840 ng/g d.w. (Nam et al., 2008). Concentrations in soils were distributed from highest to lowest in Europe > North America > Asia > Oceania > Africa > South America. Higher values were observed at locations close to long-term emission sources and at locations susceptible to large atmospheric deposition inputs. In addition, as found by other authors, a large positive correlation was obtained between population density and PAHs concentrations in soil, as well as between the amount of SOM and black carbon (Nam, Sweetman and Jones, 2009).
Despite the countless datasets available in the literature, it is difficult to obtain an accurate picture of soil pollution by PAHs, mainly because most studies focus on single scenarios or smaller geographic regions and studies differ in sampling techniques, analytical methods, and quality control schemes applied or the set of PAHs counterparts studied (Zeng et al., 2019). However, some general findings can be concluded from the synopsis in Table 2: total PAHs concentrations in soil increase in the sequence remote area < agricultural land < pasture land < forest < urban area < roadside land < industrial area / petrochemical industry < landfill / polluted sites. This ascending order is in agreement with the large literature review study conducted by Zeng and co-workers, in which they reviewed 306 articles with more than 30 000 datasets from 1 833 sampling sites (Zeng et al., 2019).
DDT can serve as a reference case for studying the exposure and fate of environmental organic contaminants because of its massive global use, its widely discussed advantages and disadvantages, the great amount of data and empirical studies on the effects on nature and human health and the number of legal measures taken to curtail its use.
DDT emissions occur during production and formulation (Meijer et al., 2003), during handling, especially in the case of inadequate knowledge and expertise (Lekei, Ngowi and London, 2014), but also as a result of inadequate waste disposal of unused material (Kishimba and Mihale, 2009; Mahugija, 2013; Mahugija, Henkelmann and Schramm, 2014). On the other hand, the abundance of DDT metabolites depends largely on their use in terms of time and scale (i.e. when, what amounts, where they are used) (Aichner et al., 2013; Elibariki and Maguta, 2017; Heinisch, Kettrup and Wenzel-Klein, 1993; Kurt-Karakus et al., 2006; Mishra, Sharma and Kumar, 2012; Zeng et al., 2019).
The global environmental fate of DDT is regulated by its physical-chemical parameters (mainly vapour pressure, water solubility, octanol-water partition coefficient) and soil properties such as soil organic carbon content, sorption coefficients and redox potential, but also by external conditions such as climate. Additionally, temperature dependencies of these properties are to be considered (Mackay et al., 2009; Miglioranza et al., 1999).
The Stockholm Convention aims to reduce the production and use of DDT and ultimately wants to eliminate DDT from global markets. However, the Convention includes an exemption on the use of DDT production and use for control of malaria and leishmaniosis, although it has experienced a significant decrease of more than 30 percent in the last decades, from 5 144 to 3 491 metric tonnes of active ingredient (Van Den Berg, Manuweera and Konradsen, 2017). In line with the exemption regulation, countries are obliged to report their intention to produce or use DDT and maintain a register. DDT is the most frequently regulated pesticide, with 319 regulatory guidance values (RGVs) established for its presence in soils; 140 of these RGVs belong to the U.S. legislation (Li and Jennings, 2017).
Under the Stockholm Convention’s DDT phase-out, there was a change in the geographical application pattern of DDT between 1970 and 1990 (Figure 12) that caused significant changes in the environmental fate of DDT: the number of completed atmospheric cycles increased and residence times in terrestrial compartments decreased accordingly. Semeena & Lammel (2003) argued that as a result of the north-south shift in the pattern of DDT application, its dispersion around the world increased, also observed by other scientist (Schenker, Scheringer and Hungerbühler, 2008).
Polychlorinated biphenyls (PCBs) are on the Stockholm Convention’s list of contaminants to be eliminated, which aims to achieve environmentally sound management of PCBs wastes by 2028 (PEN, 2016). However, 83 percent of PCBs produced are still in use and, as of 2017, only 17 percent of the total amount of PCBs had been eliminated (PEN, 2016), and thus PCBs can be found globally in all environmental compartments (Breivik et al., 2002a, 2002b, 2007; PEN, 2016).
Technical PCB has been marketed under different commercial products namely Arochlor (Monsanto/Inited States of America), Clophen (Bayer/Germany), Kanechlor (Kanegafuchi/Japan), Sovol, and TCB (the Russian Federation and Former Soviet Union) and cumulative global production was in the order of 1.3 million tonnes between 1930 and 1993, corresponding to 3 million tonnes of equipment containing PCBs (Breivik et al., 2002b, 2007). In addition to emissions from technical use, PCBs may be produced unintentionally during combustion processes and as by-products of industrial activities. For example, PCBs formation during secondary copper smelting was identified as a major source of PCBs in China (Jiang et al., 2015).
Considering the concentrations found in soils, and knowing the sampling depth and the bulk density of the soils, the global stock of PCBs can be estimated as 21 000 tonnes, which is about 1.6 percent of the known PCBs production (Breivik et al., 2002a; Meijer et al., 2003; Pandelova et al., 2018). Breivik et al. (2002a) and Meijer et al. (2003) estimated the global distribution pattern of soil pollution by PCBs. They identified regions of intensive pollution, corresponding to the industrialized countries of the northern hemisphere, such as North America, central Europe, Republic of Korea and Japan, which were explained by emissions during production, use, and spillage. Another source of PCBs soil pollution originates from accidental releases and by emissions from landfills, including illegal waste dump, which corresponds for 52 – 57 percent of the global PCB production (Breivik et al., 2002b). Accidental releases can also occur because of fires and spills to soils, which are estimated to account for 4-9 percent of the global PCBs releases to the environment. Due to the current use of PCB-containing products, it is expected that significant emissions will occur in the near future and that the PCB load of soils will be maintained for a long time.
Additionally, there has been diffuse global pollution caused by the LRT of air emissions and the transboundary and transcontinental handling of PCBs waste (Breivik et al., 2007). Monitoring studies attributed an urban origin to soil pollution caused by PCBs (Aichner et al., 2013; Hafner and Hites, 2003).
Despite the numerous studies on soil pollution by PCBs, no clear conclusion can be drawn on the current global status of PCBs pollution for three main reasons. First, studies do not provide a representative picture of all geographical regions. Second, the analytical methods used are diverse and results cannot be compared. Third, the studies cover a long period of time and the results may be biased by temporal trends. For these reasons, for an overview of soil pollution by PCBs, reference is made only to the study conducted by Meijer et al. (2003), in which the abundances and stocks of PCBs in topsoil were determined in 191 different locations. In this study, samples were taken in remote areas far from potential local emitters and the sampling, pre-treatment and analysis were identical. PCBs pollution in soils showed a strong relationship with the proximity to the source and the content of soil organic carbon.
Two of the key parameters that control the fate of PCBs in the environment are their volatility and lipophilicity. There are 209 PCBs congeners that differ on the chlorination degree. The lipophilicity and sorption capacity to soil organic matter increases with increasing degree of chlorination. The congener ratios in the atmosphere and in soil differ from the ratios in the original PCBs source. Figure 13 illustrates the shift of partitioning to higher chlorinated PCBs from technical product to soil in moderate climate regions and northern (boreal forests) climate regions. The technical PCB is dominated by the lower chlorinated congeners while PCB soil profiles are dominated by higher chlorinated PCB congeners (Breivik et al., 2007). The different congener patterns between the technical product and air emissions/polluted soil are triggered by the effect of temperature. The congener patterns along the latitude change from heavier to lighter congeners along the latitudes from the equator to the poles. This is explained by a decrease of the volatility of PCB congeners with the ambient temperature and the degree of chlorination (Meijer et al., 2003).
The ratio of PCB congeners found in soils tend to form an equilibrium in which the population is dominated by the hexa-substituted PCBs (Aichner et al., 2013; Meijer et al., 2003). This process is mediated by the physical-chemical properties of PCB congeners and the soil properties (soil organic carbon content) and climate conditions. On a global scale, regional differences can be derived, taking into account different scenarios of population, industrialization, climate regions and vegetation. Additionally, forests soils show higher abundances than agricultural soils due to the greater uptake of air contaminants by the forest canopy.
Due to remaining PCBs stockpiles and ongoing pollution, in combination with illegal or unprofessional waste handling, PCBs pollution of soils is a legacy problem that remains a concern today and will continue to be so in the coming decades.