Chapter 3. Sources of soil pollution
Sources of soil pollution and major contaminants in agricultural areas
Agricultural soils can be contaminated with a wide range of compounds, from both direct inputs (point source pollution) such as the application of pesticides and fertilizers and indirect inputs (diffuse pollution) such as flooding and atmospheric deposition. Polluted soils also represent a secondary emission source of contaminants to surrounding air, surface waters, groundwater, and subsequently to oceans.
The main sources of soil pollution in agricultural areas can be grouped as: i) pesticides; ii) mineral fertilizers; iii) organic fertilizers (manure and sewage sludge); iv) wastewater for irrigation; v) plastic materials such as films for mulching and greenhouses, drip irrigation tubes and empty packaging; vi) and rural wastes. Different contaminants are linked to each source.
3.3.1. Pesticides
According to the International Code of Conduct on Pesticide Management (FAO and WHO, 2014), pesticides are defined as “any substance, or mixture of substances of chemical or biological ingredients intended for repelling, destroying or controlling any pest, or regulating plant growth”. Pesticides include, but are not limited to, insecticides; fungicides; herbicides; rodenticides; molluscicides; nematicides; plant growth regulators (such as defoliants, desiccants, agents for setting, thinning or preventing the premature fall of fruit), and substances applied to crops either before or after harvest to protect the commodity from deterioration during storage and transport. In addition to agricultural use, pesticides are also used to control vectors of human or animal disease, and this widespread global use can impact soil organisms, wildlife and non-target plant species, and human health (FAO and ITPS, 2017; Kim, Kabir and Jahan, 2017; Newman, 1948; Sánchez-Bayo, 2011).
Pests and crop diseases have existed since the beginning of agriculture and humans have continuously sought practical solutions to reduce crop yield losses (Sánchez-Bayo, 2011). The first pesticides were based on inorganic chemicals such as sulphur, copper, mercury and arsenic compounds (Unsworth, 2010). By the end of the first half of the 20th century, the synthetic pesticide industry began to develop organic pest control compounds, which were seen as a miraculous solution (part of the “Green Revolution”) to the world’s growing demand for food and to major public health concerns such as malaria and typhus. The pesticide industry has continued to develop to the present day, with more than 800 active ingredients being commercialized in the global market (Sánchez-Bayo, 2011). The European Union pesticide database lists 1 426 pesticide active ingredients of which 476 are approved for use, 49 are pending approval, and the remaining 901 are banned or otherwise not approved for use (European union, 2016). Between 2000 and 2017 there has been continued growth in pesticide use on all continents, with a wide disparity between them (Table 2), ranging from around 8 percent increase in Europe to 84 percent and 104 percent in Oceania and South America respectively (FAOSTAT, 2019a). The UNEP report on Pesticides and Fertilizers (UNEP, 2021) identifies that the rate of use of pesticide active ingredient per unit of crop land has increased from 1.9 kg/ha in 1990 to 3.3 kg/ha in 2016, an increase of 75 percent (Figure 2). With the growth in global population, demand for food is increasing but due to the limited availability of new agricultural land, intensification of agricultural production will be required. However concerns over the unsustainability of the reliance on pesticides and fertilizers to provide this intensification into the future has led to calls for the adoption of more sustainable agricultural practices (FAO, 2011).
Table 2. Use of pesticides between 2000 and 2017.
Figure 2. Global agricultural pesticide use per unit crop land has increased by 75 percent between 1990 and 2016.
Pesticides reach the soil by deposition after being sprayed on foliage when the pesticide is washed off treated foliage by rainfall or overhead irrigation, by release from the surface of treated seeds or by direct application of granules or spray to soil (Sitaramaraju et al., 2014). Agricultural soils are also frequently affected by accidental releases of pesticides from leaking pipes, spills, damaged containers, waste dumps or underground storage tanks (Knapton et al., 2006; Kumar, Kim and Deep, 2015). The inappropriate disposal of unwanted or out of date pesticides, pesticide packaging and the cleaning of application equipment can also cause pollution.
Many pesticides and their degradation products are persistent in the environment due to their long half-lives and may have acute and/or chronic effects on non-target organisms including humans (Damalas and Eleftherohorinos, 2011; FAO and ITPS, 2017; Kim, Kabir and Jahan, 2017). Environmental concern about pesticide residues started in the 1960s after the publication of the seminal book Silent Spring (Carson, 1962). Pesticides and some of their degradation products may accumulate in soils (Báez et al., 2015; Jablonowski et al., 2012; Navarro, Vela and Navarro, 2007; Turgut et al., 2010), leach to groundwater (Arias-Estévez et al., 2008; Rosenbom et al., 2015) and can be transported by runoff to surface water bodies (Lefrancq et al., 2017; Wauchope, 1978). Pesticides can also volatilize and be transported globally to be deposited on soils and surface waters (Tripathi et al., 2019a). Their fate in the soil and transfer to other media (waterbodies, food or organisms) depends on:
- their chemical characteristics and half-lives;
- the properties of the soil such as organic carbon content, texture, mineralogy, pH, and soil microorganisms, which control their persistence, bioavailability and mobility (see chapter 2);
- climatic conditions (Monaco, Weller and Ashton, 2002).
Their persistence and mobility in the environment will also influence their effects on human health and their entry into the food chain (Wilkinson et al., 2000) (see Chapter 4).
In addition to the active compounds and degradation products, some pesticides are also sources of trace elements, such as copper-based fungicides applied in vineyards and orchards (Tóth et al., 2016 and references therein) or fungicides and pesticides containing arsenic, copper, manganese and zinc that were used historically to protect citrus, apples, peaches, strawberries, and other fruit crops (Alva, 1992). Organomercury compounds such as phenylmercury acetate were used as seed dressings (Byford, 1985), while organotin compounds have been used in antifouling paints for boats and have accumulated in harbour sediments (Maguire et al., 1986). Dredged marine sediments are often taken to land for disposal in landfills, which do not always have leachate containment measures, or are used as agricultural soil amendments, posing a high risk of soil pollution (Chan and Abdul Jalil, 2014). Chromated copper arsenate was used as a wood preservative (Hingston et al., 2001). The Compendium of Pesticide Common Names (Wood, 2020) holds information on more than 1 800 active ingredients, of which 219 include one or more trace element in their composition as shown in Table 3.
Table 3. Pesticide active ingredients containing trace elements
Due to the growing awareness of the risk to environmental and human health posed by the growing number of hazardous chemical compounds (including pesticides), the international community adopted two chemical conventions at the turn of the last millennium. The Rotterdam Convention on the use and trade of hazardous chemicals was adopted in 1998 and subsequently came into force in 2004 (Rotterdam Convention, 2010). The Stockholm Convention on the control and elimination of persistent organic pollutants (POPs) was adopted in 2001, and also came into force in 2004 (Stockholm Convention, 2008). POPs remain intact for long periods of time and become widely distributed in the environment as a result of natural processes involving soil water and, particularly, air. They accumulate through the food chain and are toxic to humans and wildlife (Stockholm Convention, 2008). Pesticides make up the majority of chemicals that are subject to each convention, 16 of the 26 chemicals to be eliminated under annex A of the Stockholm Convention and 36 of the 53 restricted chemicals in Annex III of the Rotterdam Convention. Since the ratification of the Rotterdam and Stockholm Conventions, slight reductions in trade of highly toxic pesticides from developed countries (OECD) to developing countries (non-OECD) have been reported (Núñez-Rocha and Martínez-Zarzoso, 2018). However, least developed countries still face difficulties in banning certain pesticides as alternatives are too expensive or stockpiles of the banned products are still present in those countries. In the early years of the millennium there were large stockpiles of obsolete pesticides in developing countries, mainly in Africa, South America and the Caribbean, Central Asia, India and East Europe. In some cases the pesticides are stored in containers that were degrading due to aging and weathering, or have been buried without the necessary containment measures. Through the concerted effort of the affected countries, the Global Environmental Fund (GEF) and other donors, international agencies, NGOs and the private sector the majority of the above-ground obsolete stocks have now been safeguarded and eliminated in an environmentally sound manner. Buried pesticides and soil that had been polluted by leakage from these stocks largely remain to be addressed. Pesticide pollution and exposure occur at storage sites, airstrip operation sites, areas where pesticides were used and where obsolete pesticides were disposed (Toichuev et al., 2018b, 2018a). However there is still the potential to generate new stockpiles of obsolete pesticides through the banning of further chemicals, poor pesticide procurement and stock management, and the introduction of counterfeit pesticides into the market.
Illegal and counterfeit pesticides also have the potential to cause soil pollution. It is estimated that between 5 percent and 15 percent of the pesticides that are introduced on the global market each year are illegal or counterfeit (UNICRI, 2016) In addition to the potential harm to the health of farmers, damage to the crop, and food safety, the application of these illegal and counterfeit pesticides can pollute the soil and potential impact on its viability of future harvests (UNEP, 2021).
3.3.2. Mineral fertilizers
Plants absorb nutrients and water present in the soil solution through their root systems. These nutrients can come naturally from the soil itself through the dissolution of minerals, desorption from minerals, decomposition of soil organic matter, fungal hyphae and activity of microorganisms. Nutrients can also be added to the soil from external inputs. Fertilizers, including those of mineral, synthetic inorganic, and organic origin, have been essential in supplying sufficient nutrients for increasing crop production globally. Globally, the value of gross agricultural production has increased by 250 percent between 1960 and 2016 (FAOSTAT, 2019b) during which time the global population increased by 145 percent. This tremendous increase in population could not have been achieved without management practices that included the provision of nutrients. From 1961 to 2002 the world-wide use of nitrogen (N) fertilization expanded 7.4-fold with 2.3-fold increase in phosphorus (P) fertilization (FAOSTAT, 2019c). According to the FAO report on global trends in fertilizer use (FAO, 2016), the demand for fertilizer nutrients has had a stable annual growth rate of 1.6 percent and was expected to reach 199 million tonnes by the end of 2019 (Figure 3).
Figure 3. Global use of inorganic fertilizers in agriculture.
The overuse of fertilizers in some regions and countries has led to worrying environmental problems such as the saturation of nutrients in soils and the loss of fertilizer via leaching to groundwater and via runoff to surface water leading to pollution of drinking water and eutrophication of freshwater rivers and lakes as well oceans. This excessive use is the result of several factors:
- low price contrasted with perceived lack of downside to excessive use, as compared with decreased yields;
- lack of updated and accurate fertilizer recommendations for many crops, particularly tree crops;
- lack of awareness and training on the use of the product.
It is important to improve fertilizer use efficiency to meet rising global needs for food, to address a decreasing supply of important nutrients such as P, and to limit environmental risks caused by excessive fertilizer use (e.g., eutrophication) (Rahman and Zhang, 2018). The need for a more sustainable use of fertilizers has led to the development of a Code of Conduct for Sustainable Use and Management of Fertilizers by FAO (FAO, 2019b). The code is a tool for implementing the Voluntary Guidelines for Sustainable Soil Management (FAO, 2017), with regard to nutrient imbalances and soil pollution. The code promotes practices including nutrient recycling and agronomic and land management to improve soil health. As part of its common agricultural policy proposals for 2021-27, the European Union is developing a new tool to help farmers manage the use of nutrients on their farm. The Farm Sustainability Tool for Nutrients aims to facilitate a sustainable use of fertilizers for all farmers in the European Union while boosting the digitization of the agricultural sector (European Commission, Agriculture and Rural Development, 2020).
Overuse of nitrogen fertilizer leads to nitrogen losses from the soil through volatilization, denitrification, leaching to groundwater, and through surface runoff and erosion. Depending on both the form of nitrogen and on soil conditions, nitrogen is transformed by microbial activity into nitrous oxide (N2O), which is one of the greenhouse gases responsible for global warming, and which contributes to depletion of the stratospheric ozone layer (FAO and IFA, 2001; US EPA, 2017a). Various nitrogen fertilizers contribute to soil acidification, acid rainfall and eutrophication through a variety of transformation processes in the soil and atmosphere. Reactions of nitrogen-bearing species in the atmosphere can result in formation of aerosol particles that pose risks for human health and the environment (Bittman et al., 2014; IPNI, 2011). As described by recent studies, soil acidification is an environmental condition that can reduce the availability of many plant nutrients in the soil for plant growth, cause direct toxic effects from soil elements such as aluminium, increase the risk of soil-borne plant diseases and limit the capacity of plants to counteract the attack from pathogens (Li et al., 2017b; Shen et al., 2018).
Phosphorus in soil is depleted due to crop uptake and erosion, and its replenishment through rock weathering and atmospheric deposition is slow. Hence, the application of inorganic and organic forms of phosphorus has become an essential practice in crop production (Liu and Chen, 2014). Modern agriculture is dependent on phosphorus derived from phosphate rock (PR), mainly from sedimentary ores (75-80 percent), which is a non-renewable resource (Prud’homme, 2016). Phosphate rock, especially in the sedimentary ores, contains variable levels of trace elements and radionuclides (El-Bahi et al., 2017; Gupta et al., 2014). In the phosphate mining process, trace elements and radionuclides are mobilized, and dust generated is a source of fluoride and radon emissions (Azzi et al., 2017; Reta et al., 2018). This leads to an addition of harmful substances such as arsenic, cadmium, chromium, lead, mercury, fluorine, and radionuclides like uranium, radium and thorium to soil when applying phosphate fertilizer, which may accumulate in plants and enter the food chain (Tiessen, SCOPE and UNEP, 1995). Repeated applications of fertilizers can lead to a significant accumulation of potentially toxic elements into the soil (Jiao et al., 2012). Soils with a long history of phosphorus fertilizer applications had high levels of copper, zinc and cadmium (Maas et al., 2010).
The world’s known reserves of phosphate rock are being depleted, and it is estimated that global demand for phosphorus will exceed its supply by 2035 (Cordell, Drangert and White, 2009; Filippelli, 2011), due to an estimated annual increase in demand of 0.7-1.3 percent (Van Vuuren, Bouwman and Beusen, 2010). However, a reassessment by The International Fertilizer Development Center of global phosphate reserves and resources and the opportunities for more efficient extraction and processing technologies indicated that reserves had the potential to last much longer (Van Kauwenbergh, 2010).
Van Kauwenbergh (2010) presented cadmium content of phosphate rock obtained from 35 sedimentary deposits in 20 countries. The average cadmium content of sedimentary phosphate rock deposits across the globe is about 69 times greater than that of non-phosphate containing rocks. Concentration of Cd can vary widely between countries and within deposits in the same country, within a range of less than 1 mg/kg to 150 mg/kg. The highest levels have been reported for phosphates rock from Idaho, the United States of America (40 – 150 mg/kg), and in Tobene, Senegal (60 – 115 mg/kg). Approximately 85 percent of global fertilizer production is from sedimentary deposits. Igneous phosphate rock has average cadmium concentrations of only 2 mg/kg. In the manufacturing process all the cadmium transfers to the product.
The overuse of phosphorus fertilizers can lead to its loss from croplands via erosion or runoff (Smith et al., 2016). Eutrophication of surrounding water bodies from agricultural fields is a result of high quantities of phosphorus lost through erosion or runoff (Withers et al., 2019). Quantifying phosphorus losses in eroding agricultural soils is particularly uncertain, as erosion rates vary widely even within a single field (Liu et al., 2018). It is also because few nations have comprehensive, periodic inventories of their soil erosion (Clearwater, Martin and Hoppe, 2016; Liu and Chen, 2014).
Phosphate extraction to produce phosphate fertilizers is done through wet acid extraction, which produces phosphogypsum as residue. It is estimated that around 100-280 Mt of phosphogypsum are produced each year (Saadaoui et al., 2017; Tayibi et al., 2009). Phosphogypsum may be enriched in radon, uranium and thorium depending on the phosphate rock source (Borrego et al., 2007; Rutherford, Dudas and Samek, 1994), and presents variable content of trace elements, such as barium, cadmium, copper, nickel, strontium, and zinc, depending on the characteristics of the parent rock (Luther, Dudas and Rutherford, 1993; Pérez-López, Álvarez-Valero and Nieto, 2007; Sahu et al., 2014). Therefore, the US EPA (2014, 2015a) has classified phosphate fertilizer production wastes as Technologically Enhanced Naturally Occurring Radioactive Material (TENORM). The residues of phosphate fertilizer production normally end up in open stockpiles and it has been observed that leachates from these stockpiles occur under ambient environmental conditions (Azouazi et al., 2001; Battistoni et al., 2006; Gázquez et al., 2014; Lysandrou and Pashalidis, 2008). Pérez-López, Álvarez-Valero and Nieto (2007) found an enrichment in trace elements associated with the organic fraction in the phosphogypsum waste and a mobilization of U from the crystalline structure in the original phosphate rock to the bioavailable fraction in the waste.
In addition, the production of high analysis phosphate fertilizers via phosphoric acid generates as a by-product phosphogypsum, which has been used as a soil amendment in many countries for several decades to improve soil properties and crop yields. Phosphogypsum is rich in calcium and therefore has a great potential for liming and immobilization of the trace elements (Campbell et al., 2006; Mahmoud and El-Kader, 2015). However there is evidence of possible accumulation of trace elements and radionuclides in soils that have been amended with phosphogypsum, pyrites and other amendments, depending on their source (Abril et al., 2008) (see section 2.5.1.1). Therefore, it is pertinent to monitor the content of these contaminants in soils and their transfer to the food chain, because the promotion of such recycling may increase phosphogypsum use (Papastefanou et al., 2006).
3.3.3. Organic fertilizers
Organic fertilizers commonly used in agriculture include animal manure, compost, septic sludge, sewage sludge, municipal biosolids and food processing wastes (Khan et al., 2018). The beneficial aspect from the use of the organic fertilizer is the improvement in soil health through organic carbon enrichment and slow release of nutrients. However, it may also become a source of soil pollution due to the mineralization of organic nitrogen, increasing the nitrate concentration, and the presence of trace elements, perfluorinated alkylated substances (PFASs), brominated flame retardants and other toxic substances (Gottschall et al., 2017; Petersen et al., 2003).
The use of organic fertilizers has been increasing due to the growing interest in organic production methods and consumption of organically produced food. The recycling of organic wastes in agriculture is also seen as a sustainable and environmentally sound disposal option (UN-Habitat and Greater Moncton Sewerage Commission, 2009).
Manure and animal wastes
Animal manure management systems can be classified into solid, slurry and lagoon systems, or a combination, based on the total solids content, collection, storage, transportation and application of manure on the fields (Jiang, Chen and Dharmasena, 2015). Animal wastes can be pre-treated through anaerobic digestion or composting. Anaerobic digestion generates methane that may be used for heating or power generation on the farm (Demirbas and Ozturk, 2005).
In intensive livestock production, the feeds may contain essential elements, such as copper and zinc, and non-essential elements, such as chromium, lead and other potentially toxic metals. The addition of essential trace elements in animal feed is a widespread method to improve production efficiency (Wang et al., 2017b). Researchers have found an increase in levels of trace elements in manures from 1990 to 2010 (Wang, Dong and Wang, 2014; Zhang et al., 2012a). The use of copper and zinc in animal feed results in higher levels of copper and zinc in manure with associated risk for agricultural soils (Mantovi et al., 2003; Xiong et al., 2010). Different trace elements are used in animal production according to specific needs. For example, copper sulphate, is used as a footbath to treat lameness in dairy cattle and as a growth promoter in pig and poultry systems. Zinc oxide is added to pig feed as a cure for scour (Poulsen, 1998).
An emerging concern comes from the increase of antimicrobials and antimicrobial resistant organisms found in the soils after treatment with organic fertilizer of human or animal origin (Jiang, Chen and Dharmasena, 2015 and references therein). Antimicrobials are routinely provided to animals prophylactically to prevent disease but also to promote growth. It is reported that 75 to 90 percent of the administered antimicrobials are not completely metabolized in human and animal bodies (Marshall and Levy, 2011), are excreted and accumulate in animal manure, municipal wastewater, wastewater sludge and biosolids (Bouki, Venieri and Diamadopoulos, 2013; Daghrir and Drogui, 2013; Wu et al., 2013).
Low concentrations of antimicrobials in the soil can lead to the spread of antimicrobial-resistance as a result of changes in the bacterial genome and transfer of the antimicrobial resistance genes (ARGs) and associated mobile genetic elements (MGEs) between and among microbial populations. This results in an overall change in the microbial sensitivity to antimicrobials (Cycoń, Mrozik and Piotrowska-Seget, 2019; Grenni, Ancona and Barra Caracciolo, 2018). The spread of antimicrobial-resistance is now considered a great risk worldwide for human health (Berendonk et al., 2015).
Sewage sludge and biosolids
The diversity of the quality of wastewater and the technologies used to treat them produces a variety of semi-solid and solid residues that can potentially be used as organic fertilizers. These residues include: septic sludge that is untreated faecal matter from septic tanks; wastewater sludge that has undergone primary and secondary treatment including biological digestion and settling; and biosolids which have undergone tertiary treatment and are stabilized. Biosolids are produced in large quantities as a soil amendment for agriculture (e.g., 70, 30, and 6 million tons in Japan, China, and the United States of America, respectively) (Rai et al., 2019). Septic sludge still contains potentially harmful microbes and therefore has safety issues if used as a fertilizer, while biosolids have been tested to be appropriate for use as fertilizer (UN-Habitat and Greater Moncton Sewerage Commission, 2009). Depending on the sources, the wastewater could include industrial effluents and other pollution which may contain contaminants that are either lipophilic, which associate with the solid fraction or hydrophilic and remain in the treated aqueous fraction.
Biosolids contain high concentrations of organic matter and biogenic compounds, especially nitrogen and phosphorus, necessary for plant growth. As a result, applying biosolids as a fertilizer to agricultural fields can be a useful approach to improve soil nutrient content and organic matter and allow the reuse of a by-product from the wastewater treatment plants (Lloret et al., 2016). However, biosolids contaminated with lipophilic trace elements when applied to land are one of the most important soil contributors of trace elements in soils (Srivastava et al., 2015, 2016). Biosolids are also a source of nano- and microplastics. It is estimated that of all the microplastics that go through the wastewater treatment plant, 95 percent is contained in the biosolids (Ziajahromi, Neale and Leusch, 2016).
Besides trace elements, wastewater sludge and biosolids can be contaminated with POPs including polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F), polychlorinated biphenyls (PCBs), chlorinated paraffin (CPs) and perfluorinated alkylated substances (PFASs) like perfluorooctane sulfonate (PFOS) or perfluorooctanoic acid (PFOA), which has resulted in the pollution of agricultural soils (Brambilla et al., 2015; Rideout and Teschke, 2004; Umlauf et al., 2004; Venkatesan and Halden, 2013; Weber et al., 2018a; Zeng et al., 2013). During recent decades while PCDD/F and PCB levels have been decreasing in sewage sludge and compost, the levels of other POPs such as PFOS/PFOA and CPs are increasing (Brambilla et al., 2015; UNEP, 2013; Zennegg et al., 2013).
Pharmaceuticals and personal care products that are excreted and emitted into sewage systems following use include medicines, cosmetics and personal hygiene products, i.e. moisturizers, lipstick, shampoos, hair colours, deodorants and toothpastes. The drugs such as analgesics, antibiotics, antiepileptic, β-blockers, blood-lipid regulators, hormones antiretroviral, and steroids are persistent, bioactive and bio-accumulative substances that are released into the environment through wastewater and wastewater sludge. Some of these substances are also endocrine disruptors (Payá Pérez and Rodríguez Eugenio, 2018).
Compost
Biological wastes, such as green waste and food waste from households, municipalities and industries, can be manufactured into organic fertilizers by fermentation and composting. However, recent studies have found that such fertilizer is a source of microplastic particles that are very challenging to remove (Gionfra, 2018; Weithmann et al., 2018). Municipal solid waste compost used as an organic fertilizer in agricultural soils has also been found to be a source of trace elements (Bhattacharyya et al., 2005; Madrid, López and Cabrera, 2007).
Reliance solely on manures and other organic fertilizer with their particular ratio of NPK contents, may result in one of the elements being applied in excess. Excess application of phosphorus can lead to leaching and eventual eutrophication of fresh waterbodies. Excess application of nitrogen can lead to leaching of nitrogen, ammonia volatilization, and nitrous oxide emissions (Cameron, Di and Moir, 2013), and the eventual eutrophication of marine ecosystems (Smolders et al., 2010). Whether using organic or inorganic fertilizer it is important to apply the correct ratio of NPK to avoid these effects (FAO, 2019b). In China, 55 percent of 394 groundwater samples had nitrate levels exceeding the WHO drinking water standard of 11.3 mg/L N for nitrate (Chen et al., 2010).
3.3.4. Wastewater for irrigation
The treatment of wastewater is discussed in Section 3.3.3 above. The types and levels of contaminants in the wastewater depend on the sources of the effluents and the technologies used to treat them. Wastewater has been widely used for irrigation in agriculture in developing countries, especially in or close to cities, where it is plentiful (Scott, Faruqui and Raschid-Sally, 2004). According to the latest data available about 0.31 109 m3/year of treated municipal wastewater is directly used for irrigation purposes worldwide. Urban and peri-urban agriculture benefits from the use of this low cost resource for irrigation, and the organic contaminants minimize the requirements for artificial fertilizer (Qadir et al., 2010). However, irrigation with untreated wastewater can contribute to the accumulation of trace elements and organic contaminants such as POPs and PFAS, as well as represent a source of dangerous pathogens, and pharmaceuticals and personal care products (Islam et al., 2018; Woldetsadik et al., 2017). Only a few countries have reported the direct used of non-treated wastewater for irrigation, whose volume is greatly variable, reaching 4 million of cubic meter per year in Mexico (Figure 4). Consumption of food irrigated with untreated wastewater can pose a high risk for human health due to the potential exposure to pathogens and uptake and accumulation of trace elements in the edible parts of the crops (Zia et al., 2017). Untreated wastewater is also a source of microplastics (Prata, 2018). Irrigation with untreated wastewater can also negatively impact the physical properties of soil, such as the development of soil water repellency (Travis et al., 2010; Wallach, Ben-Arie and Graber, 2005).
Figure 4. Direct use of non-treated municipal wastewater for irrigation purposes.
Irrigation with secondary and tertiary treated wastewater is generally not widespread in developed countries, although it is a common practice in countries like China, Mexico and countries of the Near East and North Africa, where water shortages are frequent (Figure 5). In Israel, about 50 percent of agricultural production stems from irrigation with treated wastewater (Reznik et al., 2017). For the most part, contaminants are left behind in the residual biosolids, such that treated wastewater has low contaminant and microplastics content.
Figure 5. Direct use of treated municipal wastewater for irrigation purposes.
Fire-fighting foams often used to contain perfluoralkyl substances (PFAS) until they were listed as POPS under the Stockholm Convention. Their pollution of groundwater and water used for irrigation is now a major concern (Ghisi, Vamerali and Manzetti, 2019).
3.3.5. Agricultural plastic waste
The use of plastics in agriculture has increased significantly in recent decades. Of the 708 000 tonnes of non-packaging plastic consumed in agriculture in the European Union in 2019, 56 percent was used in livestock production for baling (twine, nets and stretch film) and silage. Crop production consumed the remaining 44 percent for greenhouse films, mulching films, small tunnels, irrigation pipes and drippers, and protective nets and fleeces (Eunomia and Deloitte, 2020 and references therein). Plastic is also used for seed and fertilizer bags, pesticide containers, seedling trays and pots. Many of these products are for single use. Without effective mechanisms for their recovery from fields, collection and environmentally sound recycling or disposal after use, agricultural plastics represent a significant source of soil pollution and the wider environment (Vox et al., 2016). In recent years, the presence of plastic pollution in agricultural areas has attracted the attention of the scientific community and authorities (Chae and An, 2018; Gionfra, 2018).
Mulching films
Of all the plastic products used in agriculture, mulching films represent the greatest potential for soil pollution due to their intimate contact with the soil and the risk of their incorporation into the soil as they degrade or during post-harvest management.
Plastic mulch is used to increase yield by protecting the seedlings by providing insulation, limiting evaporation and to reduce weed and pest pressure. It is often used in conjunction with plastic drip irrigation tubing underneath. For rotated crops a mulching film is generally only used for a single planting season (Figure 6). At a worldwide scale, China is the largest user of plastic film mulching and registered a fourfold increase in use between 1991 and 2011 (Daryanto, Wang and Jacinthe, 2017; Steinmetz et al., 2016b). While estimates for Europe indicated that 427 000 hectares of agricultural land were covered with plastic mulch film in 2011 (Jansen, Henskens and Hiemstra, 2019), in the same year, an estimated 20 million hectares were covered with plastic film in China (Yan, He and Turner, 2014; Zhang et al., 2016a). Although more recent data have not been found, considering the upward trends recorded in previous years, it is estimated that the area covered by mulching is likely to have increased further (Jansen, Henskens and Hiemstra, 2019).
Figure 6. Plastic mulching on Round Hill, the United Kingdom of Great Britain and Northern Ireland. © Derek Harper (via Wikimedia Commons, CC BY-SA 2.0).
The major source of plastic to agricultural soil is the widely used polyethylene conventional plastic mulch films introduced in 1978. Its attractiveness is due to its durability and flexibility; however, it is non-biodegradable and recovery from the fields and recycling is complex and costly (Gionfra, 2018; Kasirajan and Ngouajio, 2012). While polyethylene has been the most used polymer for the past decades, new biodegradable plastic films are being introduced to substitute this non-degradable synthetic plastic to eliminate the costs of their removal post-harvest (Lamont, 2017). However, there are significant gaps in knowledge regarding their complete degradation (Martín-Closas et al., 2016; Sintim and Flury, 2017) and the effect of by-products and additives (such as plasticizers) during the biodegradation process on both soil structure and soil organisms (Corbin et al., 2013; Kasirajan and Ngouajio, 2012). Plastics do not degrade completely, but are fractionated into smaller and smaller particles (micro- and nano-plastics), capable of entering the food chain (Astner et al., 2019). A significant risk associated with the presence of plastics in soil is due to the presence of micro- and nano-plastics, which can be ingested by organisms that feed on soil particles and transfer them to the food chain (Huerta-Lwanga et al., 2017). These issues could prevent the adoption of these alternatives (Goldberger et al., 2015). There are two main types, oxo-degradable plastics and biodegradable bioplastics. Oxo-degradable plastics are traditional synthetic plastics such as polyethylene with additives that encourage oxidation and fragmentation of the films. The concern that these films contribute to microplastic pollution (EC, 2016) and has led to their ban in some European countries. It is claimed that mulching films made from biodegradable bioplastics do completely degrade in the soil and so can be safely ploughed into the soil post-harvest (BASF, 2020).
The extensive use of conventional plastic mulching, especially in China, has led to large accumulations of plastic residues in soils with a resultant decrease in yields. The residues smaller than 5 mm are usually referred to as microplastics, and their further breakdown most probably leads to the formation of nanoplastics, which are more difficult to detect (Rocha-Santos and Duarte, 2015). Plastic mulch pollution negatively affects plant growth by altering the activity and community of microorganisms; soil structure; seed germination; root development. It can also increase greenhouse gas emissions (Boots, Russell and Green, 2019; Chang-rong et al., 2014; Gao et al., 2019; Kim et al., 2014).
Another concern about plastic mulch residues relates to their additives, some of which are carcinogenic and endocrine-disrupting (Steinmetz et al., 2016a). They are a major source of phthalates in soils (Li et al., 2016a; Wang et al., 2015), but other highly toxic organic compounds could also be adsorbed on their surfaces and subsequently released during their alteration or interaction with organisms (Peng, Wang and Cai, 2017; Teuten et al., 2009; Thompson et al., 2009). Their release into the soil can pose a high risk for environmental and human health, although further information is needed to understand the threat (Ramos et al., 2015; Revel, Châtel and Mouneyrac, 2018).
Greenhouse film and protective netting
The use of greenhouses is widespread in all continents to ensure continuous supply of food over the year (Chang et al., 2013). China has the largest greenhouse-covered area (Chang et al., 2013), with about 3.3 million hectares (University of Rhode Island, 2019), while estimates for Europe in 2015 were about 175 000 hectares of protected horticulture areas (EC, 2017). High-technology greenhouses made with glass are replacing plastic greenhouses in developed countries (National Geographic, 2017), however, plastic continues to be used for other applications inside the greenhouse and has a very low recycling rate, which generates plastic waste (EC, 2017). In less developed countries, plastic films are still the major low-cost selected option (Gruda and Popsimonova, 2017). Greenhouse films and protective netting are designed to last multiple harvests, but eventually weathering and photodegradation require them to be replaced and the old materials safely recycled or disposed of.
Other agricultural plastic wastes
As discussed above there are many other agricultural plastic products that become wastes on the farm that, unless they are collected and managed in an environmentally sound way, represent a potential source of pollution of soils. Non-biodegradable mulching films, irrigation tubing, small tunnel films and protective fleeces need to be carefully and completely removed from the fields to avoid the risk of being incorporated into the soil during its post-harvest management. Polystyrene seedling trays need to be collected and removed from the fields after planting. Used silage and bale films, nets and twine need to be collected and stored. Empty pesticide containers represent a particular hazard due to the presence of residual quantities of the pesticide. In developing countries, pesticides for small-scale farmers are often sold in single dose sachets. The collection and safe disposal of contaminated sachets is a particular challenge and they can be found littering fields (Figure 7). The International Code of Conduct on Pesticide Management (FAO and WHO, 2014) is supported by technical guidelines on the sustainable management of empty pesticide containers (FAO and WHO, 2008). Empty packages of seeds could similarly be contaminated with pesticidal seed dressings and should be managed similarly.
Figure 7. Improperly discarded empty pesticide containers may be an important source of soil pollution. © Daniels Gene (public domain, via Wikimedia Commons)
Many countries have legislation on extended producer responsibilities that require manufacturers and their extended supply chain to collect plastic wastes from farmers and to organize the environmentally sound recycling or disposal of the plastic waste. Many of such schemes commenced with voluntary support of the pesticide industry for empty containers and have broadened to manage a wider range of agricultural plastic wastes. Example of such schemes are those run by Adivalor in France (ADIVALOR, 2020) and inPEV in Brazil (inPEV, 2018). The European Commission under its EPI-Agri programme has established a focus group to investigate the reduction of the environmental footprint of agricultural plastics (EC, 2020).
Where such collection schemes do not exist, there is a danger of soil pollution if the plastic waste is disposed of inappropriately on the farm. The open burning of plastics has long been recognized as a potential source of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans both of which are POPs and subject to the Stockholm Convention (Stockholm Convention, 2008) and other potentially polluting and harmful contaminants.
3.3.6. Rural community waste
Municipal waste is discussed in detail in Section 3.4.6. Rural communities generate similar types of waste but at smaller quantities, which makes it more complex and costly to collect and recycle or dispose of in an environmentally sound manner. In the event that the wastes are not collected for sound environmental management, it is likely that they will be dumped indiscriminately in the countryside or disposed of in an uncontrolled open dump. In order to reduce volumes, the wastes may be burnt in the open with the uncontrolled fires generating toxic emissions including partially combusted particulates containing contaminants such as PCDDs and PCDFs, which can pollute soils at both short and long range.