Chapter 5. Global status of soil pollution
Global actions to tackle soil pollution: legislation and monitoring
5.6.1. International legal instruments to tackle soil pollution
The examples mentioned in the previous sections highlight the global nature of soil pollution. Sources of pollution exist on all continents, and in addition there is planetary mobilization and transport of many contaminants. Soil pollution is not a local problem, and nor is it limited by administrative boundaries, and therefore it must be addressed globally in the same way that we work together to combat other global threats, such as climate change, land degradation and biodiversity loss.
Although there is currently no binding international agreement focusing specifically on soil pollution prevention, control and remediation, a series of binding conventions partially address these objectives. The Basel, Rotterdam, Stockholm and Minamata Conventions are multilateral environmental agreements, all of which aim to minimize the adverse effects of chemicals and hazardous wastes on human health and the environment. While they all have similar goals, each of these conventions addresses one specific issue: the Basel Convention deals with hazardous wastes, the Rotterdam Convention covers pesticides and industrial chemicals, the Stockholm Convention tackles Persistent Organic Pollutants (POPs) and the Minamata Convention focuses on mercury and mercury compounds. The scope of these Conventions and the countries currently ratifying them are examined in detail below.
5.6.1.1. Basel Convention on the Control of Transboundary Movements of Hazardous Wastes and their Disposal
The Basel Convention on the Control of Transboundary Movements of Hazardous Wastes and their Disposal (‘Basel Convention’) was adopted in 1989 and entered into force in 1992. The Convention aims to reduce the generation and movement of hazardous wastes unless they are deemed to be consistent with the principles of ‘environmentally sound management’. By environmentally sound management of hazardous wastes, the Convention means ‘taking all practicable steps to ensure that hazardous wastes or other wastes are managed in a manner in which will protect human health and the environment against the adverse effects which may result from such wastes’ (Basel Convention, 2011).
The Convention focuses on transboundary movements, preventing Parties to the Convention from exporting hazardous wastes and other wastes to a country that is not a Party to the Convention, or to a Party that has prohibited the import of hazardous waste (Art. 4). Parties may nevertheless conclude bilateral or multilateral agreements on hazardous waste management with other Parties or non-Parties if the principles of ‘environmentally sound management’ are respected.
Associated with the reduction of hazardous wastes movements, the Basel Convention also deals with the generation of hazardous wastes at the national level. Indeed, Parties must adopt appropriate measures to reduce domestic waste, provide adequate disposal facilities and ensure that those involved in waste management take the necessary steps to prevent pollution from hazardous wastes and, ‘if such pollution occurs, to minimize the consequences thereof for human health and the environment’ (Art. 4).
Currently, the Convention has 53 signatories, 188 parties ratified and implemented it, making it the most ratified Convention of the four chemical Conventions (Figure 15). It should be noted that in addition to Member Nations, one Member Organization, the European Union, has also signed, ratified and implemented the Convention. Vanuatu and Tuvalu are the latest countries to ratify it and to put the Basel Convention into force in 2019 and 2020, respectively. Although Haiti and the United States of America signed the Convention in 1989 and 1990, they have not ratified or implemented it to date (Figure 15). Fiji, Niue, Solomon Islands, South Sudan, Timor-Leste, and Tokelau are not parties of the Basel Convention to date.
Figure 15. Parties and status of ratification of the Basel Convention.
Source: UN, 2020 modified with data from the Basel Convention, 2021.
Protocol on Liability and Compensation for damage resulting from transboundary movements of hazardous wastes and their disposal
The Basel Protocol on Liability and Compensation, adopted in 1999, aims to establish a comprehensive regime of liability and compensation for damage resulting from transboundary movements of wastes and their disposal, including from illegal dumping and accidental spills. As such, the Protocol addresses the question of who is financially responsible in the event of an incident during a transboundary movement of hazardous wastes, from the point of loading of the waste on the means of transport to its export and final disposal (Basel Convention, 2011).
Although the discussions on the Protocol began in 1993 and was adopted in 1999, the Protocol is not yet in force, as it has not yet reached the deposit of the twentieth instrument of ratification, acceptance, formal confirmation, approval or accession. To date, the Protocol has been signed by 13 countries and ratified by 12 Parties (Basel Convention, 2020). Most ratifications took place between 2003 and 2009, with the exception of Saudi Arabia and Palestine, which ratified it in 2013 and 2019, respectively. While Denmark, Finland, France, Hungary, Luxembourg, Monaco, North Macedonia, Sweden, Switzerland and the United Kingdom of Great Britain and Northern Ireland signed the Protocol in 2000, no European country has yet ratified it. North American and Asia-Pacific countries have not signed or ratified the Protocol either.
This protocol could be relevant in the occurrence of soil pollution by hazardous waste and the application of the “polluter-pays principle”.
5.6.1.2. Rotterdam Convention on Prior Informed Consent for Hazardous Chemicals and Pesticides in International Trade
The Rotterdam Convention on Prior Informed Consent for Hazardous Chemicals and Pesticides in International Trade (‘Rotterdam Convention’) was adopted in 1998 and entered into force in 2004. The Convention covers pesticides and industrial chemicals, and aims to promote shared responsibility and cooperative efforts between Parties in international trade of specific hazardous chemicals, by facilitating the exchange of information on their characteristics and by improving national import and export regulations (Rotterdam Convention, 2010, Art. 1).
Once a chemical is included by the Convention, Parties have nine months to prepare a response concerning the future of the chemical concerned. This response includes either the permission, prohibition or restriction of the import of the chemical (Art. 10). If a Party bans or restricts the import of the chemical, the Party is then obliged to do the same at national level by reducing or prohibiting domestic production of the chemical (Art. 10). Each Party’s responses for each chemical are then circulated among the Parties to the Convention so that they can ensure that exporters under their jurisdiction comply with each response (Art. 11). Furthermore, the Convention encourages the exchange between Parties of scientific, technical, economic and legal information on chemicals covered by the Convention (Art. 14).
Since the adoption of the Rotterdam Convention, the Conference of the Parties (COP) has adopted a series of decisions to amend Annex III, which lists all the chemicals banned or severely restricted by the Rotterdam Convention. As a result of these amendments, 9 new pesticides and 6 industrial chemicals were included in the list (Rotterdam Convention, 2020a). Currently, 52 chemicals are covered by the Convention: 35 pesticides, including 3 severely hazardous pesticide formulations, 16 industrial chemicals, and 1 chemical present in both the pesticide and industrial chemical categories (Rotterdam Convention, 2020b). Only one chemical was removed from Annex III of the Rotterdam Convention in 2015, following the recommendations of the Chemical Review Committee (CRC) suggesting that the inclusion of the chemical was no longer justified (Rotterdam Convention, 2020a). Six new chemicals will be considered for inclusion in Annex III of the Rotterdam Convention at the tenth meeting of the Conference of the Parties that will be held in 2021 (Rotterdam Convention, 2020c).
To date, the Rotterdam Convention was ratified and implemented by 164 parties, including the member states of the European Union (Figure 16). The latest countries to ratify and implement the Rotterdam Convention are Algeria and Tuvalu in 2020 and Barbados in 2021. Angola, Saint Lucia, Seychelles, Tajikistan, and the United States of America have all signed the Convention but have not ratified or implemented it. To date, 25 countries have not signed, ratified or adopted the Rotterdam Convention (Figure 16 Note: The map has been prepared for illustrative purposes only, the boundaries shown on this map do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. Figure 16).
Figure 16. Parties and status of ratification of the Rotterdam Convention.
Source: UN, 2020 modified with data from the Rotterdam Convention, 2019.
5.6.1.3. Stockholm Convention on Persistent Organic Pollutants
The Stockholm Convention on Persistent Organic Pollutants (‘Stockholm Convention’) was adopted in May 2001 and entered into force in 2004. The main objective of the Stockholm Convention is to protect human health and the environment from persistent organic pollutants (POPs) by prohibiting and reducing the release of POPs in the environment (Stockholm Convention, 2008). The chemicals covered by the Stockholm Convention are categorised in three annexes, based on their nature and degree of restrictions.
Annex A lists all the chemicals whose production and use, as well as their import and export, must be prohibited by the Parties to the Convention. Currently, there are a total of 26 chemicals in Annex A, 14 pesticides, 10 industrial chemicals and 2 chemicals in the pesticides and industrial chemicals categories (Stockholm Convention, 2021a). It is worth noting that there are specific exemptions for the use and production of some chemicals listed in Annex A. Annex B currently includes two chemicals and requires each party to restrict the production and use of these chemicals. Similar to Annex A, the import and export of chemicals listed in Annex B can still take place, but under specific restrictive conditions. Last but not least, parties are required to reduce unintentional releases of the seven chemicals listed in Annex C to the Convention.
Any Party to the Convention can submit a proposal to add a new chemical to Annex A, B or C of the Stockholm Convention. Initially, only twelve POPs were recognised as having adverse effects on human health and the ecosystem (Stockholm Convention, 2019a). Through the adoption of amendments, sixteen new chemicals have been added to the Stockholm Convention (Stockholm Convention, 2017) and three are currently under review by the POPs Review Committee. More than half of the new chemicals added to the Annexes occurred at the fourth meeting of the Conference of the Parties to the Stockholm Convention in 2009, but new chemicals continue to be added, the most recent being Dicofol and perfluorooctanoic acid (PFOA), its salts and PFOA-related compounds, both listed in Annex A since 2019 (Stockholm Convention, 2019b).
The Stockholm Convention is the most signed Convention among the four Conventions, with 152 signatories, and is currently ratified by 184 Parties, including the member states of the European Union (Figure 17). Equatorial Guinea is the most recent country that ratified and implemented the Stockholm Convention (in 2019 and 2020 respectively). Brunei Darussalam, Haiti, Israel, Italy, Malaysia, and the United States of America signed the Convention but have not ratified and implemented it yet (Figure 17). Bhutan, South Sudan, Timor-Leste, Tokelau, and Turkmenistan have not signed, ratified or adopted the Stockholm Convention to date (Stockholm Convention, 2021b).
Figure 17. Parties and status of ratification of the Stockholm Convention.
Source: UN, 2020 modified with data from the Stockholm Convention, 2021b.
5.6.1.4. Minamata Convention on Mercury
The Minamata Convention on Mercury, adopted in 2013 and entered into force in 2017, is a global treaty to protect human health and the environment from mercury and mercury compounds. Among others, the provisions of the Convention require each Party to prohibit the establishment of new mercury mines and the reduction and eventual elimination of all mercury mining within 15 years of the Convention’s entry into force in the territory of the Party (Minamata Convention on Mercury, 2019, Art. 3). At the same, Parties are required to take appropriate measures to reduce the manufacture, import and export of mercury-added products listed in the Convention (Art 4). The Convention also addresses the use of mercury and mercury compounds in artisanal and small-scale gold mining, its storage and its disposal to control and reduce mercury releases in the environment. The remediation of sites already polluted by mercury or mercury compounds is also covered (Art 12).
The Minamata Convention has 128 signatories and 129 parties ratified the Convention, including the European Union (Figure 18). Due to its recent adoption, the number of Parties to the Minamata Convention is slightly lower compared to the other Conventions (Basel, Rotterdam, and Stockholm). However, the number of parties has grown rapidly since 2017, with 93 countries having ratified the Convention (Minamata Convention, 2021). Italy, Cameroon, and Burundi are the latest countries to ratify the Convention on the first trimester of 2021. To date, 32 countries have not yet signed or ratified the Minamata Convention.
Figure 18. Parties and status of ratification of the Minamata Convention.
Source: UN, 2020 modified with data from the Minamata Convention, 2021.
5.6.2. International voluntary agreements to tackle soil pollution
In addition to the international conventions presented above, the adoption of non-binding instruments by Member Nations in the framework of international organisations also reflects the willingness of countries to tackle all forms of pollution, including soil pollution. These agreements have been concluded mainly in the framework of the FAO and the WHO and can serve as a basis for countries that currently have no or inadequate national legislation on specific issues related to soil pollution.
5.6.2.1. The Global Soil Partnership
The Global Soil Partnership (GSP) established in 2012 as a mechanism to develop a strong interactive partnership and enhanced collaboration and synergy of efforts between all stakeholders. The GSP has the mandate of improving governance of the limited soil resources of the planet in order to guarantee healthy and productive soils for a food secure world, as well as support other essential ecosystem services, in accordance with the sovereign right of each State over its natural resources. The GSP addresses all soil threats and seeks on the one hand to improve knowledge and available data and information for informed decision making, on the other hand to increase awareness of all stakeholders, and finally to implement field actions aimed at sustainable soil management and soil restoration and protection.
5.6.2.2. Voluntary Guidelines for Sustainable Soil Management
The Voluntary Guidelines for Sustainable Soil Management (VGSSM) endorsed by FAO Council in 2016 provide technical and policy recommendations for achieving sustainable soil management. The VGSSM identify ten threats to soil functioning and health, including soil pollution, and propose a set of principles to minimize and control these threats (FAO, 2017). Within the framework of these recommendations, Member Nations are encouraged to implement or strengthen inclusive regulations conducive to sustainable soil management in order to prevent and minimize the accumulation of contaminants in soil, as well as to promote soil information by testing, monitoring and assessing potentially contaminated soils in order to reduce risks to human health and the environment. The VGSSM also focus on the identification and remediation of already polluted soils to prevent the use of polluted soils for food and feed production, as well as controlling their physical degradation to avoid the diffusion of soil contaminants. Appropriate measures to minimize the adverse effects of pesticides and fertilisers are also addressed in the recommendations to combat nutrient imbalance, soil acidification and soil organic carbon loss.
More broadly, the VGSSM recommend eleven actions to address and prevent soil threats and implement sustainable soil management (FAO, 2017). These actions include:
- establishing or strengthening inclusive policies to support sustainable soil management;
- increasing responsible investment and positive incentives to promote sustainable soil management;
- promoting secure land tenure rights according to the Voluntary Guidelines on the Responsible Governance of Tenure;
- encouraging and strengthening targeted soil research;
- preventing or minimizing soil degradation and restoring/rehabilitating degraded soils;
- promoting effective education programmes;
- ensuring adequate inclusion of sustainable soil management in extension services;
- promoting principles and practices of sustainable soil management by agricultural extension services;
- establishing or strengthening soil information systems;
- encouraging international cooperation on soils;
- promoting communication on sustainable soil management practices.
The sustainable use of soils, including the judicious use of agricultural inputs and the proper management of waste, is of paramount importance to prevent soil pollution in agricultural land. Likewise, practices to prevent and control other soil degradation processes, such as the loss of soil organic carbon and biodiversity or soil erosion, contribute to the prevention and control of soil pollution.
5.6.2.3. International Code of Conduct for the Sustainable Use and Management of Fertilisers
The “International Code of Conduct for the Sustainable Use and Management of Fertilizers” (Fertilizer Code) was endorsed by FAO Conference in 2019 and provides a set of voluntary practices to serve the various stakeholders involved in fertiliser management. The Fertilizer Code defines roles, responsibilities, and actions to prevent fertiliser misuse and its potential impacts on human health and the environment (FAO, 2019). The Fertilizer Code can be used as a basis for Member Nations to implement regulations, awareness raising activities and other concrete actions to contribute to the wide dissemination of the principles addressed in the Code. It encourages Member Nations to set standards, limits and guidelines on the harmful contents of contaminants in fertilisers products (Art 3 and Art 6). It also includes provisions on monitoring, training, research and development, and public access to information.
5.6.2.4. International Code of Conduct on Pesticide Management
The International Code of Conduct on Pesticide Management (Pesticide Code), adopted by FAO Member Nations in 2013, establishes voluntary standards of conduct for the various stakeholders involved with pesticide use to ensure the judicious use of pesticides. The Pesticide Code is intended to serve as a basis for countries who currently have no or weak national legislation to regulate and control the quality and suitability of pesticide products (FAO and WHO, 2014). The standards set out in the Pesticide Code aim to ensure that pesticides are used effectively and efficiently in a sustainable manner to minimize adverse effects on human health and the environment while contributing to the sustainable improvement of agriculture. The Pesticide Code emphasizes the adoption of a life-cycle approach to pesticide management, which means that ‘all the stages a pesticide might pass through from production to its degradation in the environment after use, or its destruction as an unused product’ should be taken into account’ (Art. 2). As such, Member Nations should ensure that the treatment, storage and disposal of hazardous pesticide wastes are carried out in an environmentally sound manner, in accordance with international agreements such as the Basel Convention (Art. 10).
5.6.2.5. Global Action Plan on Antimicrobial Resistance
As its name suggests, the Global Action Plan on Antimicrobial Resistance aims to prevent and combat antimicrobial resistance, which undermines human and animal health and hampers medicine advances to effectively treat infectious diseases. Endorsed in 2015 by the WHO Assembly, the Action Plan sets out five goals to combat antimicrobial resistance: raising awareness, increasing understanding and knowledge of antimicrobial resistance, reducing the incidence of infections, optimizing the use of antimicrobial medicines, and increasing investment in new medicines (WHO, 2015). As part of these efforts, Member Nations are encouraged to comply with international standards, such as the World Organization for Animal Health (OIE) Terrestrial and Aquatic Animal Health Codes and the FAO/WHO Codex Alimentarius Code of Practice and Guidelines related to antimicrobial resistance, in order to strengthen animal health and agricultural practices.
By promoting the judicious use of antibiotics and other veterinary drugs and thereby controlling the emergence of antimicrobial resistant bacteria in livestock, the burden of antibiotic residues, and antimicrobial resistant bacteria and genes, released into the soil through faeces and urine is significantly reduced, contributing to prevent soil pollution from these sources.
FAO, OIE and WHO have established a tripartite collaboration on antimicrobial resistance to coordinate measures to prevent the emergence and spread of antimicrobial resistance and to strengthen animal health, agricultural production and human health (WHO, 2020a) under the framework of the One Health approach (WHO, 2020b).
5.6.2.6. Codex Alimentarius
Founded in 1963, the Codex Alimentarius sets international food standards, guidelines and codes of practice to contribute to the safety and quality of international food trade. Although Codex standards and related texts are voluntary and must be translated into national legislation to be applicable, Codex standards often serve as the basis for national legislation (FAO and WHO, 2020a). Currently, the Codex Alimentarius Commission has 189 Members, 188 Member Nations and a Member Organization, the European Union (Figure 19). San Marino and Timor-Leste are the latest country to join the Codex Alimentarius Commission, in 2016 and 2018 respectively (FAO and WHO, 2020b).
Figure 19. Members of the WHO/FAO Codex Alimentarius.
Source: UN, 2020 modified with data from FAO and WHO, 2020b.
The relevance of the Codex to soil pollution lies primarily in its provisions on food additives, residues of pesticide and veterinary drug residues, contamination of food goods, methods of contaminants analysis and sampling, and import and export inspection and certification. As such, the Codex sets maximum residue limits for pesticides and veterinary drugs for all food at the point of entry into the country or at the point of entry into the channels of commerce within the country (FAO and WHO, 2020c). By becoming the reference in international food trade, Codex Alimentarius standards support the harmonization of national standards on maximum residue limits for pesticides and veterinary drug in food.
5.6.2.7. International Nitrogen Initiative
Established in 2003, the International Nitrogen Initiative (INI) is a science-policy platform for responding to the growing concern about nitrogen pollution. INI seeks to ‘optimize nitrogen’s beneficial role in sustainable food production and minimize nitrogen’s negative effects on human health and the environment resulting from food and energy production’. To achieve this, the initiative coordinates regional efforts to improve and raise awareness on nitrogen management (INI, 2017). The INI is not an official initiative of the United Nations but formed by international scientist and experts on nitrogen fertilisers’ management.
5.6.3. Soil pollution guidance values
Hazardous chemicals exhibit a wide variety of emission patterns and physico-chemical properties that determine their fate in the environment and, in turn, can cause diverse harms to human health and the environment. Consequently, a broad range of risk assessment and risk management strategies have been developed. Contaminants cannot be identified by a unique and generally valid checkbox system nor can a single and widely accepted toolbox of regulatory guidance values (RGVs) be derived (Jennings, 2008; Li and Jennings, 2017).
To assess or prevent soil pollution and manage the risk of adverse effects from soil contaminants, regulatory jurisdictions have promulgated guideline values.
From a global perspective, no coherent picture can be drawn, either in terms of definitions or in terms of the rationale for deriving such values. First of all, the terms used differ between countries and even within the same country depending on the regulatory body. Some examples of terms that refer to RGVs are soil guideline values, soil standards, screening values, trigger values, threshold values, acceptable concentrations, target values, intervention values, clean-up values, action values (Carlon, 2007; Jennings and Li, 2015).
Second, the values themselves differ in the approaches used for scientifically sound derivation and target different toxicological or ecological endpoints. Indeed, comparative studies applied worldwide to soil jurisdictions have shown that regulatory guidance values (RGVs) can vary by as much as ten orders of magnitude (Jennings, 2008), whereas even for an individual pesticide, RGVs could vary by eight (e.g. lindane), or even nine (e.g. dieldrin) orders of magnitude (Li and Jennings, 2017). Possible explanations for these discrepancies are that some values were not derived conservatively enough to avoid risk to human health, that not all major pathways of human exposure have been taken into account, or that in some cases only human health has been taken into account while in others damage to ecosystems is also taken into account (Jennings and Li, 2017).
When comparing globally the legal practice to describe the concentrations of a contaminant in soil different terms are used, such as normal, natural, characteristic, typical, baseline, ambient, widespread, or background (Ander et al., 2013; Federal Ministry of Justice and Consumer Protection, 1998; Matschullat, Ottenstein and Reimann, 2000; Ministry of the Environment, Finland, 2014; Reimann and Garrett, 2005). Since there are some subtle differences between these terms, they can lead to significantly different assessment, which in turn explains besides the different understanding of human health risk the worldwide observed huge variation of regulatory guidance values.
The term background or normal background is used in geostatistics (Matschullat, Ottenstein and Reimann, 2000; Reimann and Caritat, 2005; Reimann and Garrett, 2005). ISO 19528 defines background and includes both geogenic concentration in the soil due to geological and pedological processes and anthropogenic diffuse pollution (ISO, 2015). The definition by ISO is adopted by several statutory guidance and ordinances (see e.g. United Kingdom, Environmental Protection, Contaminated Land Statutory Guidance (United Kingdom Department for Environment, Food and Rural Affairs, 2012); Germany, Federal Soil Protection Act (Federal Ministry of Justice and Consumer Protection, 1998).
It is assumed that normal levels of contaminants in soil typically do not cause land to qualify as contaminated land (see e.g. Finland, Government Decree on the Assessment of Soil Contamination and Remediation Needs; Germany, Federal Soil Protection Act; United Kingdom, Environmental Protection, Contaminated Land Statutory Guidance, (Federal Ministry of Justice and Consumer Protection, 1998; Ministry of the Environment, Finland, 2014; United Kingdom Department for Environment, Food and Rural Affairs, 2012). Therefore, and for reasons of clarification in many legal acts as cited before terms such as ´widespread concentration levels` or ´concentration levels at similar sites´ are used.
The first step in deriving RGVs is to establish a conceptual model. Figure 20 shows some of the basic aspects to be considered when developing the conceptual model. The risk assessment of hazardous substances usually includes both the assessment of the likely exposure and the identification of potential hazards (Van Leeuwen et al., 1996).
Figure 20. Rationale to derive regulatory guidance values includes, in particular, consideration of the type and concentration of the pollutants concerned, detailed examination of the possible exposure routes, the possibility of being ingested, inhaled or absorbed by people, animals and plants, and hazard identification considering compound specific toxicological data.
RGVs are usually derived for different land uses, such as residential, industrial, agricultural or even natural reserves. Flow-chart in Figure 20 can be used to derive any kind of guidance values. In case of ecological modelling of ecotoxicity of soil contaminants on soil-dwelling organisms the direct route of exposure is of key significance and has to be considered (Gyldenkærne and Jørgensen, 2000).
Taking precautions against the negative impacts of soil pollution and taking measures to prevent or mitigate the adverse effects of polluted soil are fundamentally different starting points. When the objective is remediation of a polluted site or assessment of the contamination, the key point is to identify the severity of the pollution by comparing it to maximum acceptable levels of contaminants and to establish actual contaminant concentrations relative to a background site showing “normal” or “natural” concentrations.
Depending on the intended use of a polluted site, different levels of contamination corresponding to different land uses and different soil types may be acceptable. On the other hand, when setting precautionary values for soil, a specific land use is not envisaged, but a multifunctional land use is considered (Carlon, 2007). Consequently, RGVs should be set at the lowest level of all suspected uses, and background values should be considered. Setting a RGV close to the background value, but significantly lower than an adverse effect of concern, in turn means taking into account the principle “as low as reasonably achievable” (ALARA). Whenever health risks are considered, personal habits and the food basket should be taken into account (Figure 21).
Figure 21. Basic aspects of a conceptual model to define soil guidance values.
In addition to establishing a conceptual model that considers all the fundamental parameters for deriving RGVs, a concise and validated database of contaminant concentrations in soil is needed. One of the challenges in the characterization of a single contaminant is to identify the chemical structure and its speciation, so the chemical form in which it is found. For example, some inorganic elements such as chromium, lead or mercury exist in different oxidation states or may occur as organometallic compounds, and each of these species has different bioavailability and hence exhibits different toxicity. Therefore, it is not sufficient to know the total concentration of the contaminant in the soil to derive its potential damage.
In addition to determining the chemical state of the contaminant, it is necessary to identify possible by-products derived from its degradation and interaction with the soil matrix, as it is the case with glyphosate and its by-products aminomethylphosphonic acid (AMPA), N-methylaminomethylphosphonic acid (MAMPA) and methylphosphonic acid (MPA), which have different ecotoxicity (Gomes et al., 2014) and genotoxic effects (Woźniak et al., 2018).
As mentioned in Chapter 2, it is difficult to find a single contaminant in environmental matrices, but on the contrary, soils are often contaminated or polluted with a mixture of contaminants associated with each of the local and diffuse emissions. The information available on these mixtures of contaminants is still very limited (Backhaus and Faust, 2012; Chen et al., 2015), but it is essential to consider the cocktail effect of such mixtures for determining protective RGVs (Chen et al., 2015), since the interactions among contaminants do not only have additive effects (Sarigiannis and Hansen, 2012).
In addition to approaching the risk posed by a contaminant or mixture of contaminants in different organisms and at different stages of development in order to determine values that are protective and ensure no or acceptably low harm, harmonized sampling methods and analytical procedures, including standardized QA/QC plans, are needed to globally compare and map the distribution of contaminants in soil, to establish monitoring systems for soil pollution and to identify hotspots of risk where prevention and control interventions must take place. In this regard, the International Organization for Standardization (ISO) Technical Committee 190 on Soil Quality has developed a significant catalogue of standard methods for the collection and analyses of contaminants in soils, as well as the design and implementation of soil sampling of contaminated sites. ISO also includes methods to assess the toxicity of contaminated soils to plants, microorganisms, earthworms, insects and other biota (ISO, 2021). However, this extensive catalogue is only available on a standard purchase basis, which makes it difficult to access, especially in developing countries. With the aim of facilitating universal access to internationally developed and agreed standards, the Global Soil Partnership works in collaboration with experts from around the world to identify, agree and make harmonized sampling and analytical procedures available worldwide, through the global networks of soil laboratories (GLOSOLAN1) and soil information institutions (INSII2).