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Pesticides

Irene Domínguez, Rosalía López Ruiz, Antonia Garrido Frenich, and Roberto Romero González

Department of Chemistry and Physics, Andalusian Center for the Assessment and Monitoring of Global
Change (CAESCG), University of Almeria, Agrifood Campus of International Excellence, Almeria, Spain

1.1 Overview of Pesticides

There are thousands of chemicals that can enter the environment and pesticides are among the most significant. They have been used in the last decades in several areas, but agricultural activity is the main source of the impact of pesticides in the environment and therefore they can be present in soil, water, crops as well as in the atmosphere [1].

1.1.1 Properties

Pesticides are a class of chemicals used to limit, inhibit or prevent the growth of harmful animals, insects, weeds or fungi [2]. They can be classified according to different criteria, such as target organism, origin or chemical structure, but the most common is considering the target organism, being herbicides, fungicides, insecticides, etc., among others [3]. There are more than 800 active components and they are available in different accessible products [4]. Despite the benefits of the use of these compounds, they can be toxic to humans and many of them have been classified as endocrine disruptors, and carcinogenic effects have also been reported [5].

The widespread use of pesticides in combination with their physico-chemical properties, such as water solubility, octanol/water partition coefficient, volatility and stability against degradation by abiotic and biotic factors, are the reasons for their distribution and occurrence in different environmental matrices such as water, soil, air and biota [6] by physical processes as sedimentation leaching, sorption and volatilization.

Once pesticides are in the environment, they can be transformed by biotic or abiotic process [7], increasing the number of potential transformation products (TPs) that can be detected, and most of them are still unknown [8]. In this sense, TPs could also have environmental concern and so in addition to the parent compounds they should also be monitored in order to get a comprehensive overview of the environmental fate of pesticides.

1.1.2 Legislation

The presence of these pollutants poses a potential risk for the environment and human health and, therefore, international organizations have set legal limits regarding the presence of pesticides in water and other environmental matrices, for controlling and preventing contamination of environmental ecosystems.

For instance, in Europe, the Water Framework Directive (WFD) is intended to protect transitional waters, inland surface waters, coastal waters and groundwater. Strategies against the chemical pollution of surface waters led to the Directive 2008/105/EC [9], establishing concentration limits of 33 priority substances and 8 other pollutants, including some pesticides such as simazine and trifluralin. Priority substances are considered to pose a significant risk to or via the aquatic environment, so environmental quality standards (EQSs) were set for each of them. Then, amending Directive 2013/39/EU [10] introduced 12 new compounds to the list and the need to establish an additional list of potential water pollutants (Watch List) that should be carefully monitored to support future reviews of the priority substances list. Currently, among the priority substances are 24 pesticides with Annual Average EQS (AA-EQS) values ranging from 1 × 10-8 µg l-1 for heptachlor and heptachlor epoxide to 1 µg l-1 for simazine. In 2020, the European Union (EU) established a new Watch List of substances, including azole compounds and providing maximum acceptable method detection limits for them from 29 to 199 ng l-1 [11]. Additionally, the Drinking Water Directive 98/83/EC, amended by EU 2015/1787 [12], set special quality requirements for water for human consumption. It set concentration limits for a range of hazardous substances, including pesticides, establishing a general maximum individual concentration of 0.1 µg l-1 for individual pesticides (0.030 µg l-1 in the case of aldrin, dieldrin, heptachlor and heptachlor epoxide) and 0.5 µg l-1 for the sum of all individual pesticides and relevant metabolites/TPs detected. The same values, 0.1 and 0.5 µg l-1, for individual and total pesticides respectively, are established as groundwater quality standards in Directive 2006/118/EC [13] on the protection of groundwater against pollution and deterioration.

In the same way, the Clean Water Act (CWA) in the United States (US) establishes the basic structure for regulating quality standards for surface waters discharges of pollutants into the waters. In addition, the Safe Drinking Water Act (SDWA) was aimed at protecting drinking water and its sources (rivers, lakes, reservoirs, springs and groundwater wells). SDWA authorizes the US Environmental Protection Agency (US EPA) to set national health-based standards for drinking water to protect against contaminants, such as pesticides, that may be found in drinking water [14-16]. In this case, the proposed substance priority list is based on a combination of their frequency, toxicity and potential for human exposure at National Priorities List (NPL) sites, setting criterion maximum concentration (CMC) values for each of the pollutants listed. Aldrin, dieldrin, heptachlor and heptachlor epoxide show the lowest CMC values, between 7.7 × 10-7 and 3.2 × 10-5 µg l-1.

Whereas different countries have set pesticide regulation in water matrices, regulation in soils is scarce. For instance, Spain set generic reference levels for a limited number of substances (< 60), some of them considered as persistent organic contaminants, such as dichloro-diphenyl-trichloroethane (DDT) or dichloro-diphenyl-dichloroethane (DDE), whose reference levels were 0.2 mg kg-1 and 0.6 mg kg-1 respectively [17]. These reference levels, in terms of human protection, are the maximum concentration of a substance in the soil that guarantee that contamination does not pose an unacceptable risk to humans. In addition to complying with generic reference levels, it is necessary to determine through toxicological tests that these substances do not present a serious risk to the ecosystem.

1.1.3 Reported or Potential Metabolites and/or Transformation Products

Pesticides in the environment may experience different chemical reactions, leading to the appearance of TPs and metabolites. These compounds have potentially harmful impacts on organisms, even more than their precursors [18], making their monitoring essential. However, because of the great variety of TPs, it is difficult to carry out a comprehensive analysis of their presence, and in consequence, a risk assessment evaluation.

The metabolic/transformation pathways of pesticides can be affected by biological or/and physico-chemical factors in the environment [19]. Hydrolysis is an important degradation mode of pesticides; however, multiple TPs may be produced from different processes, even after hydrolysis [20, 21].

It was noted that the number of substances that must be considered for environmental risk significantly multiplied by a factor of 7.5, just when the precursor compounds were subjected to a photolysis process [22]. This has been observed for terbutryn, mecoprop, penconazole, boscalid diuron and octhilinone pesticides in Figure 1.1, where the number of compounds that should be monitored in environmental samples considerably increase because of the presence of TPs. After evaluating genotoxicity of the proposed TPs, it was suggested that the number of substances that pose a risk onto the aquatic environment increased by a factor of >4. This fact, together with the high incidence of TPs and metabolites in natural waters, constitutes a major concern that needs to be addressed from an analytical and legislative point of view.

Figure 1.1 Forty-five TPs originating from six pesticidal parent compounds. Illustration of the multiplication of known substances that should be further investigates by an environmental risk assessment. Source [22]. Reproduced with permission of Elsevier B.V.

Several studies have revealed the presence of TPs and metabolites in waters at higher concentrations than the parent compounds [23, 24]. The physico-chemical properties (higher mobility and polarity) of the TPs and metabolites might facilitate the migration between surface water and groundwater. Since groundwater is the greatest source of freshwater in the world, the occurrence of some relevant metabolites and/or TPs led to the restriction in the use of certain pesticides, as was recently the case for chlorothalonil and previously simazine and atrazine, among others. Most of the TPs/metabolites found in natural waters are related to acetanilide and triazine herbicides [25]. Such is the case for ethanesulfonic acid (ESA) and oxanilic acid (OA), degradation products of alachlor, metolachlor, as well as acetochlor, and atrazine-desethyl (DEA), atrazine-desisopropyl (DIA), terbumeton-desethyl (TED), terbuthylazine-desethyl (TD) and terbuthylazine-2-hydroxy (T2H). Different analysis has also revealed the occurrence of 2,6-dichlorobenzamide (BAM) from dichlobenil, aminomethyl phosphonic acid (AMPA) from glyphosate, desphenyl...