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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 chloridazon and methyldesphenyl chloridazon from the herbicide chloridazon and N,N-dimethylsulfamide (DMS) formed from the fungicide tolylfluanid [23, 25–27].

Metabolites were also detected in soils, especially when dissipation studies have been carried out. For instance, nine metabolites of famoxadone were detected in soil samples [28], with IN-JS940 the metabolite detected at the highest percentage in relation to the parent compound, as can be observed in Figure 1.2. Therefore, risk assessment is needed to evaluate potential hazards to the fauna and flora. Tiwari et al. [29] evaluated the presence of endosulfan and chlorpyrifos metabolites in soils because of the higher toxicity of some of these compounds as chlorpyrifos oxon. They determined that metabolite concentrations increased throughout the study when the concentration of the parent molecule decreased. Moreover, it was observed that concentration of metabolites was higher in soil matrices than in water. In the same way, when 2,4-dichlorophenoxyacetic acid (2,4-D) is applied on crops or on soil, it will undergo chemical, biological and physical degradation processes depending on the environmental factors, which will determine the metabolites formed. For example, 2,4-D DMA (2,4-D dimethylamine salts) is dissociated to 2,4-D acid after its application on the soil [30], so in addition to the parent compound, different TPs should be monitored.

Figure 1.2 Metabolite behavior according to the concentration of famoxadone during monitoring period (100 day) for soil experiments at: (a) normal dose (2.4 mg g⁻¹ soil) and (b) double dose (4.8 mg g⁻¹ soil). Source [28]. Reproduced with permission of Elsevier B.V.

1.1.4 Occurrence in the Environment

Pesticides and their TPs/metabolites are widely distributed in the environment and they can be detected in water, soil, sediments, aquatic biota and air [31], as can be observed in Figure 1.3, because of surface runoff from arable lands, leaching from drainage systems, volatilization, etc. They can have toxic effects in population living close to these areas [32]. For instance, when pesticides are applied in agricultural areas, approximately 20–30% of the amount is lost due to the spray drift process, whereas another significant fraction is placed into the soil or surface waters [33].

      Figure 1.3 Neonicotinoid insecticides in the environment: sources, pathways, receptors and related process. Source [31]. Reproduced with permission of Elsevier B.V.

Pesticides have been widely detected in the aquatic media. Several scientific papers and technical reports have revealed the presence of pesticides in surface water, groundwater, drinking water as well

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