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 as treated wastewater which is intended to be discharged in surface waters [34].
In 2018 the European Environment Agency (EEA) indicated that chemical status of surface waters, related to pesticides, has improved between the 1st and 2nd River Basin Management Plants (RBMPs) assessments [35]. However, pesticides listed as priority substances were still detected. Among them, isoproturon and hexachlorocyclohexane were the most frequently reported, followed by endosulfan and chlorpyrifos. Furthermore, the presence of pesticides in groundwater is a failure to achieve good chemical status. As reported by EAA, several groundwater bodies exceed the permitted concentrations set by EU of total pesticides, as well as the level set for desethylatrazine, atrazine and simazine.
Herbicides such as atrazine and fungicides such as metalaxyl have been detected in aquifers for drinking water in US [25]. The most found pesticide in drinking water in Ireland was 2-methyl-4-chlorophenoxyacetic acid (MCPA), widely used for rush control in grassland [36].
A recent study in The Netherlands, intended to assess the occurrence of pesticides in ground and surface water used as drinking water sources, has revealed the detection of 15 recently authorized pesticides, such as fluopyram and thiamethoxam, which demonstrates the importance of keeping routine monitoring methods [36].
Non-agricultural uses of pesticides are also common in urban environments, such as indoor uses or applications in gardens, roads and sealed areas, among others. As a result, triazine herbicides (atrazine, simazine and prometon) and organophosphate insecticides (chlorpyriphos and diazinon) are frequently detected in U.S. urban surface waters [34].
Pesticide occurrence in soils was inversely related to the occurrence in surface water, and one of the main factors is the hydrophobicity of the compounds, expressed by the n-octanol/water partition coefficient (Kow). Thus, pesticides with log Kow > 3 were more detectable in soils than in water. In a study performed in Argentine ecosystem, it was observed that the most detected herbicide residues (>30% of detection frequency) were acetochlor, atrazine and its metabolite (hydroxyatrazine), flurochloridone, glyphosate and its metabolite (aminomethylphosphonic acid, AMPA) and metolachlor, whereas the most frequently detected insecticides were chlorpyrifos and imidacloprid [37]. In this study, glyphosate and its metabolite AMPA were also found in all the environmental samples (soil, sediments and surface water), detecting AMPA up to 713 mg kg−1, glyphosate at 32 mg kg−1 and 5 mg l−1 in sediments and water respectively.
Another group of pesticides really known for their persistence are 2,4-D-based herbicides. The two chlorine that are present in their molecules confers persistence with an estimate half-life between 7 and 312 days, which depends on the environmental conditions [38]. Several studies have reported the presence of 2,4-D and derivates in the environment because of different activities, such as agricultural activities and rain and irrigation water. Discharges from manufacturing plants, leaching and accidental spills [39] represent an important source of the aforementioned herbicide.
The key properties that determine the presence of pesticides and their accumulation in biota are hydrophobicity and persistence. Thus, if water solubility is <1 mg l−1 or log Kow is >3, they have the potential to accumulate in biota and it may be an indicator of contamination [40]. For instance, earthworms are used as indicators to the response of pesticides [41], but fish are considered as major reservoirs of pesticides and their concentration in fish tissues has been used as an indicator of bioaccumulation [42]. High concentrations of organochlorine pesticides (OCPs) have been found in fish tissues from aquatic environments in Africa [43], detecting DDTs, lindane, mirex and endosulfan at concentrations up to 100 µg kg−1. Other studies, developed in South-America (southeastern Brazil and the coast of the Argentinean Pampas), showed that lindane and endosulfan levels could cause long-term or short-term damage to biota [44]. Additionally, it was observed that OCPs were highly bioaccumulated in soil mesofauna (up to 260 µg g−1) [45].
In the last years, several river basins were monitored in the Iberian Peninsula, and in addition to water and sediment, biota was also analyzed. Thus, 50 pesticides were monitored in Guadalquivir river basin, but none of them were detected in biota, although there were detected in water and sediments, especially organophosphorus and triazines at concentrations up to 13 ng l−1 in water and 13.2 ng g−1 dry weight (dw) in sediment [46]. However, azinphos-ethyl, chlorpyriphos, diazinon, dimethoate and ethion were detected in different species of fish from Jucar river basin, observing that the maximum average concentration was found in European eels (up to 0.024 ng kg−1), whereas in water, dichlofenthion, imazalil pyriproxyfen and prochloraz, commonly used in farming activities, were mainly detected [47]. In biota from Llobregat river basin, chlorpyrifos and azinphos-ethyl were detected at 44.75 ng g−1 dw and 105.81 ng g−1 dw, indicating possible bioaccumulation. Nevertheless, authors concluded that these values do not represent a high risk to biota. On the other hand, triazines, organophosphorus and neonicotinoids were mainly detected in water, at concentrations >600 ng l−1, as can be observed in Figure 1.4, with chlorpyrifos the compound most widely detected in soils, at concentrations up to 130 ng g−1 dw [48]. Additionally, the Ebro river basin was also monitored, detecting imazalil and diuron at the highest concentrations in water (410 and 150 ng l−1 respectively), chlorpyrifos, diazinon and diclofenthion in sediments, whereas the only compound detected in biota was chlorpyrifos, which was detected at concentrations up to 840.2 ng g−1 [49].
Figure 1.4 Pesticide families detected in (A) water, (B) sediment and (C) fish samples from Llobregat basin in 2010 and 2011 according to the sampling point. Source [48]. Reproduced with permission of Elsevier B.V.
The presence of pesticides in air depends on the specific agricultural areas, as well as those pesticides that can be transported from nearby places, observing a relationship between the concentration of some of the compounds
