Introduction

The Juarez Valley is located in the state of Chihuahua, Mexico, bordering to the north by TX, USA (105° 30′ and 106° 30′ west longitude, and parallels 30° 56′ and 31° 45′ north latitude). It includes the municipalities of Juarez, Guadalupe, and Praxedis G. Guerrero, with a total extension of 25,456 ha. The climate of this region is very dry with annual rainfall and temperatures of 200–300 mm and 17 °C, respectively. Nevertheless, it should be noted that temperatures below − 5 °C in extreme winter or above 40 °C in hot summer might be reached (INEGI 2003). The Valley is an important agricultural region where, historically, crops like wheat, pepper, tomato, onion, alfalfa, pecan, pistachio, and cotton have been grown. The intensive production of these crops has entailed the use of agrochemicals such as organochlorinated pesticides (OCPs) or heavy metal–based compounds (Udeigwe et al. 2015).

During many decades, the OCPs have also been widely used in international soils to eliminate pests and their occurrence have been reported. For example, soils in China were comprehensively explored to determine the incidence of these types of pollutants (Zhang et al. 2012). Similarly, several studies reported the presence of these compounds in agricultural soils of Pakistan, India, and, Italy, among others countries (Mahmood et al. 2014; Chakraborty et al. 2015; Qu et al. 2016).

Due to the effectiveness of the pioneer 4,4′-dichlorodiphenyltrichloroethane (DDT) as a pesticide, other chlorinated substances such as isodrin, dieldrin, endosulfan, hexachlorobenzene (HCB), and hexachlorocyclohexane (HCH), were synthesized and extensively applied in agricultural soils (Ming 2011). These pesticides are classified as persistent organic pollutants because of their chemical composition. In comparison with many other “naturally” occurring substances, these materials are more resistant to chemical, biochemical, and photochemical degradation; thus, they have a relatively long half-life (Li 2018). Unfortunately, the OCPs can be accumulated in the adipose tissue, kidneys, liver, blood, brain, and spleen (Liu et al. 2017). Studies have provided evidence to associate these pesticides with diverse health issues in humans, such as metabolic syndrome, obesity, and type 2 diabetes (Salihovic et al., 2016; Rosenbaum et al. 2017). The OCPs have also been found in crocodiles that show signs of feminization and atrophy of their reproductive organs (Raloff 1995) and in birds whose eggs have shown changes in shell thickness (King et al. 2003).

These and many other deleterious effects led to the prohibition of these substances by the Stockholm Convention Treaty in 2004 (Stockholm Convention 2004). Although controls in the use of DDT were applied in the USA since the 1970s (US EPA 2016), it is reported that DDT was used in Mexico until 2000 (Díaz-Barriga et al. 2012), while others, like endosulfan and aldrin, are still used in Mexican agricultural lands to treat crops such as corn, cotton, and beans. Notably, the use of these latter compounds is restricted or prohibited in several countries that adhere to the Stockholm Convention (Tsai 2010).

Heavy metals are defined as elements that have a density greater than 5 g cm−1. Some of them are soluble in water when they are presented in their ionic forms, which may lead to their absorption by living organisms (Ming 2011). Inside cells, heavy metals interfere with a variety of metabolic processes that can promote cancer development (Vella et al., 2017) and neurological disorders (Yorifuji et al. 2011). Elements like zinc (Zn) and iron (Fe), considered essentials because of their function to human beings, can have also adverse effects when they are ingested in high concentrations. For instance, there is evidence that consumption of Fe in higher doses is linked with hypertriglyceridemia and insulin resistance in mice (Dongiovanni et al. 2013); Zn is associated with pancreatic issues in animals (Carpenter et al. 2004; Carreira et al. 2011).

The excess of heavy metals in environmental matrices such as soil and water has been linked with human activities. Particularly, the prevalence of metals in several agricultural soils is highly associated with the use of agrochemical fertilizers or antifungal/bactericides (Tan et al. 2018). In countries such as Pakistan, China, Korea, New Zealand, and Iran, the occurrence of mercury (Hg), Cr, Fe, Ni, Cd, Pb, Zn, Cu, As, Co, and Mn has been reported (Afzal et al. 2012; Xiao et al. 2017; Zang et al. 2017; Kwon et al. 2017; Martin et al., 2017; Naghipour et al. 2018).

To the best of the authors’ knowledge, there are no reports regarding the presence of OCPs in the soils from the Juarez Valley and the studies on heavy metals in this area are still very limited. Therefore, the aim of this work was to perform an analysis on the soils of the region to diagnose their compounds and element architecture.

Materials and methods

Sample collection and processing

Soil samples (0–30 cm deep) were collected in June 2018, following the criteria of the Mexican Norm NMX-AA-132-SCFI-2006. Administratively, the Juarez Valley is divided into three modules (Fig. 1) and 10 composite samples were taken for each module (Table 1). The soil samples were sieved (mesh size no. 10) and stored in paper bags to be transported to the laboratory.

Fig. 1
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The geographic location of the Juarez Valley and modules I–III

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Table 1 Location of the sampled sites
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Pesticide analysis by GC-μECD

Pesticide determination was done following the method reported by Cantú-Soto et al. (2011). First, 0.5 g of soil samples was dispersed in a mortar with 0.6 g of Al2O3 (aluminum oxide) (J.T. Baker, Pittsburgh, PA, USA). Then, the samples were purified by incorporating them into a 5-mL glass syringe packed with 2.6 g of Al2O3 and glass fiber at the bottom; the organic compounds were eluted with 40 mL of hexane GC grade (Honeywell, NJ, USA). The eluted was collected in a 50-mL conical tube where the hexane was evaporated to dryness at 40 °C using gaseous N2, and the concentrate was reconstructed with 100 μL of hexane. A gas chromatograph with Network GC system (Agilent Technologies 7890A, Palo Alto, CA, USA), equipped with an electron micro-capture detector (μECD) with a DB-5 column (30 m in length × 0.25 mm i.d., the thickness of 0.25 μm), was used for the analysis. The column temperature was initiated at 110 °C for 1 min, then it was increased to 280 °C at a rate of 15 °C min−1; the final temperature was maintained for 2 min. The temperatures of the injector and detector were 270 °C and 340 °C, respectively. Helium was used as carrier gas at 2.3 mL min−1. A negative control (0.5 g of Al2O3) and a positive control of soil fortified with 100 μL of a 500-μg kg−1 mix of pesticides of interest: DDT, 4,4′-dichlorodiphenyldichloroethylene (DDE), 4,4′-dichlorodiphenyldichloroethane (DDD), aldrin, dieldrin, endosulfan, HCB, isodrin, lindane, methoxychlor, and mirex (PESTANAL® Sigma-Aldrich, St. Louis, MO, USA) were also analyzed. The method was validated by analyzing the following parameters: linearity, recovery percentages, and limits of quantification and detection (Table 2).

Table 2 Parameters validation for pesticide analysis
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Determination of heavy metals by atomic absorption spectrometry

Soil samples were subjected to chemical digestion before the instrumental analysis, following the method reported by Meza-Figueroa et al. (2007), with some modifications. Briefly, 2 g of each sample was mixed with 50 mL of 0.5 N HCl (hydrochloric acid) (Reactivos Meyer, Ciudad de Mexico, Mexico) in 250-mL beakers to extract bioavailable metals. This process was carried out for 24 h at room temperature with sporadic agitation. A negative (0.5 N HCl) sample and a positive sample of fortified soil with 0.5 ppm of a mix of metals of interest, Fe, Mn, Cu, Cd, Pb, and Zn (Merck, Darmstadt, Germany), were also processed. After the digestion process, samples were filtered (Whatman #42), and the liquid phase was brought to a volume of 100 mL with 0.5 N HCl. Samples and controls were analyzed by flame atomic absorption spectrometry using an Agilent 240 FS spectrometer (Agilent Technologies 7890A, Palo Alto, CA, USA), equipped with hollow-cathode lamps. Argon was used as the carrier gas. The method was validated by analyzing the following parameters: recovery percentages, linearity, and limits of quantification and detection as previously reported (Cota-Ruiz et al. 2018).

pH and electrical conductivity analysis

The pH and electric conductivity (EC) measurements were done following the criteria of the Mexican norm NOM-021-SEMARNAT-2000. Soil pH was determined by shaking 10 g of soil with 20 mL of distilled water in plastic containers for 5 min at room temperature, following by a non-shaking incubation of 10 min. The operation was repeated two more times. The pH analysis was done using a potentiometer (HANNA model HI 207, Wooncocket, RI, USA). EC was done by mixing 10 g of soil and 50 mL of distilled water, at the same above-mentioned conditions. After this period, the analysis was done using an EC meter (Thermo model Orion 3 star, Waltham, MA, USA).

Statistical analysis

To evaluate differences in pesticides and heavy metal content among the different modules, an ANOVA analysis was performed. Tukey’s HSD was used to distinguish mean significant differences (p ≤ 0.05). When data did not follow a normal distribution, they were Box-Cox transformed. Descriptive statistical parameters were also estimated for pH and CE. All the analyses were done with IBM SPSS statistics version 23 for Windows (Armonk, NY, USA).

Results and discussion

Pesticides in soils of the Juarez Valley

The concentration of DDE, DDT, isodrin, endosulfan, and DDD for modules I, II, and III of the Juarez Valley soils are shown in Table 3. The agrochemical DDE was fully detected in all sampled sites, registering average values of 4.86 ± 6.2 μg kg−1, 18.27 ± 31.04 μg kg−1, and 5.90 ± 4.81 μg kg−1, for modules I, II, and III, respectively. These mean values did not significantly differ among them (p ≤ 0.05). Meanwhile, the chemical DDT was identified in all analyzed sites from modules I and II, and in 90% of the sampled soils in module III, with average values of 0.83 ± 0.56 μg kg−1, 19.08 ± 53.71 μg kg−1, and 1.67 ± 0.79 μg kg−1, respectively. These mean values were not significantly different (p ≤ 0.05). Additionally, isodrin was found in 60% of the sampled sites in module I (12.29 ± 3.6 μg kg−1). However, it was only identified in one site in module II (0.71 μg kg−1), and no presence was detected in module III. Contrarily, endosulfan was encountered in 60% of the sample sites (1.44 ± 0.4 μg kg−1) for module III, while it was detected in only one site in module I (1.19 μg kg−1) and also in one point at module II (10.76 μg kg−1). Finally, the agrochemical DDD was detected only in two sites in module II (24.37 and 0.42 μg kg−1) and in one site in module III (0.84 μg kg−1).

Table 3 Concentrations of organochlorine pesticides residues in soils of the Juarez Valley (μg kg−1)
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The presence of OCP compounds in Mexican agricultural soils has been previously reported in the literature. For instance, Wong et al. (2008) described the presence of OCPs in agricultural soils in three Mexican states: Chiapas, Tabasco, and Veracruz. They found residues of DDT in almost all the sampled sites, Chiapas being the state that had the highest concentrations (an average of 360 μg kg−1), in comparison with Veracruz (6.5 μg kg−1) and Tabasco (0.17 μg kg−1). In the Yaqui and Mayo Valleys, Cantú-Soto et al. (2011) found DDT, DDE, DDD, and endosulfan at 17.9, 11.2, 1.6, and 6.7 ppm, respectively. These two regions in the state of Sonora, Mexico, perform very intensive agriculture activities that have been recognized as the cradle of the “Green Revolution.” In another study where soils from northern and central Sonora were analyzed, Leal et al. (2014) reported residues of DDT, DDE, DDD, endosulfan, and isodrin, with averages of 2.86, 14.63, 2.22, 4.35, and 3.30 μg kg−1, respectively. These values are similar to those reported in the present study. Only one study was found that analyses the presence of OCPs in agricultural areas (Díaz-Barriga et al. 2012) in the state of Chihuahua, Mexico. This study specified that the maximum amounts of DDT and DDE were 0.79 mg kg−1 and 0.642 mg kg−1, respectively. These values are greater than those found in the present study.

The presence of pesticides such as DDT, endosulfan, HCH, methoxychlor, chlordane, dieldrin, endrin, and aldrin have also been found in soils from China (Zhou et al. 2013; Han et al. 2017), India (Chakraborty et al. 2015; Kulangaravalappil and Chenicherry 2018), Pakistan (Mahmood et al. 2014), Romania (Tarcau et al. 2013), and Italy (Qu et al. 2016); this demonstrates that agricultural lands polluted with OCPs is a widespread issue. The differences between the amounts of OCPs in different agricultural regions, and even within the same region, are closely related to dissipation and volatilization processes, as these parameters directly influence the persistence of pesticides in soils. In addition, these processes might be affected by weather conditions, such as temperature and humidity (Zhang et al. 2012).

The OCPs can be accumulated in agricultural lands by their direct application (Wong et al. 2008) and also by the use of treated wastewater containing OCPs (Sun et al. 2009). In the case of the Juarez Valley, it has been published that some treated water contains OCPs, such as DDT and DDE (Palomo-Rodríguez et al. 2013). Inopportunely, these waters are mixed with waters of the Rio Grande River and then used to irrigate crops. Although the concentrations of the OCPs reported in this research were found to be within the FAO regulations (FAO 2000), it should not be discounted that they might bio-accumulate in crops (Adeyeye and Osibanjo 1999). On the other hand, even at low-environmental amounts, they may elicit deleterious cell responses leading to diseases or cell malfunctions (Androutsopoulos et al. 2013). In addition, these compounds could infiltrate into groundwater, affecting human and animal health (Grondona et al. 2019). Moreover, there is evidence about the possible incorporation of pesticides into the food chain due to their bioaccumulation in some vegetables (Zhang et al. 2015).

Heavy metals in soils of the Juarez Valley

The heavy metals Cu, Zn, Fe, Pb, Mn, and Cd examined in this study were found at different concentrations in the three analyzed modules of the Juarez Valley (Fig. 2). The content of Cu in module I (11.04 ± 3.83 mg kg−1) was higher than that in module II (7.79 ± 3.32 mg kg−1) and module III (6.2 ± 1.51 mg kg−1) (Fig. 2(A)) (p ≤ 0.05). The obtained values for the module I are similar to the values previously reported in soils belonging to this district (Cota Ruiz et al. 2019). In the latter study, the authors reported an average value of 20.06 ± 3.39 mg of Cu per kg of soil. In module I, it was also found that the concentration of Zn was significantly higher (38.36 ± 26.57 mg kg−1) compared with the soils from module II (23.29 ± 18.94 mg kg−1) and module III (11.80 ± 2.68 mg kg−1) (Fig. 2(B)) (p ≤ 0.01). A recent investigation conducted by Adagunodo et al. (2018) reported that Nigerian soils associated with agricultural activities had the presence of Cr, Cd, Ni, V, Cu, Co, Sb, Pb, and Zn, with Zn being the one that exhibited the higher concentrations, with values of up to 61.30 mg kg−1, similar to the values reported in this study for agricultural soils (Fig. 2(B)). Additionally, Martin et al. (2017) found Cd, Cr, Cu, Hg, Ni, As, Pb, and Zn in agricultural soils from New Zealand, where Zn showed some of the highest concentrations with values up to 163 mg kg−1. Remarkably, all tested soils in the current study presented concentrations of Cu and Zn within the permissible ranges established by the Mexican regulation and by the World and Health Regulation (WHO) standards.

Fig. 2
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Heavy metal concentration (mg kg−1) from the three modules of the Juarez Valley. The concentration represents the mean value ± SD of the sampled soils per module. Different letters indicate significant differences at p ≤ 0.05

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The highest values of Fe and Pb were found in module II, registering quantities of 1902.7 ± 332.2 mg kg−1 (p ≤ 0.01) and 23.48 ± 6.48 mg kg−1 (p ≤ 0.01), respectively (Fig. 2(C, D)). These values did not exceed the limits found in Mexican laws nor the established limits by WHO. Studies performed in different Mexican agricultural lands have also reported the presence of these and other heavy metals. For instance, Flores-Magdaleno et al. (2011) found that agricultural soils from the state of Hidalgo, in the central region of the country, had concentrations of heavy metal(oid)s Pb, Ni, Cd, As, Cr, and Hg up to > 2.5, < 1.40, < 0.20, > 0.10, > 0.016, and > 0.010 mg kg−1, respectively. A similar case was reported in Nigeria by Adagunodo et al. (2018), where they found that agricultural soils exhibited Pb in concentrations up to 43.89 mg kg−1. Another study performed by González et al. (2012), in an agricultural area in the state of Zacatecas, Mexico, located in the north-central region, found that Pb presented the highest concentrations (> 600 mg kg−1) compared with other analyzed heavy metals (González et al. 2012; Kamunda et al. 2016). In a study regarding the Juarez Valley, Cota-Ruiz et al. (2019) sampled four soils belonging to module I of this current study and they did not detect the presence of Zn nor Pb. These differences can be explained in terms of distance and location between their sampling sites and the ones presented in this study, since module I has an extension of over 3000 ha (INEGI 1999).

Additionally, the most elevated concentration of Mn was found in module III (392.43 ± 74.43 mg kg−1), compared with module II (326.96 ± 75.17 mg kg−1) and module I (231.55 ± 28.21 mg kg−1) (Fig. 2(E)) (p ≤ 0.01). Interestingly, it has been reported that Mn and Fe oxides may adsorb other heavy metals such as As and Cd (Suda and Makino 2016; Xu et al. 2017). Thus, the soils from module III could have more possibilities to reduce the heavy metal uptake by agricultural crops. However, functional experiments to demonstrate this are needed. In this study, none of the registered values for Mn concentration exceeded international standards.

The levels of Cd did not significantly differ among the tested modules (Fig. 2(F)) (p ≤ 0.05). The mean Cd levels in the three modules did surpass the values allowed by Mexican law (0.35 mg kg−1) based on crop tolerance, but not the values specified by the WHO (3 mg kg−1) for agricultural soils (Kamunda et al. 2016). It is important to note that in some cases, in the three modules, results exceeded the permissible values specified by WHO. In consequence, some of these agricultural soils represent a threat since Cd may accumulate in crops.

In the current study, the analyzed soils were classified as neutral to slightly alkaline and as saline to highly saline (Table 4). The pH of soils may induce the acquisition of heavy metals. For instance, it was previously reported that the transfer of Cd, Hg, Pb, Hg, Cu, and Zn from soil to shoots in grains of rice (Oryza sativa) was increased when the soils were acidic (Mao et al. 2019). This latter finding could indicate that the heavy metals in the soils of the Juarez Valley cannot be easily integrated into the trophic networks. However, plants like Acacia tortilis, Calotropis procera, Convovalus sp., Prosopis juliflora, Salsola sp., Ochradenus baccatus, Convolvulus sp., and Pergularia tomentosa are capable of absorbing As, Cd, Cu, Pb, and Zn from alkaline soils (Al-Farraj and Al-Wavel, 2007). Thus, many other variables, such as salinity, type of soil, plant species, among others, affect the mobility of heavy metals in soils (Acosta et al. 2011). Further heavy metal translocation studies using agricultural crops should be performed to determine their consequences to the living systems.

Table 4 Physicochemical parameters of soils from the Juarez Valley
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Conclusions

DDE and DDT were the most widespread agrochemicals in the Juarez Valley since they were found in 100% and 97% of the tested soils. Isodrin prevalence was higher in module I, as 60% of the sampled soils presented the pesticide. Meanwhile, endosulfan was detected in 60% of the soils in module III. The lowest occurrence was exhibited by DDD since it was detected in 20% and 10% of the tested soils from modules II and II, respectively. Although the concentration of all analyzed pesticides did not exceed the international regulations, their potential adverse effects cannot be discounted until more studies on biological systems have been done. As for heavy metals, Fe and Mn presented the highest concentrations in comparison with the rest of their analyzed counterparts (p ≤ 0.05). The heavy metals Fe, Mn, Zn, Pb, and Cu were found in all tested soils; however, they did not surpass the limits specified by Mexican legislation nor the ones by the WHO. The content of Cd in some analyzed soils did exceed the limits proposed by Mexican and international regulations. Given the physicochemical properties of the soils in the Valley, along with the potential adverse effects of xenobiotics to biological systems, more studies are needed to monitor the concentrations and to determine the fate of these compounds in the environment.