Introduction

Pesticides are used intensively in agriculture for pest and disease control to ensure an adequate food supply. However, their degradation leads to the production of residues that can persist in terrestrial or aquatic ecosystems for years, posing a threat to the environment. Indeed, soil polluted with pesticide residues has become a ubiquitous problem that has led to the deterioration of food quality and agricultural sustainability. Moreover, because of their volatile nature, pesticides can be transported in soil, water, and air, eventually resulting in widespread environmental contamination owing to their reported toxicity (Ecobichon 2001; Rittman et al. 2013; Yang et al. 2012; Duke et al. 2013; Davis and Landolt 2013; Schuler and Berenbaum 2013).

A number of toxic pesticides have recently been detected in the groundwater of agricultural areas (Johnson et al. 2001; Kadian et al. 2008). There is a strong relationship between the detected pesticide groups, their concentrations in groundwater, and their relatively intensive usage in the agricultural practices (Belmonte Vega et al. 2005). Furthermore, the chemical nature of the pesticide and its rate of degradation in aquatic or terrestrial systems are determining factors for estimation of its potential for groundwater contamination (Di et al. 1998). Soil is considered the main reservoir and source of pesticides released into groundwater and food crops, which imply that most applied pesticides eventually find their way into groundwater and terrestrial and aquatic food chains, where they cause long-term adverse effects on health (Chiron et al. 2000).

In Saudi Arabia, agricultural land use has increased by 10 % during the last two decades (Al-Harbi 2010). According to the Ministry of Agriculture of Saudi Arabia, the annual average import of pesticides has increased tremendously. For instance, a sevenfold increase in pesticide application in agriculture from 1976 to 1998 was reported (Al-Saleh et al. 1999). This high application rate has led to the accumulation of pesticide residues in soil and vegetation growing thereon. Occurrence of pesticide residues has recently been reported in agricultural products such as vegetables and food grains of various agricultural areas of Saudi Arabia (Osman et al. 2010). Al-Saleh et al. (2012)) reported the occurrence of the p,p′-dichlorodiphenyltrichloroethane (DDT) metabolite, p,p′-dichlorodiphenyldichloroethylene, in the serum and follicular fluid samples of 77 % of tested humans. Traces of organochlorine pesticides were also detected in human breast milk from the Al-Kharj region in Riyadh (Al-Saleh et al. 1998). Additionally, risk assessment of Saudi Arabian agricultural soils conducted in different regions revealed pesticide residues including DDT and hexachlorocyclohexane in soil and groundwater (Al-Wabel et al. 2011). However, comprehensive small, large, and catchment level modeling studies are required to determine the spatial distribution of pesticide residues in groundwater. Although the Ministry of Agriculture of Saudi Arabia has restricted the use of pesticides for which alternatives are available, these chemicals are still being used to control vector-borne disease in agricultural areas.

In the Al-Qassim region of the Kingdom of Saudi Arabia (KSA), different types of pesticides are used extensively by farmers in considerable quantities. Therefore, the present study was conducted to (i) monitor the presence of pesticide residues in groundwater in the agricultural area of Al-Qassim and (ii) investigate the correlation between vital physiochemical properties of pesticides and their leaching from soils into groundwater. This approach is expected to yield important findings that can further be applied to understanding the fate and modeling of the pesticide movement in agricultural soils and underground water with low soil organic carbon.

Materials and methods

Characteristics of the area and sampling procedure

Groundwater samples were randomly collected from water wells at 34 locations in the Al-Qassim region, located in the center of Saudi Arabia (Fig. 1). The well depths ranged from 300 to 500 m, and the groundwater is used for irrigation and drinking purposes without any pretreatment. The pumped groundwater of Al-Qassim is part of Saq Aquifer which starts from the Jordanian border with a total surface area of about 65,000 km2 and subsurface area of about 160,000 km2. Saq Aquifer is mainly composed of layers of a medium-to-coarse sandstone, with local areas of fine sandstone (Al-Salamah et al. 2011). Approximate surface extent and direction of groundwater flow is shown in Fig. 2 modified from the Ministry of Agriculture and Water (1984). Samples were collected into clean polythene bottles, transported to the laboratory in ice bags, and stored at 4 °C until analysis for physicochemical properties and different pesticides including bromoxynil, carbofuran, chlorpyrifos, cyfluthrin, diazinon, and ioxynil. The detected contents were compared with the guidelines and limits set by the European Economic Community (Directive 98/83/EC).

Fig. 1
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Map of the groundwater sampling locations Al-Qassim

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Fig. 2
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Aquifer in Al-Qassim and surrounding area (after Ministry of Agriculture and Water, Water Atlas of Saudi Arabia 1984)

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Chemicals and materials

For this study, pesticide standards with certified purities ranging from 95 to 99 % were purchased from the Environmental Protection Agency (USA). Table 1 shows the tested pesticides and their main chemical groups and characteristics. The certified purities were used to correct the concentrations of the standard solutions. HPLC grade acetone, methanol, and ethyl acetate were obtained from BDH, UK. Ultra-pure deionized water with a resistivity of 15 MΩ cm (Purelab Option-R, ELGA, UK) was used to prepare the standards. Extracts were cleaned using a solid-phase extraction (SPE) column (Water SPE TM, C18, Sep-Pak Vac, 500 mg per column, 6 cm3), with a surface area of 1200 m2 g−1, a particle size of 40–120 mm (Waters, Sunnyvale, CA, USA), and a 20-port vacuum manifold.

Table 1 The usage, chemical group, physicochemical properties, molecular weight (MW), retention times (Rt), and fragment ions selected (M/Z1, M/Z2, M/Z3) of the tested pesticides by GC-MS
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Sample preparation and solid-phase extraction

SPE was applied to concentrate and purify the groundwater samples. Briefly, samples were filtered using a 0.7-μm pore-size glass microfiber filter to prevent blocking of the SPE columns by fine suspended solid particles. The SPE columns were conditioned with 5 mL of dichloromethane and methanol (8:2), 2 mL methanol, and 10 mL distilled water containing 10 g L−1 ascorbic acid, after which 500 mL of the filtered water was passed through the column at 10–15 mL min−1. Next, the column was rinsed with 7 mL of distilled water and dried under a vacuum (10 min, intensive flow). The column was subsequently eluted at a very low flow rate (drop by drop) with 1 mL of methanol, followed by 1 mL of dichloromethane and methanol (8:2). Combined eluates were concentrated to a volume of approximately 0.1 mL under nitrogen gas flow, after which 2 mL of iso-octane was added to the concentrated extract, and the solution was evaporated to a final volume of 1 ml at 23 °C using a TurboVap 500 closed cell concentrator. Finally, the sample was subjected to gas chromatography coupled with mass spectrometry (GC-MS). The same procedure was applied to extract all pesticides.

Gas chromatography and mass spectrometry

An HP 5890 series II Plus GC coupled with an HP 5972 Mass Selective Detector was used to analyze the pesticide residues of the extracts. The GC column (J&W Scientific, Folsom, CA) was a DB-5 fused silica capillary column (30 m × 0.32 mm ID, 1 μm film thickness). A total of 1 μl of the aqueous extract was injected into the GC-MS via a splitless injector at a temperature of 250 °C. The temperature program for analysis was as follows: 1 min at 100 °C, followed by an increase at 10 °C min−1 to 240 °C, which was held for 15 min. Helium was used as the carrier gas (1.2 mL min−1). Quantification was performed using a four-point calibration curve plotting peak area versus concentration, and the results were expressed as the percentage recovery of the pesticide (Fig. 3). Samples with residues below the detection limits were assigned a value of zero and included in the mean calculations.

Fig. 3
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GC-MS chromatograms of groundwater sample spiked with 0.50 mg L−1 of mixed standard solution of the tested pesticide

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Quality assurance/quality control

Recoveries were determined for all samples by spiking the distilled water with a mixture of tested pesticide standards prior to extraction. Analytical method recoveries for the given set of pesticides were determined by repeating the entire analytical procedure using reference groundwater enriched with a standard mixture of pesticides of known concentrations.

Results and discussion

Method efficiency and pesticides recovery

SPE was employed to pretreat the groundwater samples. SPE is generally used to concentrate and purify minute amounts of organic micropollutants from water and soil and to carry out simultaneous extraction and concentration of many pesticides and their metabolites in aqueous samples (Font et al. 1993; Sabik et al. 2000). The most widely used sorbents in SPE are C8 and C18 chemically bonded to silica, carbon black, and polymeric resins (Sabik et al. 2000). We used a certified C18 cartridge containing a hydrophobic, reverse-phase, silica-based bonded phase. C18 is an efficient sorbent for determining both polar and non-polar analytes at very low levels of detection by reducing the background interference. The extract from SPE was analyzed using a GC-MS equipped with an electron impact ionization detector. Analytical method recoveries for the given set of pesticides ranged from 85 to 105 % for all tested pesticides (Table 2), while reproducibility calculated based on replicate analyses revealed a relative standard deviation (%) of less than 2.48 %. All laboratory blanks were below the detection limits of different pesticides (Table 2). Taken together, these results confirmed the analytical precision and accuracy of the SPE technique used in this study.

Table 2 Recovery, detection limits (LODs), and maximum residue limits (MRLs) of the tested pesticides
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Water quality and samples clusters

The groundwater quality of the Al-Qassim region is presented in Table 3. According to the FAO guidelines for water quality for agriculture (Ayers and Westcot 1985), most groundwater samples from the region were suitable for irrigation purposes with no or moderate restrictions. Sodium adsorption ratio values ranged from 1.5 to 15 with a mean value of 3.95, indicating a slight effect on sensitive crops. Water pH was neutral to slightly basic (7.04–7.90), falling within the normal range of 6.50–8.50 for irrigation water. Chemical water analysis parameters (EC, pH, SAR, cations, and anions) of each sample were employed to perform cluster analysis (correlations) using the Unscrambler X 10.2 software. The groundwater samples fell into four clusters (viz. 0, 1, 2, and 3), with minimal water variation in each cluster (Figs. 4 and 5). Locations in each cluster were plotted using a contour map generated by SigmaPlot (version12.0), where the City of Buraydah (the capital of Al-Qassim Province) represented the point of origin (0,0) on the map (Fig. 6). The contour map clearly showed specific trends based on the similar properties of groundwater owing to the similar soil properties and agricultural patterns of the regions).

Table 3 Chemical properties of ground water
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Fig. 4
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Cluster analysis for groundwater based on chemical quality parameters

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Fig. 5
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Contour map of groundwater chemical quality parameters based on the cluster analysis, showing four clusters (0, 1, 2, and 3)

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Fig. 6
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spatial distribution of the detected pesticides residue in groundwater of Al-Qassim (ng L−1)

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Pesticide residues in groundwater

The Al-Qassim region is one of the largest agricultural areas in Saudi Arabia, producing the majority of vegetables consumed locally. Therefore, it is essential to monitor pesticide levels in the region to ensure water quality and evaluate the public health risk. A multiresidue GC-MS method was applied to screen seven different pesticide residues (Table 1), and the levels, ranges, frequency, and identity of pesticide residues in the Al-Qassim region were evaluated. In the analyzed samples, residues of six pesticides, bromoxynil (herbicide), carbofuran (nematicide), chlorpyrifos (insecticide), diazinon (insecticide), ioxynil (herbicide), and metribuzin (herbicide), belonging to the hydroxybenzonitrile, carbamate, organophosphate, organophosphate hydroxybenzonitrile, and triazinone chemical groups, respectively, were detected. Cyfluthrin (insecticide), which belongs to the pyrethroid chemical group, was not detected in any of the analyzed samples. Additionally, eight groundwater samples showed no pesticide contamination, which could have been due to the adsorption of pesticides by soil particles prior to infiltration into the groundwater or the low use of the tested pesticides at the sampled locations.

Among the tested pesticides, diazinon occurred most frequently in groundwater (15 locations) of the Al-Qassim region, showing the second highest level of 2.33 ppb (Fig. 6) and a mean value of 0.39 ppb. Chlorpyrifos was the second most frequently observed pesticide, being found in 12 water samples with a mean value of 0.53 ppb, while metribuzin was detected in the groundwater of six locations with a mean value of 0.76 ppb. Carbofuran was detected in eight locations with a mean value of 0.30 ppb, while bromoxynil and ioxynil were detected in four locations with mean values of 0.26 and 0.08 ppb, respectively. The calculated total pesticide residue and their distribution are shown in Fig. 6. The total pesticide residue and the levels of individual pesticides in the groundwater of the studied area exceeded the allowable limits of 500 and 100 ng L−1, respectively, set by the European Economic Community (EC Directive 1998). The presence of these pesticides is likely due to their intensive use by farmers of this region and indicates that groundwater consumption by drinking or irrigation may cause pesticides exposure to the community of the Al-Qassim region, thereby posing a health hazard. Toxicity of agricultural pesticides has previously been reported (Dawson et al. 2010), and organophosphate pesticides (e.g., chlorpyrifos and diazinon), which were the most commonly detected pesticides in the studied area, can cause severe damage to the nervous system (Saunders et al. 2012; Dahlgren et al. 2004). This result is confirmed by the findings of Osman et al. (2010)) who found residues of chlorpyrifos in vegetables produced at Al-Qassim. This might be due to the relatively long half-life of chlorpyrifos in soils and water bodies (al-Mihanna et al. 1998) and the widespread use of chlorpyrifos in the agricultural activities in Saudi Arabia (Al-Dawood et al. 2009).

Spatial distribution of pesticides

Spatial variations of the contaminant concentrations provide adequate information regarding the source of contamination and actions needed for their remediation or management. The spatial distribution of the pesticides detected in the groundwater of the Al-Qassim region is shown in Figs. 6 and 7. Comparison of the map of total detected pesticides (Fig. 6) with satellite image of the Al-Qassim region (Fig. 1) revealed higher concentrations of pesticides in areas of intensive agriculture. The area most heavily affected by pesticides in groundwater was located 50–60 km west of Buraydah. The same trend was observed for the individual pesticides (Fig. 7), although the contamination levels were more intense for chlropyrifos, diazinon, and metribuzin. The spatial distribution clearly indicated that the source of contamination in groundwater is leaching of pesticides from agricultural areas. The pesticide residue levels are high in wells located near these agricultural lands, while they slowly decrease as the water flows out toward the southwestern sides. It was previously believed that aquifers in the KSA were deep because of the occurrence of deserts. However, several recent investigations reported that about 50 % of the wells in the studied region are shallow, with depths ranging from 50 to 200 m, thus increasing the risk of pesticide contamination (Alabdula’aly et al. 2010).

Fig. 7
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spatial distribution of the individual detected pesticides residue in groundwater of Al-Qassim (ng L−1)

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The extent of pesticide transport from the soil to groundwater appears to be a function of several factors, including the chemical properties of the pesticides, soil chemical and physical characteristics (texture, structure, O.M. content, dominant clay minerals, iron oxides), hydrological soil properties, and agricultural management systems (e.g., time of pesticide application). The mobility and leaching potential of hydrophobic pesticides are related to their weak sorption on the soil matrix, as quantified by the organic carbon partitioning coefficient (KOC) (Rittman, et al. 2013; Yang et al. 2012; Duke et al. 2013; Davis and Landolt 2013; Schuler and Berenbaum 2013). A smaller KOC is associated with weaker pesticide binding to the soil and therefore increased leachability into the groundwater. However, pesticides with larger KOC values such as chlorpyrifos have also been observed in groundwater and drainage water (Elliott et al. 2000; Lari et al. 2014), presumably as a result of leaching caused by heavy rainfall shortly after application of the pesticide to wet soils with preferential flow paths. Saudi Arabian soils are characterized by low amounts of organic carbon owing to scant rainfall and the arid environment.

Conclusions

Seven pesticides were monitored in groundwater samples from 34 wells in the Al-Qassim region using the SPE technique and GC-MS analysis. Groundwater was contaminated with multiple pesticide residues exceeding the permitted levels of 0.5 μg/L for total pesticides and 0.1 μg/L for individual pesticides. Chlorpyrifos and diazinon were the most frequently detected pesticides in the region. Extreme concentrations of pesticides were related to intensive agricultural activity in the area, and the spatial distribution indicated that the source of pesticide contamination of the groundwater is about 55 km southwest of Al-Buraydah. However, the pesticides are evenly distributed towards the outflow of groundwater. Overall, the results presented herein indicate an urgent need for pesticide application management in the agricultural area of Al-Qassim to avoid toxic health effects.