Occurrence and seasonal variation of organochlorine pesticides in selected vegetable farmlands in Lagos State, Nigeria
Article information
Abstract
Pest infestation in crop production have increased farmers' interest in pesticides use with short and long term consequences. This study investigated the occurrence and seasonal variations of organochlorine pesticide residues in vegetable farms in selected areas of Lagos State. Non carcinogenic and carcinogenic risk assessment was also evaluated. Soil samples were collected during the wet and dry seasons at soil depth of 0-30 cm. Gas chromatography coupled with an Agilent mass spectrometer was used to analyse organochlorine residues (alpha-lindane,lindane, delta.-lindane, aldrin, heptachlor epoxide, alpha.-endosulfan, p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE), endrin, endosulfan, m,p'-dichlorodiphenyldichloroethane (m,p'-DDD), endosulfan sulfate, o,p'-dichlorodiphenyltrichloroethane (o,p'-DDT) and endrin ketone) in soil. Heptachlor epoxide showed maximum concentration of 43.03 mg/kg in Station 19 in Western zone during the dry season while m,p'-DDD and endosulfan had minimum value of 0.004 mg/kg in Station 2 and Station 5 respectively during the wet season in the Far eastern zone. The concentrations of organochlorine residues were intermediate in the Eastern zone in both seasons. There was significant (p < 0.05) increase in dry season concentrations when compared to wet season. The risk assessment indicated Hazard Quotient (HQ) > 1 for non-cancer risk and cancer risk > 10-6. Thus a need for stringent monitoring programs for pesticides.
Introduction
Agriculture is the foundation of the Nigerian economy and the main source of livelihood for the majority of the rural population [1], who, as farmers, produce food for consumption and raw materials for production and exportation [2]. However, migration of people from the rural areas has led to increased population in the urban areas [3]. In order to meet the huge demand on food due to increased urban population, urban farming has become essential.
Lagos State is divided into three agricultural zones in terms of the spatial distribution of urban farming communities by the Agricultural Development; these are Far eastern, Eastern and Western zones [4]. There are various farming activities in Lagos State but majority of crop farmers cultivate vegetables [5]. However, pest infestation has been observed to have significant impact on vegetable production, accounting for 20-60 % losses before harvest [6, 5]. Consequently, farmers rely heavily on pesticides to control these pests [7, 8].
Pesticides are used globally at a rate of approximately two million tonnes per year, with Europe accounting for 45 %, the United States for 25 %, and the rest of the world for 25 %, with Africa accounting for 5 %. They have been accredited with success in both agricultural and public health sectors [9]. However, their usage over the years have caused contamination of the environmental matrices such as air, soil, groundwater, surface water and undesirable residues accumulating in soil, soil organisms and food crops [10]. Therefore, posing a threat to human health and the ecosystem as a whole. The continuous use of organochlorine pesticides remains global issue due to their persistence and long-distance dispersal via ocean currents and atmospheric transport [11]. Resulting in their banned by the United Nations Stockholm Convention [12]. However, despite the prohibition, most of the noxious organochlorine compounds (OCPs) are still used both on agricultural crops and domestically in many underdeveloped countries [13].
Exposure to pesticides could occur through dietary route such as consumption of foods contaminated with pesticides and non-dietary route which are accidental ingestion of soil, inhalation of dust and dermal contact [14]. These exposures could result in both non-carcinogenic and carcinogenic effects such as endocrine system disruption, immune system suppression, neurological damage, cancer and death in children and adults [15, 16]. Some organochlorine pesticides such as dichlorodiphenyltrichloroethane (DDT) and its metabolites, aldrin, heptachlor and it metabolite are classified as probable human carcinogens [17]. OCPs exposure have been associated with brain, kidney, leukemia, lymphoma and breast cancers [18-20].
Studies have shown that organochlorine pesticide residues are widely distributed in Nigerian soil, water and cultivated food [21-26]. Conversely, there is dearth of information on organochlorine residues in Lagos State vegetable farmlands and non-dietary health risk assessments of these residues in children and adults. Hence, this research assessed some of the organochlorine residues in the agricultural soils to provide information that would be useful in environmental monitoring of pesticides in the soil. With a view of evaluating the health risk connected to the non- dietary exposure of these residues.
Materials and Methods
Study area
The study was carried out in selected areas in Lagos State with Latitude 6 ° 23′ N and 6 ° 41′ N, and Longitudes 3 ° 09′ E and 3 ° 20′. The sampling sites were selected based on the three zonal agricultural divisions of Lagos State by the Lagos State Agricultural Development Agency. From each zone, three blocks were selected where vegetables are cultivated. The Far eastern zone samples were collected from Epe (Origanringan), Ejirin, and Agbowa; were coded ST1-ST6; control site was designated as FC (ST7). The sampled sites in the Eastern zone were chosen from Imota, Gberigbe, and Laspotech; were coded ST8-ST13; control site was designated as EC (ST14). The Western zone sampled sites were chosen from Ikeja, Ojo and Badagry; were coded ST15 - ST20; control site was designated as WC (ST21). Six sampling locations were identified using the global positioning system and three sampling points were established for each location and zone. The control sites were locations within the zones where there were no obvious agricultural and industrial activities. A total of sixty three (63) soil samples were collected from different locations and the control sites. For each zone, seven (7) composite samples were analysed in triplicates for each season. The map of the study areas is shown in Figure 1.
Map of the sampled stations
Sampling schedule and soil sample collection
Soil samples were collected between June/July and November/December, 2021 depicting the wet and dry seasons. Soil samples were collected using a clean stainless steel auger at a depth of 0 to 30 cm. Soil samples were placed in aluminum foil, kept in zip-lock bags in order to minimize possible contamination and stored at 4 °C as described by Opeolu et al. [27].
Organochlorine pesticides residues extraction from Soil
Pesticides extraction from soil samples was performed using EPA 3550C method described by USEPA [28] with minor adjustment. The soil sample (10 g) was weighed into a 100 ml Erlenmeyer flask, 5 g of anhydrous sodium sulphate was added, followed by 50 ml of a mixture of acetone and n-hexane (1:1 v/v) to the mixture in the Erlenmeyer flask, sonicated for 10–15 minutes in a high-frequency ultrasound bath and decanted into a round bottom flask. The extraction process was repeated with another 50 ml (acetone and n-hexane). The extract was concentrated to 10 ml using a rotary evaporator.
Extract clean-up process
A column was filled with cotton wool containing anhydrous silica gel and sodium sulphate (2:1). The column was conditioned with a mixture of 2.5 ml of n-hexane and acetone (1:1). The extract was then poured through the column and later eluted with 5 ml of the mixture of n-hexane and acetone. The eluate was concentrated to 2 ml using a nitrogen concentrator or by air-drying with exclusion of light. The eluate was poured into a GC vial for GC-MS analysis.
Determination and quantification of Organochlorine pesticides in Soil samples
Gas chromatography (GC) was operated in selective ion monitoring (SIM) and scanning mode to ensure detection of target components at low concentration. The stationary phase for the separation of the compounds was an HP-5M capillary column (30 m length × 0.32 mm diameter × 0.25 m film thickness) coated with 5 % phenylmethylsiloxane (Agilent Technologies). The carrier gas was helium at a constant flow of 1.2 ml/min at an initial nominal pressure of 0.26 psi and an average velocity of 40.00 cm/sec. The samples were injected in splitless mode at an injection temperature of 250 °C. The flush flow used was 30.0 ml/min at 0.35 min with a total flow of 31.24 ml/min. The initial oven temperature was 50 °C (1 min) and then increased from 25 °C/min to 100 °C (3 mins) and from 5 °C/min to 300 °C (5 mins). The run time was 16 mins with a solvent delay of 3 mins. The mass spectrometer was set to electron impact ionization mode at 70 eV, with the ion source temperature set at 230 °C, the quadrupole temperature at 150 °C, and the transmission line temperature at 300 °C. Quantification of OCPs involved external calibration curves prepared from reference standard solution. The concentrations of organochlorine residues were determined by comparing the peak heights of the samples with the peak heights of the reference standards of known concentrations.
Health risk assessment
Three possible routes of human exposure to pesticide residues in soil samples from the three Agricultural zones were considered in determining the risk to human health. The three exposure routes were ingestion, inhalation and dermal contact. This represents the non-dietary risk assessment.
Chronic daily intake (CDI)
All routes of exposure were subjected to non-carcinogenic and carcinogenic risk assessments in children and adults using the risk assessment models [29, 30]. CDI of contaminated soil samples for each population (children and adults) were estimated using the equations 1 to 5 below [31,32] to evaluate the non-carcinogenic risk. Then, the carcinogenic risk for each population (children and adults) was estimated using the cancer slope factor (equation 6) [33]
Where Csoil is the concentration organochlorine residue in soil (mg/kg); InR is the ingestion rate, rate (200 mg.d-1 for a child and 100 mg.d-1) CF is the conversion factor (106 mg/kg), ED is the exposure duration (yr), EF is the exposure frequency (350 d/yr for a child and adult), IhR is the inhalation rate (12 m3/d for child and 13.25 m3/d), AT is the average life span for (non-cancer risk: 2190 d for child and 9490 d for adult; cancer risk:25550 for both child and adult), BW is the average body weight (17.4 kg for a child and 80 kg for adult); PEF is the particle emission factor (1.36 x 10-9 m3/kg), ABS is the dermal absorption factor (chemical specific), AF is the skin adherence factor for soil (0.2 mg/cm2 for a child and 0.07 mg/cm2) for adult, SA is soil surface area (cm2), ABS is dermal absorption factor (chemical specific), RfD is reference oral dose (chemical specific),CDI: Chronic daily intake, HQ is Hazard Quotient, HI is Hazard Index, CR is Cancer Risk, CSF is Cancer Slope Factor
Data analysis
The mean and standard deviations (mean±standard deviation) of obtained data on organochlorine and their residues in soil samples were computed. Two way ANOVA was carried out to compare the means and Duncan multiple range test was used to separate the means at level of significant (p < 0.05). The data were processed using the Software Package for Social Sciences (version 26.0).
Results
Far Eastern zone
Maximum concentration of Heptachlor epoxide (34.69 mg/kg) was detected in ST5 during the dry season while m,p'-DDD and endosulfan sulfate had the minimum concentration of 0.004 mg/kg in ST2 and ST5 respectively during the wet season. The concentrations of the OCP residues in the sampled stations during the wet and dry seasons varied significantly (p < 0.05) but there was no significant (p > 0.05) variation in OCP concentrations in some sampled stations and control site at both seasons (Table 1).
Wet and dry season’s organochlorine distribution (mg/kg) in the Far eastern zone
Eastern zone
Heptachlor epoxide had 21.74 mg/kg had the highest concentration in ST12 during the wet season while m,p'- DDD and endosulfan sulfate had the lowest concentration of 0.004 mg/kg in ST8 and ST9 during the dry season. There was significant (p< 0.05) variation in some OCP residue concentrations across the sampled stations and control sites during the wet and dry seasons (Table 2).
Wet and dry season’s organochlorine distribution (mg/kg) in the Eastern zone
Western zone
In the western zone, heptachlor epoxide had maximum concentration of 43.03 mg/kg in ST19 during the dry season while alpha-endosulfan and endosulfan in ST15 and ST16 respectively with minimum concentration of 0.004 mg/kg in wet season. There was significant (p < 0.05) variation in the concentrations of some of the OCPs detected in the sampled stations when compared with control site during the wet and dry season (Table 3).
Wet and dry season’s organochlorine distribution (mg/kg) in the Western zone
Risk assessment
The results of the non-carcinogenic and carcinogenic risk of organochlorine pesticide residues in children and adults from the three agricultural zones are presented below.
Far Eastern zone
Heptachlor epoxide had the highest HQ of 5.34 × 107, 4.91 × 1014 and 1.76 while endosulfan sulfate had the lowest HQ with values of 1.89 × 102, 1.73 × 109 and 6.22 × 10-6 in children for ingestion, inhalation and dermal exposure routes respectively. Likewise, heptachlor epoxide had the highest HQ in adults with values of 1.21, 1.18 × 1014 and 4.61 × 107 while endosulfan sulfate had the lowest HQ with values of 4.28 × 10-6, 4.17 × 108 and 1.63 × 102 for the three exposure routes respectively. HQ for other organochlorine residues had intermediate values. All the organochlorine residues had HI > 1 for all exposure routes in children and adults (Table 4). The total cancer risks in children for inhalation and ingestion routes was significantly greater than the stipulated potential cancer risk values of 10-1 while the dermal route in children had cancer risk value within the stipulated values of 10-6 and 10-4. Adults had cancer risk value within the stipulated values of 10-6 and 10-4 for inhalation and dermal routes while the inhalation route had value significantly greater than the stipulated potential cancer risk values of 10-1 (Table 4).
Estimation of non-carcinogenic and carcinogenic for organochlorine residues in Far eastern zone
Eastern zone
Heptachlor epoxide had the highest HQ of 4.31 × 106, 3.96 × 1013 and 1.42 × 10-1 for ingestion, inhalation and dermal exposure routes respectively while p,p'-DDE had the lowest HQ with values 5.29 × 101, 4.86 × 108 and 1.74 × 10-6 for ingestion, inhalation and dermal exposure routes respectively in children. Also, heptachlor epoxide had the highest HQ in adults with values of 9.76 × 10-2, 9.51 × 1012 and 3.71 × 106 for ingestion and inhalation and dermal routes respectively while p,p'-DDE had the lowest HQ with values of 1.2 × 10-6 and 1.17 × 108; 4.56 × 101 for ingestion, inhalation and dermal exposure routes respectively. HQ for other organochlorine residues had intermediate values. All the organochlorine residues had HI < 1 for dermal exposure in children while adults had HI < 1 for ingestion route (Table 5). The total cancer risks in children and adults for ingestion and dermal exposure routes were within the stipulated potential cancer risk values (10-6 to 10-1) while children and adults had values greater than the stipulated value (10-1) for inhalation exposure route (Table 5).
Estimation of non-carcinogenic and carcinogenic for Organochlorine residues in Eastern zone
Western zone
Heptachlor epoxide had the highest HQ of 3.57 × 107, 3.28 × 1014 and 1.18 while endosulfan sulfate had lowest HQ of 1.11 × 102, 1.02 × 109 and 3.66 × 10-6 for ingestion, inhalation and dermal exposure routes respectively in children. In adults, heptachlor epoxide had the highest HQ of 8.09 × 10-1, 7.89 × 1013 and 3.08 × 107 while endosulfan sulfate had lowest HQ of 2.52 × 10-6, 2.45 × 108 and 9.57 × 101 for ingestion, inhalation and dermal exposure routes respectively. HQ for other organochlorine residues had intermediate values. All the organochlorine residues had HI > 1 for all exposure routes in children. Adults had HI < 1 in ingestion route while inhalation and dermal routes had HI > 1 (Table 6). The total cancer risk values in children and adults were greater than the stipulated potential cancer risk values (10-1) for inhalation exposure route, (< 10-4) for ingestion and dermal exposure routes respectively (Table 6).
Estimation of non-carcinogenic and carcinogenic for Organochlorine residues in Western zone
Discussion
Pesticides especially organochlorine pesticides are extremely toxic, less than 1 % hit the target, when applied [34]. The remaining pesticides lingers in the soil for a long time before breaking down into metabolites, which can be dangerous depending on their chemical composition [35-37]. Several studies have shown pesticide residues in different environmental matrices and organisms [26, 38-39]. These concentrations showed the indiscriminate and injurious use of these pesticides in Nigeria regardless of government policy on their usage [40].
Heptachlor epoxide which was the most prominent OCP residue detected in this study is the degradation product of heptachlor has low susceptibility to biodegradation, oxidation and hydrolysis or photolysis in the environment [41]. Serious heptachlor epoxide poisoning could cause central nervous system effects as such seizures, convulsions and hyperexcitability, liver damage and cancer [42-43]. The high concentration of heptachlor epoxide in this study could be as a result of recent application or accumulation of its massive usage for a long period of time in the past. The value obtained was higher than the findings of Gopalan and Chenicherry (2018) [44] who detected 7.5 ng/g of heptachlor epoxide in soil during the monsoon season. Also, Huang et al. [33] detected lower concentration of heptachlor epoxide (0.51 ng/g) in their study.
Pollution from organochlorine pesticides occurs more frequently through transport and atmospheric deposition, which are responsible for their transport to distant locations [45]. The detection of these pesticide residues in the control sites in this study could be attributed to their trans boundary properties and this suggests drift during pesticide application, this is in agreement with the study of Tariq et al. [46].
In Nigeria clime, seasonality is differentiated by rainfall pattern [47]. Due to high intensity of rain during the wet season, these pollutants could be easily leached to the aquifer or runoff to the surrounding water bodies. Thereby, reducing their concentrations in wet season compared to the dry season. Seasonal variation, which was observed in the concentrations of OCP residues in this study could be attributed to the rainfall pattern. In addition, it also suggested their intensive use during the dry. This agrees with the study of Nyantaky et al. [48].
The Stockholm Convention identified and classified some of the organochlorine pesticides as probable carcinogens to human [49-51]. This shows that they could be detrimental to human and the environment. Non-carcinogenic health effects are shown by hazard index. HI > 1 specifies high potential non-carcinogenic health effects of such pesticide while HI < 1 indicates low non-carcinogenic health risk of the pesticide to human [52, 53]. ATSDR (1996) and (2004) [54, 55] standard proposed the following qualitative ranking of cancer risk: very low (value < 10−6), low (10−6 ≤ value < 10−4), moderate (10−4 ≤ value < 10−3), high (10−3 ≤ value < 10−1), and very high (value ≥ 10−1). The non-carcinogenic and carcinogenic risk assessment of organochlorine pesticide residues showed that most of the children and adults exposed to pesticides in the study areas could be at non-cancer and cancer health risk at different levels. This contradict the findings of Yao et al. [56], who observed that OCPs could have no non-carcinogenic risk to children but have some non-carcinogenic risk to adults. Their findings also showed that there was no carcinogenic risk when children and adults are exposed to OCPs through the three exposure routes.
Conclusions
These results established that some of the vegetable farmlands in selected areas of Lagos State are polluted with organochlorine pesticides. This is an indication that these banned pesticide though they persist in the environment are still in use. Consequently, this could constitutes a threat to public health because of their potential bioaccumulation and biomagnification along the food chain when the cultivated vegetables are consumed. Therefore, there is need for more stringent monitoring program such as restriction on pesticide usage. This will prevent illegal use of banned pesticides and limit organisms’ exposure.
Acknowledgements
The Central Research Committee (CRC No: 2021/02) of the University of Lagos is highly acknowledged for funding this research.
Notes
The authors declare no conflict of interest.
CRediT author statement
FIO: Conceptualization, Methodology, Writing-Review and Editing, Supervision and Funding acquisition; OFH: Conceptualization, Methodology, Investigation, Formal Analysis and Writing- Original draft preparation; MNI: Writing- Review and Editing, Supervision and Funding acquisition; AGU, IA, MF, PA, AJ and OO: Investigation.