In a quest to increase agricultural productivity there has been an increase in the use of pesticides. Pesticides include insecticides, herbicides, rodenticides, and fungicides (Chopra et al., 2010). These persistent organic pollutants (POPs) have become an integral part of the Nigerian society and are being used for diverse activities ranging from crop protection from insect pests, weeds, rodents and fungal diseases to animal husbandry and public health applications (Zhou et al., 2006). Despite these overwhelming benefits, chemical pesticides are usually not target-specific and may  cause  harm  to  non-target organisms. Most pesticides are very persistent, remaining in the environment for long periods (Chopra et al., 2010). Due to their stable structure and lipophilic character, pesticides, especially organochlorine pesticides (OCPs), tend to bioconcentrate and biomagnify in the food chains particularly those associated with fatty tissues, leading to vertebrate and non-vertebrate toxicity in non-target organisms and even humans (Okoya et al., 2013; Masia et al., 2013; Ize-Iyamu et al., 2007). Upadhi and Wokoma (2012), Williams  (2013),   Ezemonye   et   al.  (2008a, b,   2009),

 

Ize-Iyamu et al. (2007), Adeboyejo et al. (2011), Okeniyia et al. (2009) and Adeyemi et al. (2011) are few studies that have provided evidence of pesticide contamination in various environmental compartments in some rivers in Nigeria. Protecting Nigeria’s fresh water bodies from pesticides and other pollutants is of utmost importance due to their prevailing toxicity even at trace levels. Thus, data pertaining to pesticide concentration in rivers are very critical.

 

The purpose of this research is to determine the presence of selected pesticides in water, sediment and biota of the Ogbesse River. To our knowledge, few studies have assessed the risk of pesticide occurrence to non-target organisms and humans in Nigeria. Therefore this study assesses the ecotoxicological risk to non-target organisms accrued by the presence of these pesticides and also establishes the risk of exposure to humans as a result of the eventual consumption of pesticide contaminated fishes.

Site description and sampling

 

Ogbesse River is located in Ogbesse town (N 06° 45’ 40’’, E 005° 46’ 07.4’’) within the forest belt ecological zone of Edo State, Figure 1. It is prominent for cocoa, plantain and pepper production. The river separates Ondo and Edo States. The surrounding area of this river is typified by the presence of commercial cocoa farms, which constantly make use of agrochemicals to enhance their production. Water samples (n=216) were collected from three sites along the stretch of the river, 0.3 m below the water surface with a pre–cleaned glass bottle using hydrobios sample based on the method by Ezemonye et al. (2008a,b).

 

 

Sediment samples (n=216) were also collected using methods described by Ezemonye et al. (2008a,b). Samples were taken from the positions where an accumulation of fine-texture substrate took place. The upper 2 cm of bed sediment at each site were collected with a Teflon-coated spoon and wrapped in aluminum foil. Samples of Clarias gariepinus and Tilapia zilli were captured randomly and/or bought from fishermen around each river (Ize-Iyamu et al., 2007). Samples were collected monthly for 18 months.

 

Extraction and analysis

 

Sediment samples were extracted and cleaned up according to the method described by USEPA (2004). Edible portions of C. gariepinus and T. zilli samples were extracted and cleaned up according to Steinwandter (1992), while water samples were extracted and cleaned up based on Osibanjo and Adeyeye (1997) and USEPA (2004). The cleaned up extracts were analyzed for pesticides (α HCH, γ - HCH, β- HCH, Heptachlor, Heptachlor Epoxide, Aldrin, Dieldrin, Endrin, p,p DDT, Endosulfan I, Endosulfan II, Endosulfan aldehyde, Endosulfan sulfate, Atrazine, Glyphosate and Carbofuran).

 

Results were obtained using a Hewlett-Packard (hp) 5890 Series II equipped with 63Ni Electron Capture Detector (ECD) of activity 15 mCi with an auto sampler. The efficiency of the analytical method (the extraction and clean-up methods) was determined by recoveries of an internal standard. Peak identifications were conducted by comparing the retention time of standards and those obtained from the extracts. Concentrations were calculated using  a four-point calibration curve. Method detection limits (MDLs) ranged from 0.005 µg/l for pesticides in water and 0.01 µg/g/dw for pesticides in sediment and biota. A simple, rapid solvent extraction method was used to determine total lipids in each fish tissue based on the method described by Randall et al. (1998) and lipid normalized concentrations were obtained using the ratio between pesticide concentration in tissue and lipid fraction in the tissue.

 

Risk assessment

 

Risk Quotient Method (RQ) was used to determine the risk of pesticide exposure in aquatic organisms. RQ is the ratio of the Measured Environmental Concentration (MEC) to the Predicted No Effect Concentration (PNEC). The predicted no effect concentration (PNEC) was obtained by multiplying the LC₅₀ with an assessment factor (AF) of 100. LC₅₀ was obtained from Munn et al. (2006). The assessment factor takes into account the uncertainty in extrapolation from laboratory toxicity tests for a limited number of species to the real environment (Sangchan et al., 2012).

 

PCA analysis

 

Residues obtained from water, sediment and fish analysis was subjected to Principal Component Analysis (PCA) using SPSS 20 statistical software to infer the hypothetical sources of pesticides. PCA was performed by varimax rotation because it minimizes the number of variables with a high loading on each component, facilitating the interpretation of PCA results (Frimpong et al., 2013).

 

Health risk estimation

 

To assess the risk of pesticide contained in each fish species on consumers, the guidelines for potential risk assessment drawn up by the US EPA were used. Estimated Average Daily Intake (EADI) was obtained by multiplying the residual pesticide concentration (ug/kg) in each fish species by the consumption rate in Nigeria (kg/day) and dividing the product by the body weight (kg) (WHO 1997; Fianko et al., 2011). The food and agricultural organization (FAO, 2011) quotes the per capita consumption of fishes in Nigeria as 9 kg per year.

 

Concentration of pesticides in water

 

Concentrations of pesticides in water are shown in Table 1 and Figure 2, with Heptachlor Epoxide having the highest mean concentration (0.4 µg/l). Overall, HCHs (31%) were the most dominant pesticide recorded in water samples. The concentration of pesticide residues was significantly higher (p ≥ 0.05, t= 0.41) during the wet season when compared with the dry season.

 

 

 

The presence of these pesticides residues in water samples from Ogbesse River is indicative of the possible contamination of the river through various anthropogenic sources within the region especially agriculture. It was also observed that concentrations of pesticide residues in this study were higher than recommended guideline value for pesticides residues in fresh water bodies set by European Union (EU) (0.1 ug/l) (Hamilton et al., 2003). 

 

While HCH, Endosulfan, Endrin, DDT, Dieldrin and Carbofuran, were above Canadian EPA and WHO guideline values for domestic and drinking purposes, concentrations observed in water samples were also higher than the allowable residual limit of individual and total concentration of pesticides residues in drinking water at 0.1 and 0.5 µg/l, respectively set by NISERA (Adeyemi et al., 2008). Further comparison with  previous studies showed that concentrations observed in this study were lower than concentrations of pesticides residues observed in Lagos lagoon (Adeyemi et al., 2011), Benin rivers (Ogba, Okoro and Ovia) (Ize-Iyamu et al., 2007) and Warri rivers (Ezemonye et al., 2008a) in a study in Warri rivers. However concentrations observed in this study were higher than those observed in the Niger Delta (Upadhi   and   Wokoma,  2012),   Lagos   Lagoon (Williams, 2013; Adeboyejo et al., 2011); Rivers in the South West Nigeria (Okeniyia et al., 2009); Northern River (Darko and Babayo, 2013), and Ondo Rivers (Okoya et al., 2013). 

 

Concentration of pesticides in sediment

 

All the selected pesticides were detected in sediment samples obtained from Ogbesse River (Table 1, Figure 3). It was observed that alpha HCH had the highest mean concentration (2.4 µg/kgdw) while overall HCHs (29%) was the predominant pesticide. It was also observed that the concentration of pesticide residues in sediment samples were relatively higher during the dry season compared with the wet season. However, there was no significant difference (p ≥ 0.05, t = 0.83) between the concentrations of pesticides residues observed during the dry season and the wet season. There was a higher concentration of pesticide residues in sediment samples compared to water samples, which indicates the hydrophobic nature of the pesticides detected (Ezemonye et al., 2008b; Ezemonye et al., 2009; Williams, 2013).

 

 

Hexachlorocyclohexane (HCH) showed predominance in this river, indicating the introduction of HCH to sediments via runoffs from surrounding agricultural areas, where HCH pesticides have been used or are currently being used (Olatunbosun et al., 2011). Furthermore, the half-life of HCH and its isomers in the environment ranges from 15 months, 50 days and 20 days for alpha HCH, gamma  HCH  and  beta  HCH  respectively  (FAO, 1997); hence, the detection of these pesticides is indicative of the fact that they were recently applied. This is further confirmed by the ratio of α-HCH and γ -HCH. In the entire river sampled, α-HCH /γ -HCH ratio in sediment samples were below 3, indicating that the source of HCH in the environment is a fresh input of technical HCH (Chen et al., 2009). There was also a positive correlation between the total organic carbon content and pesticide concentration in sediments, which indicates that the sediment organic carbon could enhance adsorption and deposition of these pesticides compounds (Figure 4). This finding is similar to Devi et al. (2013), who reported significant correlations between organochlorines and TOC in India, but does not agree with the studies by Olatunbosun et al. (2011) and Liu et al. (2008), who reported that there was no correlation between OCPs and TOC in sediment from water bodies in Niger Delta and China respectively, suggesting that TOC has no influence on the distribution of OCPs in the sediments.

 

 

Concentration of pesticide in biota

 

Lipid normalized concentrations of pesticides in the two fish species collected from Ogbesse River is presented in Table 1 and Figures 5 and 6. In C. gariepinus, Beta HCH had the highest mean concentration (2.34 µg/gdw) while p,p DDT had the lowest mean concentration (0.04 µg/gdw). Seasonal distributions indicate that each pesticide residue in C. gariepinus had high concentrations during  the  wet  season.  However, there was no significant difference (p ≥ 0.05, t=0.09) between the concentrations of pesticides residues during the dry season and the wet season.

 

On the other hand, beta HCH had the highest mean concentration (1.73 µg/gww) in T. zilli samples and seasonal distribution showed higher concentrations of each pesticide during the wet season. However, there was no significant difference (p ≥ 0.05, t= 0.61) between the concentrations of pesticides residues observed during the dry season and the wet season.  Overall,  HCHs were predominant in C. gariepinus and T. zilli. The presence of pesticides in C. gariepinus and T. zilli could be attributed to uptake either through bioconcentration from water through gills or epithelial tissues and through bioaccumulation through water and through food leading to eventual biomagnification in different organisms, occupying successive trophic levels (Murty, 1986). Hence, the presence of pesticide residues implies that there is clear evidence of the bioconcentration and bioaccumulation   of   pesticides   from   the   surrounding environment.

 

The levels and occurrence of residues in fish samples seem to be governed by feeding mode, age and mobility of the biota; consequently, higher concentrations of pesticide residues observed in C. gariepinus may be attributed to the feeding  mode  of  the fish (Mwevura et al., 2002). This is corroborated by Biego et al. (2010) and Mwevura et al. (2002) who related the habitation and feeding habit of C. gariepinus to an increased concentration of pesticide residues compared with other pelagic fish species. Murano et al.  (1997)  and Kidwell et al. (1990) equally adds that pesticide accumulation in fish was due to their lipid content; this implies that due to the high lipid content in C. gariepinus, more pesticide residues tend to be trapped in their lipid stores.

 

 

 

 

 

Risk assessment of pesticide residues

 

Risk assessment using the Risk Quotient deterministic method, where MEC (Measured Environmental concentration) was divided by the PNEC (Probable No Effect Concentration), is presented in Table 2. Risk Quotient for pesticide residues in water samples for both fish species showed that Endosulfan I, Endosulfan II, Endosulfan Aldehyde, Endrin, and Dieldrin are likely to cause adverse effects to each fish species (RQ≥1). Fatemeh et al. (2012) and Sanchez-bayo et al. (2002), report that RQ≥1 indicates a high risk, while 0.1≤ RQ≤1 indicates medium risk, and 0.01≤RQ≤0.1 indicates low risk.  

 

Concentrations  of  pesticide  residues  in  sediment were below Freshwater Sediment Screening Benchmark (FSSB). However, Dieldrin and Endrin in sediment samples were above the Effect Low Range (ERL), indicating potential risk to benthic organisms due to exposure to contaminated sediments (Table 2). This is similar to studies carried out in India where DDT concentrations were reported to be above the ERL, and also similar to a study on ecological effects of pesticides in Nigeria Delta region of Nigeria, where DDT values were above the ERL (Olatubosun et al., 2012).

 

 

Human health risk assessment of pesticide residues in biota

 

C. gariepinus and T. zilli are commercial aquatic products for Ogbesse community in Edo State. Hence, assessing the potential human health risk from the consumption of these fishes is an extremely important step towards public health safety. Table 3 shows the Estimated Average Daily Intake (EADI) and the Hazard Quotient for each pesticide. Three population groups with varying body weights were used for this study. EADI for Aldrin, Dieldrin, Endrin and Heptachlor epoxide in C. gariepinus were higher than the ADI for all weight groups, while EADI for gamma HCH exceeded the ADI for 10 kg category. Table 3 also showed that Hazard Quotient (HQ) values  were  above  one  suggesting   that   health   risks associated with intake of Aldrin, Dieldrin, Endrin and Heptachlor epoxide through the consumption of C. gariepinus was high. On the other hand, human health risk estimates presented in Table 4 showed that EADI for Heptachlor epoxide, Endrin, Aldrin and Dieldrin in T. zilli were higher than the ADI for 10 kg weight groups, EADI for Heptachlor epoxide and Endrin, exceeded the ADI for 30 kg category, while EADI for Heptachlor epoxide exceeded the ADI for 70 kg category.

 

 

 

All pesticides with EADI above the ADI equally had their HQ greater than 1, suggesting potential health risk through the consumption of T. zilli. For C. gariepinus and T. zilli the hazard index estimations showed that mixtures of organochlorine pesticides were above one (1) for all the population groups, while hazard estimates for Glyphosate, Triazine and Carbamate pesticides were below one. It has been reported that if the hazard index >1, the mixture has exceeded the maximum acceptable level (Tsakiris et al., 2011) and there might thus be risk. Therefore, results from this study suggest a great potential for chronic toxicity through the consumption of pesticide contaminated C. gariepinus and T. zilli obtained from Ogbesse River because hazard index >1.

 

The results of Human Health Risk estimations obtained from this study, conforms with studies by Darko and Akoto (2008), who reported that the life time consumption of eggplant and tomatoes from Kumasi market could pose some health risks because HQ>1, and also agrees with Sohair et al. (2013) who reported potential health risk from consumption of fruits from Egypt because Health hazard >1.

 

In similar studies, Fianko et al. (2011) and Andoh et al. (2013) reported  potential  risk  to  humans  through  the consumption of fish, maize and cowpea from Ghana because estimated hazard quotient was above 1.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

This study presents evidence of pesticide contamination of water, sediment and fish species, arising from various agricultural practices in Ogbesse River. Overall, sediment samples had the highest concentrations of pesticides residues, with the concentration of pesticide residues in the following order; Sediment ? C. gariepinus ? T. zilli ? water. Ecological and human health risk assessment provided evidence of potential risk to aquatic organisms and humans. Therefore, there is the need for continuous monitoring of Ogbesse River and its vital fish resource for pesticides in other to ensure the safety of consumers. Proper orientation exercises for farmers are needed to ensure the proper handling of pesticides and its application instruments. Finally, public health educational programs should be introduced to prevent or reduce exposure of the population to pesticides from different sources.

The authors have not declared any conflict of interests.

The authors express their appreciation to the Japan Society for Promotion of Science and the National Centre for Energy and Environment (NCEE) for partial funding and travel grants this project.