Environ Monit Assess (2011) 177:51–61 DOI 10.1007/s10661-010-1617-y

Determination of organochlorine pesticides in the agricultural soil of Oke-Osun farm settlement, Osogbo, Nigeria John Adekunle Oyedele Oyekunle · Aderemi O. Ogunfowokan · Nelson Torto · M. S. Akanni

Received: 31 December 2009 / Accepted: 9 July 2010 / Published online: 3 August 2010 © Springer Science+Business Media B.V. 2010

Abstract This study was conducted to evaluate the levels and seasonal variations of some organochlorine pesticides (OCPs) in the cultivated land of Oke-Osun farm settlement, Osogbo, Nigeria. A field sampling programme was conducted in the rainy and dry seasons for 4 months each resulting in the analysis of a total of 40 samples. Soil samples collected at 20-m intervals were air-dried to a constant weight, sieved through a mesh of 2.0-mm pore size and selected by coning and quartering method. Solid–liquid extraction was used to extract OCPs from the soil. Qualitative identifications and quantitative evaluation of the OCPs were carried out with the aid of a Perkin Elmer gas chromatograph coupled with electron capture detector. Seasonal mean ranges of OCPs in soil (μg/kg) were 13.09 ± 21.66 βBHC–42.01 ± 17.50 p, p -DDT in rainy season and 30.74 ± 17.38 α-BHC–82.88 ± 32.24 p, p DDT in the dry season. The results obtained from

J. A. O. Oyekunle (B) · A. O. Ogunfowokan · M. S. Akanni Department of Chemistry, Obafemi Awolowo University, Ile-Ife, Nigeria e-mail: [email protected], [email protected] N. Torto Department of Chemistry, Rhodes University, Grahamstown 6140, South Africa

this study revealed that agricultural soil samples of Oke-Osun farm settlement were contaminated with persistent organochlorine pesticides mainly as a result of their applications by farmers. Higher levels of OCPs were obtained for dry season than the rainy season. There were indications from this study that pesticides that have deleterious health effects on humans previously placed under legal restrictions by regulatory agencies were still being used by the farmers of Oke-Osun farm settlement and this gives cause for environmental concern. Keywords Organochlorine pesticides · Soil · Farm settlement · Gas chromatography · Oke-Osun

Introduction The use of pesticides in Nigeria for disease vector control and agricultural pests eradication has been on for quite some time. Across the globe and particularly in Nigeria, the number of people making use of pesticides in one form or the other keeps increasing. According to FAO (2005), pesticides importation rose steadily from about 13 million dollars in 2001 to 28 million dollars in 2003. There are evidences that this trend is still on the increase from year to year (PAN 2007). Soils are the most significant sink for all environmental contaminants such as trace metals

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and pesticide residues released into the environment by man’s activities (Kabata-Pendias et al. 1992). These contaminants enter the soils by a number of pathways, and their behaviour and fate in soils differ based on their source, species and degradation products or metabolites. Soil quality is tremendously affected by the types and levels of contaminants or pollutants in it. By extension, depending on the use to which the soil is put and its extent of interactions with other environmental components, a given soil affects the quality of other environmental matrices a great deal. For example, crops are planted on it for consumption and some of the pollutants may get translocated into the crop systems and invariably become part of the food chain; runoffs from the soil enter aquatic habitats near and far and stress up the aquatic ecosystem. It has been reported by Pandey and Shukla (1983) that pesticides in runoffs damage fisheries potentialities of freshwater bodies. There is an international concern in recent times based on scientific and toxicological evidence about the dangers to human health and the environment by persistent toxic chemicals such as organochlorine pesticides and their metabolites (UNEP-GEF 2002). Such dangers include formation of cancer cells (Settimi et al. 2003), neurotoxicological and immunotoxicological disorders (Donkin et al. 1996; Galloway and Handy 2003; Kamel and Hoppin 2004), reproductive and foetal developmental abnormalities (Garcia et al. 1999; Yucra et al. 2006), endocrine disruption (Pesticide Trust 1995; Barlow 2005) and enzyme inhibitor (Pesticide News 2000; Manirakiza et al. 2002). The health problems associated with the presence of traces of xenobiotics such as pesticide residues in foods and in both abiotic and biotic environments have led to greater scientific, industrial and governmental concerns internationally. Organochlorine pesticides (OCPs) and their residues come as a contaminant of an environment basically as a result of anthropogenic activities and biochemical degradation of the original compounds. They (OCPs) are known to be persistent environmental pollutants (McAloon and Mason 2003) because they are capable of long-term resistance to biodegradation and therefore they can be concentrated through food chains and produce a sig-

Environ Monit Assess (2011) 177:51–61

nificant magnification of the original concentration at the end of the chain (Sankararamakrishnan et al. 2004; Darko and Acquaah 2007). In developing countries like Nigeria, investigating and estimating the occurrence and levels of pesticide residues of agricultural soils located in rural areas have not been an object of extensive research. Hence there is paucity of data available with respect to the pesticide residues pollution status of Nigerian farm settlements including those established by state and federal governments. Whereas, the applications of certain pesticides had been outlawed as far back as 1972 in places like the USA (http://www.edf.org/article.cmf), it was not until 1991 that the Federal Environmental Protection Agency (FEPA) now Federal Ministry of Environment placed a ban on them in Nigeria. Even after the ban of 1991, some of the legally restricted products like DDT and gammalin 20 (a form of Lindane) are still being sold under different trade names and used illegally in some parts of the country. In Zaki-Biam, Benue state of Nigeria, for example, farmers who used to rely almost exclusively on the traditional systems of weed control now extensively rely on the applications of herbicides and other pesticides for weed and pests control. This affirmation was based on a descriptive cross-sectional survey which involved the utilization of in-depth interviews of farmers who are currently domiciled in Zaki-Biam environment. The present study is a preliminary work aimed at assessing the organochlorine pesticides pollution status of the agricultural soil samples from Oke-Osun farm settlement, Osogbo, Nigeria. Oke-Osun farm settlement was established in 1960 by the defunct government of Western Region of Nigeria to produce food crops and fish to compliment the efforts of peasant farmers within the region. The farming system at Oke-Osun farm settlement is a mixture of mechanized and traditional systems with agrochemicals applied from time to time to the farm lands. For over 50 years of its existence, no known scientific work has been carried out to assess the pollution status of this farm settlement. This lack of information on the pollution status of the farm settlement coupled with the adverse health effects of OCPs, made this study worthwhile.

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Materials and methods

Reagents and materials

Description of sampling location

The reagents used in this study were of spectra purity. Other materials like glass wool, anhydrous sodium sulphate and silica gel were heated in a Muffle furnace at 450◦ C for 4 h. No. 1 Whatman filter papers (0.45 μm pore size) were oven dried to constant weights at 105◦ C and cooled in a desiccator. The reagents used were dichloromethane (Rochelle Chemicals, SA); n-hexane (Ultrafine Limited, Marlborough House, London); Silica gel 60 PF254 (MERCK, Germany); sodium sulphate anhydrous (Rochelle Chemicals, SA). All glassware and sample bottles for trace organic analysis were washed with hot liquid detergent solution, rinsed with pure acetone and n-hexane mixture and then heated in an oven at a temperature of 120◦ C for 12 h prior to use (Ogunfowokan 1992; Ogunfowokan et al. 2003).

The map of Oke-Osun farm settlement indicating the sampling sites is as presented in Fig. 1. It is located about 3.5 km South of Oke-Osun Shrine in the outskirt of Osogbo metropolis. The location is a rural setting stretching over the acquired 2,500 ha. Over 90% of the total farm land consists mainly of cultivated plots allocated to farmers who plant food crops essentially while about 10% of the farm land is used for building houses by the settlers. Crops harvested from this settlement are sold to consumer intermediaries from Osogbo and its environs. The farmers here actively apply agrochemicals to improve crop yields year in year out without a follow-up assessment of how much of the agrochemicals affect the non-target components of the ecosystem.

Fig. 1 Map showing sites location within Oke-Osun farm settlement, Osogbo, Osun State, Nigeria

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Sample collection, handling and extraction Samples were collected on seasonal basis, comprising of four representative months of May to August, 2004, for the rainy season and November 2004 to February 2005 for the dry season. Five soil samples per month were collected at 0 to 15 cm depth within a regularly cultivated farm land using Auger sampler at intervals of 20 m apart adjacent to a fish pond. The collected samples were placed in aluminium foils. The samples were transported to the laboratory and preserved in the refrigerator at 4◦ C. The samples were later dried at the ambient temperature in a well-aerated cupboard to prevent cross-contamination. Final sample selection from the dried bulk was done using coning and quartering method. For the extraction of the OCPs from the dried soil samples, the method of Fatoki and Awofolu (2003) was adopted with modifications. A 20 g of the selected dried and sieved (using a 25 mm pore size sieve) sample was weighed into a pre-extracted Whatman extraction thimble. The extraction was carried out in a soxhlet extractor for 10 h using dichloromethane (DCM) as the extracting solvent. The extract was concentrated by distilling off the solvent (DCM) on a rotary evaporator at about 41◦ C to about 3 mL. The concentrated extract was cooled down to room temperature and then concentrated further to about 2 mL under a stream of high purity (99.999%) nitrogen. The reduced extract was preserved for chromatographic clean-up prior to gas chromatograph coupled with electron capture detector (GC-ECD) analysis. Clean-up experiment A column of about 15 cm × 1 cm i.d. was packed with about 5 g activated silica gel prepared in a slurry form in n-hexane. About 0.5 cm3 of anhydrous sodium sulphate was placed at the top of the column to absorb any water in the sample or the solvent. The column was pre-eluted with 15 mL of n-hexane without the exposure of the sodium sulphate layer to air. The reduced extract was placed in the column and allowed to sink below the sodium sulphate layer. Elution was done with 2 × 10 mL portions of the extracting solvent (DCM). The eluate was collected, dried

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with anhydrous sodium sulphate and then evaporated to dryness under a stream of analytical grade nitrogen (99.999%). Gas chromatographic analysis The dried eluate above was reconstituted with 1 mL n-hexane and 0.5 mL of 20 ppm hexachlorobenzene was added as an internal standard. Qualitative and quantitative analysis of the OCPs were carried out with the aid of a Perkin Elmer Gas Chromatography Autosystem (XL) coupled with electron capture detector at the Department of Chemistry, University of Botswana, Garborone, Botswana. The levels of OCPs were calculated from the relationship given by Harris (1999) as:   Ax As =F   , [X] S where Ax = area of analyte signal; As = area of internal standard signal; F = response factor; [X] = concentration of analyte and [S] = concentration of internal standard. The GC was run under the following conditions: injector temperature, 250◦ C; detector temperature, 300◦ C (held for 5 min); capillary column, Zebron ZB-1701, 30 m × 0.25 mm i.d. × 0.25 μm f.t.; oven temperature programme, 280◦ C starting from 50◦ C for 1 min and continued at 20◦ C/min to 150◦ C and at 5◦ C/minute to 280◦ C held for 4 min; injected sample volume, 1 μL, splitless mode; carrier gas, N2 at 30 mL/min; and splitless flow rate, 19.6 mL/min. Identification of the OCPs was by comparison of the retention times of the peaks with those of standard OCP compounds. Quality control work Recovery experiment Two 20-g portions each of dried soil sample were pulverized. One portion was spiked with 10 mL of 1,000 mg/L standard mixture consisting of 13 different organochlorine pesticides while the other (control) portion was left unspiked. The two portions were separately but similarly extracted and taken through the proce-

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dures outlined earlier. Also, 10 mL of the standard 1,000 mg/L mixture of the OCPs, in spectra grade n-hexane, was put into a clean oven dried sample bottle. This was dried at ambient temperature by purging with high-purity nitrogen gas and the residue redissolved in 1.0 mL n-hexane. Of each of the spiked, unspiked (control) and standard mixture, 1.0 μL was separately injected into the column of the GC-ECD and analysed one after the other. The recoveries of OCPs were determined by comparing the peak areas of the OCPs after spiking with those obtained from the evaporated standard residues. Calculation of percentage recovery (%R) was done based on: % R=



Peak area of A − peak area of A × 100 Peak area of OCP in standard

where A = OCP in spiked soil sample and A = OCP in unspiked soil sample.

ECD. The response factors (RF) were obtained from the relationship: RF =

Peak area of OCPs Peak area of internal standard.

Determination of limit of detection (LOD) The limits of detection for GC-ECD determination of OCPs were based on the empirical and more specific definition described by Miller and Miller (2000) using the relationship: yC = y B + 3sB where yC = analyte signal equivalent to detection limit; y B = blank signal; and sB = standard deviation of the blank. From the value of yC , the analyte concentration corresponding to the detection limit was evaluated.

Determination of response factor

Results and discussion

The method of Ogunfowokan et al. (2003, 2006) was used. The response factor of the standard OCPs was determined by analysing 1.0 μL of 1,000 ppm stock solution of the standard mixture containing the internal standard (I.S.) on the GC-

The reproducibility and reliability of the procedures used based on the recovery work gave values of 86.93% to 112.86% in the soil samples (Table 1). Hence, the procedures outlined for OCPs assessment in this study are adjudged

Table 1 Response factor, retention times, limits of detection and % recovery for OCPs OCP

Response factor

Retention time (min)

% Recoverya in soil

Calculated limit of detection (μg/L)

HCB A−BHC −BHC Heptachlor Aldrin B−BHC Chlordane p, p -DDE Dieldrin o, p-DDD Endrin p, p -DDD β−Endosulfan p, p -DDT

– 1.772 ± 0.011 1.161 ± 0.022 1.286 ± 0.101 1.344 ± 0.021 0.672 ± 0.005 0.603 ± 0.006 0.890 ± 0.010 0.934 ± 0.040 1.276 ± 0.059 1.076 ± 0.021 1.892 ± 0.276 0.657 ± 0.055 0.942 ± 0.019

8.58 ± 0.25 9.93 ± 0.21 11.39 ± 0.33 12.26 ± 0.42 13.28 ± 0.34 14.01 ± 0.44 17.29 ± 0.26 17.58 ± 0.13 18.21 ± 0.29 18.72 ± 0.41 19.00 ± 0.52 20.43 ± 0.36 20.72 ± 0.29 25.44 ± 0.38

89.66 ± 5.04 86.93 ± 7.19 88.15 ± 4.55 90.56 ± 9.74 92.74 ± 5.53 97.21 ± 6.70 93.35 ± 4.47 102.11 ± 2.51 91.45 ± 3.38 92.43 ± 2.99 94.58 ± 3.34 98.83 ± 6.34 87.79 ± 2.29 112.86 ± 7.56

0.673 1.087 0.097 1.090 2.107 0.100 0.483 0.183 0.895 1.110 0.307 2.003 0.067 0.056

a Values

are reported as mean of triplicate analysis ± % RSD

α-BHC

May SS1–05 25.62 ± 0.12 SS2–05 34.75 ± 0.47 SS3–05 20.67 ± 0.13 SS4–05 22.98 ± 0.24 SS5–05 21.64 ± 0.37 Monthly 25.13 ± 5.69 mean ± SD Range 20.67–34.75 June SS1–06 80.34 ± 1.25 SS2–06 13.22 ± 0.17 SS3–06 24.74 ± 0.27 SS4–06 5.93 ± 0.33 SS5–06 6.65 ± 0.17 Monthly 26.18 ± 31.20 mean ± SD Range 5.93–80.34 July SS1–07 10.14 ± 0.26 SS2–07 6.77 ± 0.27 SS3–07 17.22 ± 0.39 SS4–07 3.36 ± 0.31 SS5–07 15.96 ± 0.17 Monthly 10.69 ± 5.91 mean ± SD Range 3.36–17.22 August SS1–08 27.27 ± 0.19 SS2–08 160.82 ± 7.35 SS3–08 17.48 ± 0.15 SS4–08 16.09 ± 0.11 SS5–08 58.45 ± 2.31 Monthly 56.02 ± 61.02 mean ± SD Range 16.09–160.82 Rainy season 29.50 ± 35.91 mean ± SD Rainy season 3.36–160.82 range

Code

3.95 ± 0.40 4.27 ± 0.15 14.07 ± 0.29 ND 5.76 ± 0.29 5.61 ± 5.19

6.05 ± 0.23 3.36 ± 0.12 10.34 ± 0.11 2.21 ± 0.18 1.23 ± 0.14 4.64 ± 3.66 1.23–10.34 12.48 ± 0.37 10.45 ± 0.22 18.02 ± 0.29 ND 4.40 ± 0.16 9.07 ± 7.03 ND–18.02 27.92 ± 0.48 18.58 ± 1.12 ND 18.02 ± 0.61 25.57 ± 0.01 18.02 ± 10.95

ND ND 2.95 ± 0.81 ND ND 0.59 ± 1.32

ND–2.95

13.58 ± 0.56 12.46 ± 0.14 6.75 ± 0.28 ND 19.38 ± 0.25 10.43 ± 7.36

ND–19.38

35.03 ± 0.39 22.72 ± 0.51 ND 94.42 ± 0.32 4.77 ± 0.11 31.39 ± 37.93

64.77 ± 0.18 28.39 ± 1.45 48.73 ± 2.25 49.12 ± 0.21 31.19 ± 3.21 44.44 ± 14.89

ND–19.56

12.85 ± 0.15 ND 19.56 ± 0.15 ND ND 6.48 ± 9.19

10.46–42.58

32.57 ± 0.22 16.89 ± 0.21 29.28 ± 0.12 42.58 ± 0.35 10.46 ± 0.28 26.36 ± 12.77

12.26–31.57

16.02 ± 0.15 31.57 ± 0.18 19.03 ± 0.91 12.26 ± 0.11 27.43 ± 0.07 21.26 ± 8.03

Aldrin

15.99 ± 0.35 38.21 ± 3.02 ND 12.82 ± 0.27 10.10 ± 1.11 15.26 ± 14.12

ND–26.31

7.39 ± 0.43 10.15 ± 0.27 ND 15.27 ± 0.16 26.31 ± 0.30 11.82 ± 9.80

ND–0.67

ND ND 0.67 ± 0.08 ND ND 0.13 ± 0.30

ND–64.53

64.53 ± 0.11 33.41 ± 0.17 55.42 ± 2.03 19.41 ± 1.09 ND 34.55 ± 26.26

Chlordane

33.30 ± 0.16 75.81 ± 1.17 82.01 ± 3.26 57.55 ± 0.49 27.48 ± 0.41 55.63 ± 24.54

ND–23.23

16.55 ± 0.27 ND 23.23 ± 0.52 20.64 ± 0.14 21.30 ± 0.11 16.34 ± 9.46

26.29–70.51

55.16 ± 1.27 30.09 ± 0.10 26.29 ± 1.09 48.20 ± 0.65 70.51 ± 0.18 46.05 ± 18.24

22.15–64.55

24.25 ± 0.05 28.30 ± 1.62 ND 64.55 ± 1.45 22.15 ± 1.64 34.81 ± 19.99

p, p -DDE

84.90 ± 1.63 61.50 ± 0.52 60.47 ± 0.62 53.40 ± 0.35 39.99 ± 2.25 60.05 ± 16.33

ND–13.32

13.32 ± 0.27 8.58 ± 0.17 ND ND 10.71 ± 0.18 6.52 ± 6.19

6.93–28.06

28.06 ± 0.41 15.36 ± 0.51 23.64 ± 0.10 11.56 ± 0.36 6.93 ± 0.16 17.11 ± 8.66

28.22–65.53

65.53 ± 0.17 43.96 ± 0.32 41.87 ± 3.26 39.65 ± 0.25 28.22 ± 1.21 43.85 ± 13.56

o, p -DDD

31.85 ± 0.84 81.03 ± 4.33 72.91 ± 3.75 23.94 ± 0.52 17.27 ± 1.33 45.40 ± 29.42

10.90–27.86

10.94 ± 0.21 10.90 ± 0.15 27.86 ± 0.47 17.91 ± 0.13 12.62 ± 0.19 16.05 ± 7.20

15.42–59.41

59.41 ± 2.56 41.13 ± 0.63 15.96 ± 0.15 22.79 ± 0.21 15.42 ± 0.38 30.94 ± 19.01

18.43–67.87

67.87 ± 1.25 35.89 ± 3.09 18.43 ± 0.27 34.30 ± 0.96 41.81 ± 1.73 39.66 ± 17.99

p, p -DDD

77.11 ± 0.69 98.44 ± 6.19 77.59 ± 3.43 64.69 ± 0.19 38.38 ± 2.24 71.24 ± 22.01

ND–17.24

11.85 ± 0.21 ND 12.83 ± 0.39 ND 17.24 ± 0.36 8.38 ± 7.92

2.70–53.64

53.64 ± 6.03 25.91 ± 0.69 19.95 ± 0.31 8.37 ± 0.06 2.70 ± 0.15 22.11 ± 19.87

23.59–80.17

23.59 ± 0.31 37.07 ± 0.60 80.17 ± 3.39 40.30 ± 0.88 27.29 ± 0.41 41.68 ± 22.58

Dieldrin

25.80 ± 1.15 96.43 ± 2.28 29.93 ± 0.27 25.02 ± 0.22 16.91 ± 1.35 38.82 ± 32.55

ND–18.95

12.11 ± 0.32 ND 5.93 ± 0.66 1.88 ± 0.23 18.95 ± 0.31 7.77 ± 7.78

11.55–81.12

81.12 ± 2.34 11.55 ± 0.82 24.43 ± 0.29 18.01 ± 0.34 20.64 ± 0.31 31.15 ± 28.33

ND–84.78

43.03 ± 0.12 32.24 ± 1.30 ND 84.78 ± 2.12 ND 32.01 ± 35.20

Endrin

78.40 ± 3.49 68.47 ± 1.43 40.07 ± 0.36 30.72 ± 0.23 15.26 ± 1.22 46.58 ± 26.30

ND–37.41

21.13 ± 0.19 ND 37.41 ± 0.13 ND 17.94 ± 0.14 15.30 ± 15.80

4.29 –17.17

4.29 ±0.55 6.22 ± 0.19 10.71 ± 0.28 17.17 ± 0.33 16.16 ± 0.13 10.91 ± 5.76

15.50–51.71

16.05 ± 0.22 23.02 ± 1.42 15.50 ± 0.29 19.45 ± 0.28 51.71 ± 1.81 25.15 ± 15.15

Endosulfan

62.85 ± 3.08 81.63 ± 4.68 88.36 ± 1.76 44.52 ± 0.59 60.22 ± 4.53 67.52 ± 17.59

13.66–60.76

14.99 ± 0.23 13.66 ± 0.23 27.69 ± 0.71 60.76 ± 0.36 25.00 ± 0.37 28.42 ± 19.08

25.43–52.26

29.17 ± 0.31 26.96 ± 0.44 33.89 ± 0.53 25.43 ± 0.10 52.26 ± 1.03 33.54 ± 10.94

18.18–54.99

18.18 ± 0.53 35.43 ± 0.51 29.89 ± 2.77 54.99 ± 1.86 54.38 ± 0.94 38.57 ± 29.35

p, p -DDT

590.95 ± 13.15 883.06 ± 36.31 517.55 ± 15.85 503.17 ±4.42 382.91 ± 20.60 575.53 ± 187.42

72.97–208.31

166.01 ± 3.64 72.97 ± 1.45 208.31 ± 4.62 119.82 ± 1.33 189.81 ± 2.54 151.38 ± 54.94

194.96–433.76

433.76 ± 15.57 194.96 ± 4.03 236.92 ± 4.43 202.25 ± 2.91 208.72 ± 3.22 255.32 ± 101.01

342.73–411.17

407.52 ± 4.29 411.17 ± 10.71 351.94 ± 21.99 461.69 ± 10.96 342.73 ± 8.89 395.01 ± 46.06

Total burden per site

ND–94.42

ND–60.24

ND–51.03

ND–64.77

ND–64.53

ND–82.01

ND–84.90

10.90–81.03

ND–98.44

ND–96.43

ND–78.40

18.18–88.36

72.97–575.53

ND–94.42 ND–27.92 ND–51.03 28.39–64.77 ND–38.21 27.48–82.01 39.99–84.90 17.27–81.03 38.38–98.44 16.91–96.43 15.26–78.40 44.52–88.36 382.91–883.06 13.09 ± 21.66 16.41 ± 15.68 14.14 ± 14.31 24.64 ± 17.49 15.44 ± 19.20 38.39 ± 23.04 31.88 ± 24.33 33.01 ± 21.59 35.86 ± 29.87 27.44 ± 28.47 24.48 ± 21.28 42.01 ± 17.50 344.31 ± 183.64

25.76 ± 0.52 51.03 ± 2.26 ND 12.86 ± 0.31 37.32 ± 0.52 25.39 ± 20.01

ND–11.81

8.68 ± 0.17 ND 11.81 ± 0.63 ND ND 4.10 ± 5.72

ND–14.07

10.72–31.01

16.51–60.24

ND–21.90

15.94 ± 0.52 31.01 ± 0.27 10.72 ± 1.07 28.30 ± 0.32 21.31 ± 0.13 21.46 ± 8.42

Heptachlor

16.51 ± 0.52 27.12 ± 0.49 60.24 ± 7.87 18.82 ± 1.26 46.79 ± 0.58 33.90 ± 18.95

γ-BHC

10.40 ± 0.22 17.40 ± 0.27 ND 21.90 ± 0.14 ND 9.94 ± 9.96

β-BHC

Table 2 Levels of OCPs (μg/kg) in soil samples of Oke-Osun farm settlement during rainy season

56 Environ Monit Assess (2011) 177:51–61

November SS1–11 SS2–11 SS3–11 SS4–11 SS5–11 Monthly mean ± SD Range December SS1–12 SS2–12 SS3–12 SS4–12 SS5–12 Monthly mean ± SD Range January SS1–01 SS2–01 SS3–01 SS4–01 SS5–01 Monthly mean ± SD Range February SS1–02 SS2–02 SS3–02 SS4–02 SS5–02 Monthly mean ± SD Range Dry season mean ± SD Dry season range

Code

83.38 ± 2.10 154.61 ± 1.02 19.94 ± 0.23 ND 51.60 ± 3.21 61.91 ± 60.73

36.61 ± 1.01 228.73 ± 1.66 39.16 ± 1.22 119.88 ± 3.29 43.56 ± 1.63 93.59 ± 83.17

ND–50.45 ND 57.95 ± 3.12 40.61 ± 0.55 ND ND 19.71 ± 27.68

ND–101.97

101.97 ± 2.62 ND 100.29 ± 0.12 40.70 ± 0.35 ND 48.59 ± 50.76

ND–77.25

76.36 ± 1.13 35.58 ± 0.18 77.25 ± 1.48 ND ND 37.84 ± 38.42

ND–91.86

30.66 ± 1.21 23.08 ± 0.38 ND ND 91.86 ± 0.19 29.12 ± 37.66

Chlordane

83.72 ± 1.29 122.34 ± 4.51 112.07 ± 3.34 62.32 ± 4.23 75.43 ± 2.11 91.18 ± 25.22

31.79–104.57

70.77 ± 0.64 104.57 ± 0.25 42.33 ± 0.35 48.74 ± 2.31 31.79 ± 0.45 59.64 ± 28.88

ND–192.46

59.70 ± 0.18 192.46 ± 1.94 73.99 ± 0.57 66.91 ± 0.15 ND 78.61 ± 70.10

ND–93.02

19.01 ± 1.31 61.31 ± 0.67 ND 93.02 ± 0.94 84.30 ± 1.37 51.53 ± 40.63

p, p -DDE

56.16 ± 0.14 163.45 ± 2.87 147.28 ± 3.79 107.73 ± 5.21 164.50 ± 5.17 127.82 ± 46.19

29.72–67.09

61.23 ± 0.15 67.09 ± 0.29 45.93 ± 2.33 33.96 ± 0.37 29.72 ± 0.19 47.59 ± 16.39

42.31–95.22

80.40 ± 0.67 74.24 ± 0.35 95.22 ± 0.23 42.31 ± 0.61 54.99 ± 0.23 69.43 ± 20.93

19.96–85.29

19.96 ± 0.63 44.11 ± 0.69 42.06 ± 1.15 85.29 ± 0.53 57.46 ± 0.59 49.78 ± 23.98

o, p -DDD

ND–135.84

ND–83.99

ND–154.61

ND–228.73

ND–101.97

ND–192.46

Dieldrin

30.62–75.86

61.44 ± 0.22 75.86 ± 0.39 49.19 ± 1.38 43.11 ± 0.15 30.62 ± 0.29 52.04 ± 17.34

34.39–76.47

53.38 ± 0.42 58.75 ± 0.99 65.74 ± 0.71 34.39 ± 0.38 76.47 ± 0.54 57.75 ± 15.65

23.57–76.29

14.96–95.54

60.65 ± 0.41 95.54 ± 1.72 14.96 ± 0.58 26.63 ± 0.69 49.54 ± 1.22 49.46 ± 31.46

41.92–87.08

45.81 ± 0.30 87.08 ± 0.82 41.92 ± 0.98 45.49 ± 0.27 69.98 ± 0.37 58.06 ± 19.70

14.79–62.39

14.79 ± 2.01 29.13 ± 0.12 48.35 ± 0.34 62.39 ± 0.17 59.75 ± 0.47 42.88 ± 20.45

Endrin

23.57–176.17

34.35–176.17 61.65 ± 34.74

11.09 ± 0.26 58.72 ± 0.42 ND 84.18 ± 0.17 84.26 ± 0.39 47.65 ± 40.01

p, p -DDT

125.59 ± 1.36 79.34 ± 0.27 74.79 ± 1.05 46.45 ± 0.36 61.32 ± 0.56 77.50 ± 25.57

55.80–108.06

59.48 ± 0.25 102.15 ± 0.29 108.06 ± 0.37 55.80 ± 0.21 77.30 ± 0.26 80.55 ± 23.93

12.97 ± 6.14 62.73 ± 4.55 41.56 ± 1.14 ND 44.78 ± 1.46 32.41 ± 25.42

14.79–120.78 ND–100.97

466.03–890.71

890.71 ± 7.56 568.61 ± 3.51 579.41 ± 13.23 525.01 ± 7.73 466.03 ± 11.38 605.95 ± 165.31

446.44–1,046.35

742.08 ± 5.91 1,046.35 ± 10.10 781.69 ± 7.90 501.28 ± 3.55 446.44 ± 2.52 703.57 ± 29.98

225.70–1,019.30

225.70 ± 6.92 481.64 ± 4.63 266.65 ± 3.27 849.10 ± 5.87 1,019.30 ± 7.43 568.48 ± 345.79

Total burden per site

11.09–254.22

225.70–1,281.67

658.79–1,281.67 693.77 ± 147.02

100.80 ± 0.52 658.79 ± 18.71 92.37 ± 1.27 1,281.67 ± 29.18 52.93 ± 3.89 722.78 ± 21.02 254.22 ± 4.16 1,083.48 ± 28.52 128.75 ± 3.29 738.73 ±22.87 125.81 ± 76.73 897.09 ± 271.52

44.82–100.07 46.45–125.59

100.07 ± 0.80 74.56 ± 0.20 91.62 ± 3.41 65.58 ± 0.08 44.82 ± 0.61 75.33 ± 21.82

19.88–67.53

67.53 ± 0.39 37.01 ± 0.11 19.88 ± 0.36 63.75 ± 0.17 25.91 ± 0.12 42.82 ± 21.76

13.12–100.97 11.09–84.26

13.12 ± 0.12 49.49 ± 0.25 88.69 ± 0.77 100.97 ± 0.25 96.82 ± 0.88 69.82 ± 37.69

Endosulfan

30.83–120.78 ND–62.73 52.93–254.22 57.20 ± 28.37 55.09 ± 31.22 82.88 ± 32.24

96.87 ± 2.41 123.22 ± 2.37 111.45 ± 2.37 91.66 ± 3.10 76.20 ± 1.15 30.83 ± 2.37 34.04 ± 1.83 34.35 ± 0.68 76.21 ± 2.10 137.27 ± 2.95 176.17 ± 3.52 120.78 ± 2.24 67.37 ± 2.55 66.20 ± 1.41 42.28 ± 0.23 90.48 ± 41.95 88.09 ± 57.95 78.41 ± 33.64

28.05–64.73

35.27 ± 0.24 37.19 ± 0.28 64.73 ± 3.74 36.90 ± 1.34 28.05 ± 1.75 40.43 ± 14.08

25.41–64.00

52.10 ± 0.58 64.00 ± 0.24 36.72 ± 0.42 25.41 ± 0.43 35.61 ± 0.19 42.77 ± 15.22

ND–105.75

18.92 ± 0.31 28.56 ± 0.21 23.05 ± 0.21 23.57 ± 0.44 ND 41.83 ± 0.16 75.88 ± 0.46 73.31 ± 0.72 105.75 ± 0.43 76.29 ± 1.01 44.72 ± 44.25 48.71 ± 24.75

p, p -DDD

19.96–164.50 ND–137.27

41.02 ± 0.25 ND 18.97 ± 1.01 ND 33.40 ± 1.46 18.68 ± 18.80

ND–41.96

50.45 ± 0.47 ND ND 33.13 ± 1.01 42.24 ± 2.48 25.16 ± 23.77

36.80–99.04

74.38 ± 0.22 99.04 ± 0.75 45.59 ± 0.51 36.80 ± 0.26 70.34 ± 0.32 65.23 ± 24.73

ND–102.21

13.43 ± 0.11 47.41 ± 0.33 ND 56.81 ± 0.15 102.21 ± 0.22 43.97 ± 40.11

Aldrin

ND–79.15

ND 135.84 ± 3.31 81.08 ± 1.05 73.25 ± 1.88 ND 58.03 ± 58.21

12.59 ± 0.11 64.96 ± 0.25 24.58 ± 0.19 31.86 ± 1.04 20.86 0.35 30.97 ± 20.23

ND–58.85

ND ND ND 23.05 ± 0.46 41.96 ± 2.34 13.00 ± 19.02

ND–49.29

37.73 ± 0.43 43.71 ± 2.26 49.29 ± 0.28 15.46 ± 0.11 ND 29.24 ± 20.79

6.14–59.06

6.14 ± 0.13 11.19 ± 0.19 24.43 ± 0.53 36.09 ± 0.67 59.06 ± 0.31 27.38 ± 21.22

Heptachlor

56.16–164.50 34.04–137.27 73.65 ± 42.72 54.60 ± 36.43

ND–95.57

ND–79.15

58.85 ± 0.15 ND ND 33.66 ± 0.36 54.62 ± 1.15 29.43 ± 28.51

ND–80.36

44.71 ± 0.28 80.36 ± 0.59 53.23 ± 0.65 26.72 ± 0.24 ND 39.00 ± 26.88

ND–83.99

11.62 ± 0.51 28.42 ± 0.14 ND 57.34 ± 1.34 83.99 ± 0.72 36.27 ± 34.30

γ-BHC

12.59–64.96 ND–135.84 ND–41.02 ND–154.61 36.61–228.73 ND–57.95 62.32–122.34 30.74 ± 17.38 58.13 ± 44.78 30.85 ± 26.65 32.88 ± 37.10 56.99 ± 52.19 33.82 ± 37.85 70.24 ± 44.13

ND–135.61

85.27 ± 0.27 ND 95.57 ± 0.27 70.19 ± 0.14 17.45 ± 0.17 53.70 ± 42.48

63.09 ± 0.75 135.61 ± 1.44 82.82 ± 1.12 68.00 ± 0.33 ND 69.90 ± 48.53

27.24 ± 0.31 46.36 ± 0.14 31.98 ± 0.22 20.24 ± 0.39 35.84 ± 0.49 32.33 ± 9.76

79.15 ± 0.23 34.46 ± 0.11 ND 22.91 ± 0.11 32.9 ± 0.17 33.88 ± 28.80

ND–91.97

16.89–34.70

20.24–46.36

14.30 ± 0.00 65.27 ± 0.52 ND 91.97 ± 0.27 82.85 ± 0.46 50.88 ± 41.37

β-BHC

24.10 ± 0.11 16.89 ± 0.27 21.29 ± 0.32 31.85 ± 0.20 34.70 ± 0.39 25.77 ± 7.39

α-BHC

Table 3 Levels of OCPs (μg/kg) in soil samples of Oke-Osun farm settlement during dry season

Environ Monit Assess (2011) 177:51–61 57

58

ary to 75.33 (21.82) in January and p, p’-DDT 47.65 (40.01) in November to 125.81 (76.73) in February. Overall dry season mean of the OCPs ranged from 30.74 (17.38) for α-BHC to 82.88 (42.93) for p, p -DDT. These values were generally higher than the rainy season range values of 13.09 (21.66) for β-BHC to 42.01 (51.56) for p, p -DDT (Tables 2 and 3). These lower levels reported for the rainy season may be due to dilution resulting from precipitation and leaching as a result of surface wash-off. Out of the 20 soil samples collected and analysed during the dry season, the percentage OCPs in the samples were in the following order: 55% for chlordane, 70% for γ -BHC, 75% for β-BHC and heptachlor, 85% for aldrin, 90% for p, p -DDE, 95% for α-BHC, p, p -DDD and p, p -DDT while o, p -DDD, dieldrin and endrin were found in all the soil samples. The OCPs studied in the soil samples fell into three categories: dichlorodiphenylethanes (p, p -DDE; o, p -DDD; p, p -DDD and p, p DDT), cyclodienes (heptachlor, aldrin, chlordane, dieldrin, endrin and endosulfan) and chlorinated benzenes/cyclohexanes (α-BHC, β-BHC and γ BHC). Figure 2 shows that for both seasons, the levels of the three classes of OCPs studied in the soil followed the same decreasing trend viz: dichlorodiphenylethanes > cyclodienes > chlorinated benzenes/cyclohexanes. However, their dry season levels were higher than their rainy season levels. From Fig. 2, it is obvious that during the rainy season, the dichlorodiphenylethanes had the highest total mean of 36.32 ± 4.74 μg/kg

Levels of OCPs (ug/kg)

reliable and efficient. The range of response factors of 0.603 to 1.892 showed the separation efficiency of the programmed method for the GCECD identification and quantification of OCPs while the LOD values for the OCPs ranged from 0.056 to 2.107 μg/L. Rainy season levels of OCPs in the soil samples are represented in Table 2 (May to August). During the rainy season, the mean range of the pesticides (μg/kg) were: α-BHC 10.69 (5.91) in July to 56.02 (61.02) in August; β-BHC 0.59 (1.32) in June to 31.39 (37.93) in August; γ -BHC 4.64 (3.66) in June to 33.90 (18.95) in May; heptachlor 4.10 (5.72) in July to 25.39 (20.01) in August; aldrin 6.48 (9.19) in July to 44.44 (14.89) in August and chlordane 0.13 (0.30) in June to 34.55 (26.26) in May. Others include p, p -DDE 16.34 (9.46) in July to 55.63 (24.94) in August; o, p -DDD 6.52 (6.19) in July to 60.05 (16.33) in August; p, p DDD 16.05 (7.20) in July to 45.40 (29.42) in August; dieldrin 8.38 (7.92) in July to 71.24 (22.01) in August; endrin 7.77 (7.78) in July to 38.82 (32.55) in August; endosulfan 10.91 (5.76) in June to 46.58 (26.30) in August and p, p -DDT 28.42 (19.08) in July to 67.52 (17.59) in August. Generally, β-BHC occurred in 60%; chlordane in 70%; heptachlor in 75%; aldrin and endrin in 85%; γ -BHC, p, p DDE, o, p -DDD, dieldrin and endosulfan in 90%; and α-BHC, p, p -DDD and p, p -DDT in 100% of the 20 soil samples analysed for rainy season. Levels of OCPs in the soil during the dry season are represented in Table 3 (November to February). The mean ranges of the pesticides in micrograms per kilogram were: α-BHC 25.77 (7.39) in November to 33.88 (28.80) in January; βBHC 50.88 (41.37) in November to 69.90 (48.53) in December; γ -BHC 18.68 (18.80) in February to 39.00 (26.88) in December; heptachlor 13.00 (19.02) in January to 61.91 (60.73) in February; aldrin 25.16 (23.77) in January to 93.59 (83.17) in February; chlordane 19.71 (27.68) in February to 48.59 (50.76) in January; p, p -DDE 51.53 (40.63) in November to 91.18 (25.22) in February; o, p DDD 47.59 (16.39) in January to 127.82 (46.19) in February; p, p -DDD 40.43 (14.08) in January to 90.48 (41.95) in February; dieldrin 48.71 (24.75) in November to 88.09 (57.95) in February; endrin 42.88 (20.45) in November to 78.41 (33.64) in February; endosulfan 32.41 (25.42) in Febru-

Environ Monit Assess (2011) 177:51–61

90 80 70 60 50 40 30 20 10 0 A

B

Rainy season

C

A

B

C

Dry season

Fig. 2 Classes of OCPs in the soil. A Diclorodiphenylethanes, B Cyclodienes, C Chlorinated benzenes

1.00 1.00 0.14 1.00 0.14 0.61a 1.00 0.62a 0.28 0.72a 1.00 0.59a 0.61a 0.17 0.71a 1.00 0.53a 0.54a 0.50a 0.33a 0.58a 1.00 0.58a 0.50a 0.34a 0.54a 0.30 0.49a 1.00 0.20 0.26 0.19 0.18 0.01 0.40a 0.12 1.00 0.22 0.53a 0.64a 0.57a 0.53a 0.27 0.22 0.53a a Values

are significant at 0.05 level (N = 40)

1.00 0.68a 0.22 0.46a 0.57a 0.47a 0.14 0.21 0.24 0.23 1.00 0.32a 0.20 0.41a 0.34a 0.18 0.20 0.11 0.19 0.36a 0.19 1.00 0.31 0.24 0.29 0.49a 0.21 0.32a 0.31 0.17 0.33a 0.34a 0.15 0.26 α−BHC β−BHC γ −BHC Heptachlor Aldrin Chlordane p,p -DDE o,p -DDD p,p -DDD Dieldrin Endrin Endosulfan p,p -DDT

1.00 0.38a 0.41a 0.63a 0.55a 0.63a 0.51a 0.33a 0.39a 0.17 0.46a 0.41a

Endo Endrin sulfan Dieldrin p, p DDD o, p DDD p, p DDE Chlordane Aldrin Heptachlor γ -BHC β-BHC α-BHC OCPs

Table 4 Correlation coefficients of OCPs in soil samples from Oke-Osun farm settlement

followed by the cyclodienes (23.57 ± 8.04 μg/kg) and the chlorinated benzenes/cyclohexanes had a total mean value of 19.67 ± 8.68 μg/kg. In the dry season, however, the dichlorodiphenylethanes occurred at a mean level of 70.34 ± 11.78 μg/kg followed by the cyclodienes with a total mean level of 49.61 ± 12.90 μg/kg and the least total mean value of 39.91 ± 15.78 μg/kg was obtained for the chlorinated benzenes/cyclohexanes. Obviously, from the results of this study, the farmers at Oke-Osun farm settlement relied more on the dichlorodiphenylethanes for pests control than on any of the other classes of the OCPs studied. Results from this study showed that those pesticides previously placed under legal restrictions by Nigerian FEPA were still being used by the farmers since concentration levels of OCPs obtained for DDT were higher than those of its metabolites like DDD and DDE. The Pearson correlation coefficients of the OCPs in the soil samples are shown in Table 4. All the 13 compounds analysed were positively correlated with 15.58% of them being significant at 0.05 levels while 41.56% of them were significant at 0.01 levels. Clearly, from the results of the correlation coefficient, the OCPs in these soil samples are likely to be from the same sources and they interact with the soil particles similarly. The relationship and association of the OCPs among the sites from which the soil samples were collected was further statistically analysed by cluster analysis using average linkage between groups and synthesized by the dendrogram plots shown in Fig. 3. In this study, Ward’s method was adopted because it has small space distortion effect and it is an extremely powerful grouping mechanism (Willet 1987; Halena et al. 1999). On the basis of their mean OCPs contents, the compound relationship and association at the sampling sites based on the dendrogram, can be grouped into three: (α-BHC, γ -BHC, heptachlor)–(chlordane, endosulfan, β-, aldrin) constituting group 1; (p,p DDE, o,p -DDD) constituting group 2 while group 3 is made up of (p,p -DDD, endrin, dieldrin, p,p -DDT). The status of pollution as well as the types and pattern of contaminations at the sites are also shown in the dendogram. Each group may be reflecting the extent of anthropogenic sources due to agricultural activities or structural relation-

59 p, p DDT

Environ Monit Assess (2011) 177:51–61

60

Environ Monit Assess (2011) 177:51–61

Fig. 3 Soil OCPs dendrogram (hierarchical cluster analysis) using average linkage (between groups)

ship of the individual OCPs (in some cases), and also their mode of metabolites formation amongst other sources.

Analytical Chemists (SEANAC) in support of his Ph.D. research which was utilized at the Department of Chemistry, University of Botswana, Gaborone, Botswana.

Conclusion

References

In conclusion, this study revealed that agricultural soil samples of Oke-Osun farm settlement, Osogbo, were contaminated with persistent organochlorine pesticides at levels higher in the dry season than the rainy season. There was an indication from our study that those pesticides previously placed under legal restrictions by the Regulatory Agency were still being used by the farmers of Oke-Osun farm settlement. Whereas the use of chemical pesticides in boosting food production remains indispensable, there is however, the need for the Federal Ministry of Environment to vigorously embark on the campaign for safe use of pesticides in the Nigerian environment. This is necessary to keep the farmers informed regarding the dangers associated with improper applications of pesticides and the need to start adopting those that are more environmentally friendly.

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Acknowledgements The authors wish to gratefully acknowledge the fellowship granted to Oyekunle, John Oyedele by the Southern and Eastern Africa Network of

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