Residues of Organochlorinated Pesticides in Soil from Tomato Fields, Ngarenanyuki, Tanzania

This work presents the concentrations of five pesticide residues, lindane, chlorpyrifos, endosulfan, p, p'-DDE and p, p'-DDD in soil samples collected from tomato fields in Ngarenanyuki, Tanzania. Endosulfan sulphate was detected in 100 % of the sample analysed with mean concentration of 0.2407 mg/kg dw. Chlorpyrifos was detected in 87 % of the samples with mean concentration of 0.1253 mg/kg dw. p, p'-DDE and p, p'-DDD were detected in 46 and 40 % of the samples analysed with mean concentrations of 0.1482 and 0.154 mg/kg dw, respectively. Lindane was the least detected pesticide. It was detected in 5 (33 %) of soil samples analysed with mean concentration of 0.2126 mg/kg dw. Low concentrations detected indicate the past usage of the pesticides. @ JASEM The increased use of pesticides in agricultural activities has caused pollution of environmental compartments; soil, water and air. Their chemical properties such as high lipophilicity, bioaccumulation potential, long half-life in the environment, and potential to long-range transport, envisaged the chance to be found in water, soil or food even decades after their applications (Abbassy et al., 1999). Agricultural pesticides most often are applied as liquids, granules or seed sprayed/treated on the crop and/or the soil (Pimentel and Levitan, 1986). Extremely small percentage (less than 0.3%) of the amount applied, goes into direct contact with or consumed by target pests, therefore 99.7% goes somewhere else in the environment (Pimentel, 1995). In Tanzania, most of the pesticides imported are used in agriculture activities (AGENDA, 2006). The aim of this work was to ascertain the levels of pesticide residues in soils from tomato fields at Ngarenanyuki. MATERIALS AND METHODS Study Area and Sample Collection: Ngarenanyuki is among the 17 wards in Arumeru District, Arusha City, Tanzania. It comprises of five villages (Uwiro, Ngabobo, Olkung’wado, Kisimiri chini and Kisimiri juu) located at the foot of Mt. Meru, north east of Arusha City (3° 9' 0'' South, 36° 51' 0'' East). 15 soil samples of 500 g each were randomly collected from 15 different locations at Uwiro village in January and February, 2009 (Figure 1). The soil samples were collected by using stainless spoon at the depth of 0 15 cm (the root zone). The sampling procedures were conducted as described by Åkerblom (1995). Chemicals: Chromatography grade dichloromethane, n-hexane, acetone, cyclohexane and ethyl acetate were used for sample preparation. Samples were quantified using pesticide standard mixture which had over 99 % certified purity. Laboratory glassware were washed with detergents, rinsed with distilled water and acetone, and then dried in an oven at 110 °C overnight prior to use. Extraction: Two sub-samples of 20 g each were subjected under different experimental treatments. For dry weight determination one sub-sample in a pre-weighed watch glass was heated in the oven at 105 C to constant weight. Another sub-sample was put in a teflon stoppered flask (250 ml) for pesticides extraction. 14 ml of 0.2 M ammonium chloride solution was added, the mixture swirled for 2 min and left to stand for 15 min. 100 ml of cyclohexane/acetone mixture (1:1 v/v) was then added, the flask was tightly stoppered and vigorously shaken for 1 min, and then less vigorously after every 10 min for 1 h. The contents were then left to stand overnight. Intermittent shaking was continued the next day for two hours and then left to settle. Distilled water was cautiously added to the neck of the flask. The organic phase was pipetted into an E-flask containing anhydrous sodium sulphate (15 g), swirled and left to stand for 15 min. The contents were then decanted through a plug of glass wool into an evaporation flask. The remaining sodium sulphate was rinsed with 20 ml of acetone/cylohexane mixture and decanted through the same glass wool. The resulting extract was concentrated to less than 2 ml using a rotary evaporator, the solvent was then changed to cyclohexane/ethylacetate (1:1 v/v, 1 ml) ready for clean up. Clean up: This was done by glass column (60 cm x 22 mm) chromatography packed with florisil (magnesium silicate), glasswool and anhydrous sodium sulphate. Cyclohexane: acetone (9:1 V/V) was added into the glass column and allowed to pass through drop by drop until very little was left on the upper part of the column. The sample concentrate was poured into the column and drained to make 3 ml of the sample. This was then transferred into the Teflon cork vial and stored at 4 °C until analysis.