METHOD OPTIMIZTION AND DETERMINATION OF SELECTED ORGANOPHOSPHATE AND CARBAMATE PESTICIDE RESIDUES IN THE RICE (Oryiza Sativa) GROWING REGION OF MWEA, KIRINYAGA COUNTY, KENYA

Abstract

Carbamate and organophsphate pesticides have been widely used in rice production in the Mwea region, Kirinyaga County, Kenya for the control and elimination of pests. This region is found in central Kenya and is the largest rice irrigation scheme in Kenya accounting for over 70% of the rice produced in Kenya. Concern over the presence of pesticide residues within the farmlands have arisen. Exposure to pesticide residues is known to course myriad of health effects to both human and animal life such as endocrine disruption or even death. The main objective of the study was to investigate for the residues of the highly toxic pesticide carbofuran, its major metabolite 3-hydroxycarbofuran and two organophosphates- diazinon and fenitrothion within the agricultural region of Mwea, Kenya. Soil, water from within the rice paddies and from two major channels supplying the paddies with water as well as rice plant samples were collected for analysis during the month of June, 2013 (n=34 in total). Extraction of pesticides from water was carried out using the solid phase extraction method. Soil and rice samples were air-dried thereafter pesticides extracted using organic solvents employing the sohxlet method of extraction. The extracts were analysed using gas chromatograph –mass spectrometer. Prior to sample analysis, method validation and optimization studies were carried out. A test on the efficiency of two columns, a fast GC column- DB-Xlb and a conventional column–Rtx-5MS resulted in the fast GC column giving better resolution and lower limit of detections (LOD) values. Carbofuran and its metabolites were below the detection limit (0.38µg/L) in all of the samples from the agricultural farmlands study site. However, the two organophosphate pesticides under study were detected in water sampled from the irrigation paddies and in soil samples. Fenitrothion was detected in soil from two sections of the study site (Karaba A and B) at levels of 0.06±0.012 and 0.11±0.054 mg/kg respectively. Diazinon was detected in both a single soil sample and single paddy water sample (from Wamumu A) at 0.24±0.033 mg/kg and 0.19±0.065 mg/L respectively. All the pesticide levels detected were above maximum acceptable total and individual levels of contamination, as set by the European Union (0.5 and 0.1 μg/L, respectively). The soil pH varied from 5.29-5.63 and was considered to be strongly acidic while water pH varied from 6.84-5.46. From the study, a simple and clear method for the analysis of carbamate and organophosphate pesticides was developed. pH was observed to be a vital

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physico-chemical parameter in determining the availability of carbamate pesticides in the environment and that carbamate pesticides residues tend to more available in acidic environment.