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
xiv
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.
