Assessment Of The Fate Of Selected Pesticides On Vegetables In Naivasha Area

Abstract

This study assessed the fate of pesticides used on vegetables in Naivasha area. The harmful effects of pesticides make them pose a serious threat to some of the non-target organisms including human and wild life. The concentrations of two organophosphate pesticides (diazinon and chlorpyrifos) and organochlorine pesticide residues (heptachlor, heptachlor epoxide, aldrin, dieldrin, α-HCH, γ-HCH, β-HCH, δ-HCH, endosulphan I, endosulphan II, p,p’- DDE, dieldrin, endrin, endrin aldehyde, methoxychlor, p,p’-DDD, p,p’-DDT and endosulphan sulphate) were determined in kales, soil and water samples from Naivasha area. Standard procedures were used in sample collection and preparation. Determination of pesticide concentrations in the water, soil and kale samples was done using a GC-MS (GC- 6890, MSD 5972-2) and a gas chromatograph (Agilent 6890N) combined with an auto sampler (Agilent 7683 Series injector), and an electron capture detector (μ-ECD). The organophosphate pesticides were not recorded in any of the samples. Varying concentrations of organochlorine pesticides were detected in the samples. In kales, methoxychlor was the highest detected pesticide with concentration of 75.41±7.71 μg/kg. aldrin recorded the highest concentration (218.47±6.76 μg/kg) in the soil samples while in the water samples, methoxychlor was the highest detected pesticide with a concentration of 0.68±0.01μg/l. The results suggest contamination of vegetables with pesticide residues that need to be monitored to reduce the risk of exposure to the unsuspecting consumers. Results of dissipation study of chlorpyrifos revealed concentration in kale leaves at 75.82±3.56 mg/kg on day 0 while on day 7 the residues were 2.82±0.03 mg/kg. In stems, roots and soil samples, initial chlorpyrifos concentrations were 61.36±7.52 mg/kg, <LOD and 42.03±0.00 mg/kg, while the final levels were 1.13±0.06 mg/kg, 1.56±0.00 mg/kg and 1.05±0.04 mg/kg, vi respectively. The half-lives in the leaves, stems, roots and soil were 0.63, 0.67, 1.1 and 0.5 days, respectively. Diazinon concentration in leaves on day zero was 49.02±6.26 mg/kg while final concentration was 3.12±0.14 mg/kg for day 11. Beyond the 11th day the concentration was below detection limit. Diazinon concentrations in stems, roots and soil on day zero were 37.88±3.32, <LOD and 38.25±0.00 mg/kg respectively while the final detectable concentrations were 5.16±0.17, 1.00±0.07 and 1.67±0.02 mg/kg for stems, roots and soil, respectively. The half-lives were 0.42, 0.62 and 0.43 days for stems, roots and soil, respectively. In both dissipation studies (diazinon and dursban), higher concentrations were observed on the leaves in day zero, followed by stems, soil and roots. From the dissipation studies, chlorpyrifos had longer persistence on the crops and soil compared to diazinon applied under the same environmental conditions. Based on the organophosphate (diazinon and Chlorpyrifos) residue levels detected farmers and consumers should be educated on post-harvest interval to be observed before harvesting of vegetables.