Characterization of selected pesticide residues in sediments of the Ugandan side of Lake Victoria

Abstract

This study investigated the role of biogeochemical processes in influencing bio-availability and biodegradation of selected pesticides in Lake Victoria. Due to the hydrophobicity nature of most pesticides, sediments which serve as sink to the less water soluble chemicals were selected as a matrix of study. To achieve the set objective, an inventory of the pesticides use in the Ugandan side of Lake Victoria catchments was made, seasonal variability of pesticide concentrations in sediments were determined, followed by sorption and pesticide degradation studies within the lake sediments. Information on the pesticides used was gathered using questionnaires and professional interviews. Surface sediment samples were collected using a corer, extracted for pesticide residues using solid dispersion methods and extracts cleaned up using gel permeation chromatograph – GPC. Analysis for pesticide residues in extracts was done using a temperature programmed Gas Chromatograph equipped with a Nitrogen Phosphorous Detector and an Electron Capture Detector. External and inter-laboratory confirmatory tests were done using a dual column and dual detector approach coupled with a sulphuric acid treatment method of selected extracts. Validation of selected samples was done using a Gas Chromatograph equipped with a Mass Selective Detector. Sorption experiments were conducted in mixed solvents (methanol-water) systems and sorption coefficients in aqueous systems were obtained using the theory of the Solvophobic model. Degradation experiments to determine the kinetics of pesticide transformation by different microbial communities were defined in terms of the extractable concentrations based on method extraction efficiency. The presence of microbial communities within the lake sediment was established using a DNA screening technique. Results from the survey indicate the major challenges associated with pesticide use in Uganda as being use of un-registered pesticides, inappropriate packaging, storage, disposal and human exposure due to poor handling of the pesticides and their containers. A total of eighteen pesticide residue/metabolites have been identified and quantified within Lake Victoria sediments with chlorpyrifos recording the highest mean value of 4.18 μg/kg (dry weight) and endosulfan sulfate being the most frequently detected (42%) pesticide residue. The study has also detected residues of pesticides which are not registered for use in Uganda. Sorption studies have indicated that sorption on containers can cause grave errors in the estimation of sorption coefficients, especially for those chemicals with very low water solubility. Chlorpyrifos has been found to have a higher affinity to the lake sediment than both the β- endosulfan and α- endosulfan in that order. Degradation studies indicate that α-endosulfan, β-endosulfan and chlorpyrifos degrade faster in unsterilized than in sterilized sediment samples of Lake Victoria, suggesting that microorganisms contribute to the degradation processes of the three pesticides within the lake sediment. A DNA screening for the presence of bacteria that degrade pesticides within Lake Victoria has indicated presence of microbes in the lake environment that are capable of degrading phenyl derived pesticides, toluene derived pesticides and xylene derived pesticides. The fate of pesticide residues within Lake Victoria has been described using the "Quantitative Water Air Sediment Interaction" (QWASI) model. With this model, it has been predicted that the residence times (years) of chlorpyrifos, α –endosulfan, β- endosulfan, endosulfan sulfate, pp- DDT, HCHgamma, and chlordane within the lake system are; 3.3 , 6.3, 6.1, 3.2, 0.4, 14.0 and 1.7, respectively.