Distribution Of Chlorpyrifos And Some Organochlorine Pesticide Residues In The Upper Tana River Catchment.

Abstract

In this study water, sediment and weed samples obtained from Tana River in the upper Tana catchment were screened for chlorpyrifos and organochlorine (OCs) pesticides. The main concern of this study was to investigate the water quality in Tana River in the Upper Tana catchment. This is because of intense crop and livestock farming activities and high rate of industrialization processes taking place in the area. The pesticides residue levels were then correlated with the physico-chemical parameters in water samples. A total of 720 samples of water, sediment and weed were analysed for two years from ten sampling points. The river profile was divided into three sections; upstream, mid-stream and downstream. The extraction of water samples was done by liquid-liquid partitioning method using dichloromethane, while sediment samples were extracted in hexane and acetone solvents by soxhlet extraction method. The extraction of weed samples was done using an orbital shaker in acetone. The analysis of the pesticides was done using Gas chromatography-mass spectrometer and the data analysis conducted using Microsoft excel and Pearson’s correlation Statistical Package for Social Scientists. The concentration of the OCs in water samples ranged from <0.00012 to 107.33 μg/L with p,p’-DDT with the highest mean residue level observed at Point 7 (Kiganjo) located at the mid-stream section. Sediments samples had OCs concentration ranging from < 0.00024 to 190.07 μg/kg. These levels were greater than those found in water samples. On the other hand the residue levels of the OCs detected in weed samples ranged from < 0.00012 to 28.82 μg/kg. Generally, there was an increasing trend in levels of individual OCs as well as the total OCs in the mid-stream in all the three matrices. The concentrations were higher during the dry seasons than in the wet seasons. Chlorpyrifos mean residue levels varied between < 0.0001 and 6.80 μg/kg. The highest mean residue level of 6.80 μg/kg was detected at Point 5 (Tetu). On the other hand chlorpyrifos mean residue levels found in sediment samples (< 0.0001 -1.43 μg/kg) were generally lower than those detected in water samples. Mean Chlorpyrifos levels in weed samples ranged between < 0.0001 and 2.57 μg/kg, with the highest concentrations observed at point 7 (Kiganjo) which is situated at the midstream. pH in water samples ranged between 6.71 and 7.54 which falls within the range of 6.5-8.5 levels for natural water bodies recommended by the European Union. Electrical conductivity ranged between 57.02 μS and 373.43 μS, with high mean levels observed at the mid-stream. The correlation of water samples from the upper Tana River showed high positive values for organochlorine pesticides and the physico-chemical parameters. For example a high positive correlation coefficient of 0.925 was observed between OCs and Salinity.