Water covers 70.9% of the Earth's surface (WFB, 2008), and is vital for all known forms of life (WFB, 2008). The specific contaminants leading to pollution in water include a wide spectrum of chemicals like Persistent Organic Pollutant 1(POPs), pesticides and pathogens. Chlorpyrifos is an organophosphate insecticide used to kill variety of insects, especially ticks and was introduced in Kenya in 1965 (Meister, 1992). This study set out to determine the chlorpyrifos concentration residue levels in some selected samples from the upper Tana River, to confirm their state of contamination. Water, sediment and weed samples obtained from the upper Tana River in Kenya were screened for chlorpyrifos and found to be contaminated. The river profile was divided into three sections representing upstream, middle stream and downstream. Water samples were sampled using the amber bottles while the sediment samples were collected by scooping it with a shovel. Weed samples were plucked using hands, while the hard ones were cut with a clean stainless steel knife. The extraction of water samples was done by liquid-liquid partitioning method using dichloromethane, while sediment samples were extracted in acetone and hexane 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 (GC-MS) and the data analysis conducted using Microsoft excel, 2ANOVA and Pearson’s correlation Statistical Package for Social Scientists, (SPSS). Chlorpyrifos mean residue levels found in water samples 3(BDL to 6.73µg/L), were generally higher than those detected in weed samples of BDL-2.57 1.43 µg/kg, while sediment samples had the lowest levels of BDL and 1.43 µg/kg, as shown in Figure 1. The mid-stream samples had higher levels than both the up stream and the down stream, as can be observed in Figure 2, probably due to intense industrialisation and farming activities at the midstream.
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