Based on the horizontal and vertical distributions of pesticides in the river and the reservoir, the fates of pesticides in these systems were investigated in this study. Four pesticides, i.e., MEP, thiobencarb, BPMC and isoprothiolane had the highest concentrations among the eighteen pesticides. These pesticides in suspended components except isoprothiolane were detected not only during stormy weather days but also during dry weather days. The percent detection of pesticides in suspended component was 59.8-61.9% in MEP, 34.6-38.9% in thiobencarb and 17.0–19.0% in BPMC. However, isoprothiolane in the suspended component was undetectable.

These pesticides in the suspended component have existed as a part of the suspended solids in the river, then disappeared by sedimentation during the flow of the river from the upper stream to the backwater in the reservoir. On the other hand, the pesticides in the dissolved components that flowed into the reservoir were uptaken or adsorbed by phytoplankton. Moreover, the concentrations of the less absorptive pesticides like isoprothiolane have decreased by dilution and diffusion in the reservoir.

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