Highlights
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Biofilm formation at the shore of river is likely to promote accumulation of microorganisms.
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Clayey materials collected at the river shore contained minerals such as muscovite, kaolinite and goethite.
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Microorganisms in the biofilms mainly attached to the mineral muscovite.
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Microorganisms accumulated in the clayey materials and were washed off during rain season.
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Clayey materials obviously promote biofilm formation and accumulation of microorganisms, increasing water contamination.