Fig. 1. Schematic diagram of floodplain filtration. If contaminated river water is sprayed over the floodplain, organic matter in the water is expected to be decomposed by microbes using O₂ as an electron acceptor and a reducing layer is expected to develop beneath the oxidizing surface layer due to the depletion of oxygen in the surface layer. Ammonium in the river water is expected to be converted to NO₃ in the oxidizing surface layer and then the NO₃ leached down in the reducing layer can be removed by denitrification with the further degradation of organic matter.