Nitrification and denitrification processes in a well mixed tidal river in Japan are modeled, based on results of long term field observation on water quality and sediments in the river. Laboratory test results and values in references are used for the determination of parameters and coefficients in the model. In modeling of the processes, sediments, suspended solids, and overlying water are taken as elements and in computation a Lagrangian reference frame in terms of accumulated water volume is employed for reduction in numerical dispersion. Simulation results on ammonium and nitrate nitrogen, showing good agreement with long term field observation results, qualitatively explain well the processes in the river. The results indicate some countermeasures to reduce nitrate more such as the increase in the area of sediment surface. The simulation model is available for the estimation of water quality in well mixed tidal rivers and applicable for water quality control.

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