Several field surveys were conducted to investigate changes of water quality with time in the tidal River Rokkaku, where a turbidity maximum exists. Suspended solids of the turbidity maximum reach more than 20 g/l in concentration. Based on the surveys, NH4-N, NO2-N, and NO3-N have peaks in concentration at certain salinities, located in order of NH4-N, NO2-N, and NO3-N toward the river mouth within 10 to 25 km from it. The salinities were 0.5, 1.7, and 3.0 %, respectively, and the maximum concentrations were about 3.0, 0.3, and 4.5 mg/l in winter time, respectively.

Laboratory batch experiments were conducted, using suspended solids and sediments taken from the river, to study the effect of salinity on nitrification and to estimate kinetics parameters of it in the river. Suspended solids and sediments were sampled at a point in the middle stream. The sediments were collected from the aerobic layer of mud, less than 1 cm thick from the surface. Experimental results indicated much more inhibition of NO2 oxidation by salinity than that of NH4 oxidation. Nitrifying bacteria in sediments were less sensitive to salinity than those in suspended solids. The change of nitrogen concentration with time was clearly explained with the Monod growth model and the two kinetics parameters were obtained by the curve fitting method. Maximum specific growth rates of NH, oxdizing bacteria ranged from 0.015 to 0.029 (hour−1), which decreased markedly with more than 15 %. salinity. Those of NO2 oxidizing bacteria ranged from 0.015 to 0.025 (hour). Saturation constants of NH4 and NO2 oxidation were also dependent on salinity. Changes in NH4-N and NO2-N concentrations in the River Rokkaku with time were simulated well using a newly developed river model and the parameter values obtained in the laboratory tests. The mechanism of nitrification by suspended solids and sediments in the river is shown to depend on tidal effects.

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