Nutrient and carbon contents of the sediment in a drainage canal may increase under cyclic irrigation although the effluent loadings from a paddy-field district are reduced by the reuse of drainage water. In this study, a paddy-field district adjacent to Lake Biwa implementing cyclic irrigation was investigated and the nitrogen, phosphorus and carbon contents of the sediment in the drainage canal are determined. The results show no clear relationship between irrigation period and the temporal variation in nitrogen, phosphorus or carbon content of the sediment. The amounts of nitrogen, phosphorus and carbon in the upper layer of the sediment are larger than those in the lower layer. The mean nitrogen, phosphorus and carbon contents of the sediment were respectively 1.96 g·kg^-1, 2.44 g·kg^-1, and 20.0 g·kg^-1 in the upper layer and 1.36 g·kg^-1, 0.75 g·kg^-1, and 12.1 g·kg^-1 in the lower layer. In particular, the phosphorus content of the sediment in the drainage canal was higher than mean phosphorus contents of the sediments in rivers and lakes. Nitrogen in the sediment existed mainly in the organic state. In contrast, phosphorus was mainly in the inorganic state.

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Natural gas and iodine from a gas reservoir 500-2,000 m below the ground surface are dug up in the Kujukuri District, Chiba Prefecture in Japan. Underground water (salinity water) containing a high nitrogen concentration is drawn up from the earth by excavation and drained to the public water area around iodine production plants. In this study, the concentrations of nitrogenous compounds in the river and sea areas adjacent to the excavation sites were monitored to elucidate the effect of the underground water. In addition, the fate of these nitrogen compounds in the sea area was predicted by a simulation model incorporating NH₄-N diffusion and convection. Throughout the season studied, only the NH₄-N concentration in rivers increased downstream of the iodine production plants, indicating that NH₄-N, mainly discharged from the plants, is seldom oxidized to nitrite or nitrate. An apparent NH₄-N dissipation rate was estimated using a simulation model. The results of a comparison of the NH₄-N dissipation rate determined by simulation with the nitrification rate determined by a laboratory experiment suggest that NH₄-N assimilation by plankton in coastal and offshore areas prevails over nitrification.

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