Abstract
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.
