To confirm the semi-diurnal and seasonal variation in nutrient flux with the dynamics in groundwater-seawater interaction, we conducted intensive observations at 25 piezometers in a tidal flat over a distance of 100 m across a steep-sloped island. Based on the chloride balance, groundwater was very well-mixed with seawater under the tidal flat. Nitrate-nitrogen (NO 3--N) concentrations declined from >20 to near 0 mg L-1 along the groundwater flowpath from the hillslope to the tidal flat. In addition, nitrate concentrations in pore water of the tidal flat were lower (at 0.1 mg L-1) than in seawater. These results suggest that the reduction process of NO3--N occurred in both contaminated groundwater and seawater. Discharge of inorganic nitrogen by groundwater was confirmed offshore. The suspected source was nitrogen mineralization of organic compounds and seawater recirculation. Phosphorus was produced offshore and transported from the land area. Seasonal variations in nutrient dynamics at the tidal flat were also confirmed.