The seasonal variation in the nitrate-nitrogen (NO 3 --N) discharge by groundwater to the sea was evaluated from the monitoring data of hydraulic potential and NO 3 --N concentrations in the groundwater of a small coastal agricultural catchment. A large variation in the NO 3 --N concentration (<0.05 -5.3 mg L -1 ) was found in the shallower groundwater of the coastal area. These results suggest that NO 3 --N attenuation via denitrification influences the seasonal variation in NO 3 --N

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... ions. Furthermore, significant NO 3 --N attenuation was confirmed in relatively small Darcy flux (<1.8 × 10 -6 cm/s). The estimated NO 3 --N flux varied (0.006 -0.8 kg d -1 ) with changes in the NO 3 --N concentration. Annual groundwater discharge is estimated to be 1.7-fold higher than river discharge, while groundwater NO 3 --N flux accounts for approximately 70% of river flux owing to the attenuation of NO 3 --N in coastal areas.