Characteristics of seasonal NO₃^--N discharge by groundwater in a coastal agricultural catchment

The seasonal variation in the nitrate-nitrogen (NO₃^--N) discharge by groundwater to the sea was evaluated from the monitoring data of hydraulic potential and NO₃^--N concentrations in the groundwater of a small coastal agricultural catchment. A large variation in the NO₃^--N concentration (<0.05 - 5.3 mg L^-1) was found in the shallower groundwater of the coastal area. These results suggest that NO₃^--N attenuation via denitrification influences the seasonal variation in NO₃^--N concentrations. Furthermore, significant NO₃^--N attenuation was confirmed in relatively small Darcy flux (<1.8 x 10^-6 cm/s). The estimated NO₃^--N flux varied (0.006 - 0.8 kg d^-1) with changes in the NO₃^--N concentration. Annual groundwater discharge is estimated to be 1.7-fold higher than river discharge, while groundwater NO₃^--N flux accounts for approximately 70% of river flux owing to the attenuation of NO₃^--N in coastal areas.