Nitrate transport process in a small coastal alluvial fan catchment

In order to demonstrate the nitrate transport process from land to the sea, variations in the NO₃^--N flux of river water were investigated, and the NO₃^--N flux of groundwater was estimated in a small coastal alluvial fan catchment covered by orange groves. The results are summarized as follows: 1) In the base-flow period, the NO₃^--N flux of river water decreased from the midstream to the downstream area with the deduction in runoff; 2) At the peak of a storm event with 11 mm of total rainfall, the NO₃^--N flux increased up to about 16 times that in the base-flow period. The total discharge of NO₃^--N during that event was estimated to be equal to that of 4 days during the base-flow period; 3) The NO₃ ^--N concentration in groundwater was more than 20 mg L^-1 at the midstream area, whereas it had declined to less than 2.0 mg L^-1 in the downstream area. Based on these results, we estimated the NO₃^--N discharge by groundwater to the sea to be about 2.4% of the annual nitrogen input from fertilizer application, suggesting that NO₃ ^--N was removed from groundwater by denitrification in the downstream area.