Abstract
Three years of field observations conducted of the groundwater level, nitrate concentration, and isotope ratios (δ15NNO3 • δ18ONO3) of groundwater in the Sata River alluvial fan, Asakura, Fukuoka revealed that groundwater levels fluctuated seasonally and that nitrate concentration fluctuation is affected both by paddy irrigation and timing of fertilization activities. To evaluate nitrogen behavior and its budget in the groundwater aquifer system, a three-dimensional groundwater flow coupled with convection-dispersion model was developed, incorporating detailed nitrogen load information including the fertilizing rate, timing, and effects of nitrogen uptake rate by planted crops at the ground surface for each agricultural land use. Results suggest that nitrate concentration in the studied groundwater shows clear seasonal fluctuation annually with the lower nitrate concentration around 3 mg/L. Because the nitrogen budget under the present condition shows a quasi-steady state with nitrate concentrations that cannot decrease below 3 mg/L, this concentration limit has been created by the long-term cumulative effect of the nitrogen load. The nitrogen discharge rate through the discharge flow of the groundwater from the surface of the studied alluvial fan to the lowland area outside of the study area was estimated using this model as 106 kg/ha/y.
