Abstract
The study area is located at the southern part of Okinawa Island, Japan, where Ryukyu Limestone is extensively distributed. Nitrate concentrations at springs have been decreasing through a long-term monitoring since the mid-1990s. We studied the sources of nitrate in the groundwater by the emission factor method using data on the cultivated area, the number of livestock and the population. The result revealed that the trend of the nitrate concentrations corresponds only to that of the load from chemical fertilizer for upland fields, leading to the assumption that the chemical fertilizer most strongly affects the groundwater quality. The result of the balance analysis of water and nitrogen using a parallel reservoir model which takes into account cave systems supports this assumption. The calculated result by the nitrogen dynamics model that consists of the parallel reservoir model as a water balance submodel and a nitrogen balance submodel is coincident with the trend of monitored nitrate concentrations.
