Abstract
The Miyakonojo Basin is one of major agricultural and stockbreeding areas in Japan. Groundwater sampling from different type of aquifers was conducted over the area to understand nitrate concentration distribution and to reveal the process controlling nitrate concentration change along the groundwater flow. In shallow groundwater, high NO3－ concentrations were found especially in the western side of the basin. In the middle depth and deep groundwater, high NO3－ concentration areas were found in the western and southern part of the basin. In general, nitrate concentration decreased along the groundwater flow from upstream to downstream, suggesting the occurrence of natural nitrate attenuation process in this area. Considering the results of redox parameters and nitrogen isotope ratios in nitrate, it is suggested that dilution process is major nitrate attenuation controlling factor in the midstream of groundwater flow, while denitrification process is dominating in the downstream of middle and deep groundwater.
