The fate of Nitrate under the natural groundwater flow system in a volcanic aquifer composed by pyroclastic flow

Nitrate in drinking water causes health problems, and causes eutrophication. Miyakonojo basin is a well- known agricultural area in Southern Kyushu, Japan and highly depends on groundwater resources for everyday use. The detailed three dimensional groundwater flow system study has been done in the basin to understand the three dimensional distribution of Nitrate-Nitrogen in the aquifer. Field water sampling was conducted between Sep. 2006 and July 2007. The result suggests that the tributary river water mainly recharges to the shallow groundwater in the eastern part of the basin. On the other hand, the summer precipitation recharges the shallow groundwater which flows from upland down toward the river in the western part of the basin. This means that the shallow groundwater flow system is primarily affected by the surface geomorphology, which is completely different between eastern and western parts of the studied basin. For nitrate concentration, the shallow groundwater shows gradual increasing tendency along the flow line in the eastern part of the basin, while the nitrate concentration decreases and the reduction product increases along the flow in the western part. Especially in the western part of the basin, the major land use are the intensive cattle farming and dry farm land using artificial fertilizer, both are the potential sources of the nitrate for the local shallow groundwater. Also geomorphologic upland of the western part of the basin works as the recharge area of the shallow groundwater, and high concentration of nitrate is supplied at these areas. To understand this site-specific nitrate reduction process in the western basin, it is important to understand the nitrate origin and nitrogen process including denitrifcation process. For this, a multi-Isotope approach of dN and dO to tracing the sources of nitrate is applied in the study area with the relation of groundwater flow system. The field sampling was done in July 2007 and Sep. 2008 to analyze inorganic water chemistry, dD and d18O stable isotopes in water andd15N and d18O in Nitrate. We intend to report this result and related discussion at the meeting.