NITRATE NITROGEN POLLUTION BY RECHRGE IN AGRIVULTURAL AREA IN KUMAMOTO AREA, JAPAN

Kumamoto city located in the middle of the Kyusyu Island, Japan is a greatest city which uses groundwater for domestic water. There are 4 groundwater areas in Kumamoto prefecture, and the Kumamoto area is the biggest one in those 4 areas. In this area, all of domestic water for 1 million populations depends only on groundwater. In addition, this groundwater is used to agriculture and industrial too. And this groundwater is the important resource for supporting development of Kumamoto prefecture. However, groundwater level decreased year by year until 2004. It seems that paddy field having high ability for groundwater recharge is continuously decreasing. Because of that, Kumamoto city and 4 companies have performed the ponding water project on the farmland in the Middle Shira-River Area since 2004. The ponding water project is that farmer keep water on farmland which stops cropping rice by the acreage-reduction policy. In this way, groundwater level is rise up. However, recently, the nitrate nitrogen concentration of groundwater rises. And, it is said that the ponding water project is a cause of the nitrate nitrogen pollution of groundwater. Therefore the authors estimated quantity of nitrate nitrogen eluviation from ponding water on farmland in the Middle Shira-River Area. The typical crop fields are selected for analysis. As a result, it is considered that the ponding water project does not contribute to the nitrate nitrogen pollution