Abstract
The groundwater quality of all the area in Kumamoto prefecture was investigated by Kumamoto prefectural government from 1989 to 1994. As a result, groundwater with high concentration of nitrate-nitrogen was found out in U-town located in north area of Kumamoto pref. The investigation points were classified into four groups based on a relation between concentrations of nitrate-nitrogen and nitrogen isotope values of nitrate from the result of investigation of groundwater quality in 1995 and 1996. Study of nitrogen supply in each group indicated that the pollution of group containing most of investigation points was caused by infiltration of chemical fertilizer. The soils of the lands used for greenhouses and vegetable fields contained a high concentration of nitrate-nitrogen and to deeper portion compared with other types of land use, from study of concentrations of three nitrogen forms in solution dissolved soils of several types of land use. The quantities of nitrogen infiltration from several nitrogen-sources in U-town were estimated. The livestock industry and fertilizer had a large quantity of nitrogen infiltration to underground. It was conjectured that groundwater pollution by nitrate-nitrogen in U-town was greatly affected by infiltration of chemical fertilizer into a field.
