The groundwater in Toyooka basin, northern part of Hyogo prefecture, has been used not only for agriculture, industry and melting snow in winter, but also for drinking water supply. The sea water goes up to 16km distance from the estuary of Maruyama river, when the discharge of river water decreases. The chloride ion content of water samples of some wells in the basin exceeded the drinking water standard of the Ministry of Health and welfare. Thus, it is necessary to study the quality of groundwater and the regional distribution of chloride ion contents within this basin.
We have surveyed the quality of groundwater in t h e basin since 1987, in order to know the seasonal and regional variations of chloride ion content and to grasp the situation of groundwater flow in this basin. Based on the chemical composition and chloride ion content distribution, the growndwater in the basin are classified into following three types. One is typical artesian water flowing from south to north along the Maruyama river. Another is the saline spring waters leaking along the Kongouzi fault. The last is the river water mixed with sea water moving up from estuary of Maruyama river.