Abstract
High fluoride concentration up to 12 mg/l was observed in the deep groundwater of Mizunami area, central Japan. Water-rock interaction tests were performed to investigate the factors affecting the fluoride concentration. Nine granitic and two sedimentary rock specimens were collected from the core of deep borehole drilled at the study area. The fluoride concentration is changed among different type of granitic rock specimens. That change may be occurred as a result of the spatial distribution of mineral composition in the granite mass. Moreover, groundwater residence time is an important factor that affects fluoride concentration. The most difficult point was the evaluation of the coexisting ions that affects on the fluoride concentration. Coexisting ions possibly related to the mineral composition in the rock specimens and chemical circumstances such as pH, groundwater type, etc.