Hydrogeochemical factors controlling the fluoride concentration in spring/groundwater in Aso caldera in the volcanic region, Japan

Groundwater and springs are important water resources for domestic and agricultural uses in the Aso caldera in the volcanic region of Kumamoto, Japan. In addition, some springs play the role of tourist attraction. Previous studies have reported high fluoride (F-) levels in these water bodies; however, these studies were conducted more than 20 years ago and did not study the factors controlling the F- concentration in detail. In this study, to understand F- concentration in recent years and the controlling factors, water samples were collected at two artesian wells and 22 springs every 3 months from July 2020 to October 2021. Water samples showed a mean F- concentration ranging from 0.16 to 1.18 mg/L. Five of the 24 (20%) sampling sites showed a mean F- concentration above the Japanese drinking water standard of 0.8 mg/L. The water samples with higher F- concentrations were mainly distributed on the central cone sides, and represented Ca-SO4-HCO3 and Ca-SO4 types. In other words, the water samples evaluated as having an effect of hydrothermal water, based on a higher SO42- concentration or lower Cl-:SO42- molar ratio, showed higher F- concentrations. In terms of anthropogenic factors, fertilizer application was not an important factor controlling the F- concentration in spring and groundwater in the study area due to the distribution of upland fields. As a natural factor, although the general mechanisms of weathering and anion exchange of F-bearing minerals can be one of the factors controlling the F- concentration in the study area, it did not contribute to the entire study area due to the limited distribution of the strata containing the related minerals. Magmatic gases, including SO2, HF, and HCl, were considered to form hydrothermal water in the study area, and the effect of hydrothermal water could increase F-, SO42-, and Cl- in water samples. This mechanism was validated by the positive correlations between F- and SO42- and Cl-. Thus, magmatic gas constituted the main contribution to the increase in the F- concentration in the study area.