Abstract
The headwaters of the Agatsuma River system in Gunma Prefecture include four mountain bodies with distinct ages of volcanic activity. The quality of river water and groundwater in these regions is believed to reflect differences in the degree of weathering associated with each mountain’s geological age. However, few studies have compared water quality among multiple mountain bodies within a single river system. This study investigates the mechanisms of water quality formation by analyzing the chemical properties of river water and groundwater in the headwaters of the Agatsuma River system. From November 2021 to June 2024, water samples were collected from 97 sites. These samples were analyzed for pH, electrical conductivity, major ion concentrations, SiO2 concentrations, and water stable isotope ratios of oxygen and hydrogen. The results indicate that rock weathering is a major factor influencing the water quality in the headwaters of the Agatsuma River system. Distinct differences in chemical composition were observed among the four mountain bodies. The proportion of HCO3－ among anions increased with the age of volcanic activity, indicating that differences in the degree of rock weathering affect river and groundwater chemistry. In particular, the river water quality in the Mt. Azumaya watershed appears to reflect the permeability and solubility of weathered rocks distributed throughout the catchment.
