Hydrological methods for understanding the shallow groundwater flow in volcanic regions with abundant groundwater resources

Abstract

Aso volcano is an active volcano in Kyushu Island, southwest Japan. This volcano is characterized by a large caldera, extending 18 km east–west and 25 km north–south, where a central cone is located. Aso volcano has abundant groundwater resources and is an area where many spring waters have been confirmed. Regional characteristics of spring discharge mechanisms could be found using hydrogeochemical techniques such as major ions, stable isotope ratios (δ2H and δ18O), and age dating methods (3H, CFCs, and SF6) of the spring water. There is a clear difference in the type of major dissolved ions between the outer rim side (Ca-HCO3 type) and the central volcano side (Ca-SO4 type), suggesting differences in the groundwater flow system in each area. Among the springs in Aso caldera, the springs located in the area between Aso volcano and the two rivers were characterized by abundant dissolution of sulfate ions (SO4 2-). This was considered an indicator of the influence of Aso volcano. Furthermore, such springs via the Aso volcano have a higher recharge elevation due to the lighter stable isotope ratios, and the age estimation method shows that these springs have a longer residence time. These hydrogeochemical results indicate the existence of large groundwater flow system. The understanding of groundwater flow mechanisms is a fundamental issue for the proper management of abundant groundwater resources in volcanic regions. Through the application of the methodology in this study revealed that there are springs with various flow systems in the Aso caldera.