阿蘇カルデラ内における地下水の流動機構

Abstract

The groundwater flow system in Aso caldera is divided into four areas ( ( I ) somma of Aso volcano, ( II ) central cones side of the Aso volcano, ( III ) caldera low land unconfined groundwater and ( IV ) caldera low land artesian system) based on hydrogeochemical characteristics such as stable isotope values ( δ D and δ18O) and water chemistry. Estimated recharge elevations for groundwaters in areas ( I ) and ( III ) were relatively low and of Ca-HCO3 water type. On the other hand, groundwaters in areas ( II ) and ( IV ) belong to relatively a large groundwater flow system with high SO42- content and groundwaters were recharged in the high elevation area of the Aso volcano. We estimated the residence times of groundwaters collected from the four representative areas by using groundwater age tracers, tritium and chlorofluorocarbons (CFCs). The groundwater ages were estimated to be within 3 years under piston flow model by tritium analysis. Besides, longterm observed record analysis of tritium concentration were applied for areas ( I ) and ( II ) to estimate more precise groundwater age. Based on this analysis, the groundwater age was limited to approximately 20 years with piston flow model in the area ( I ) , and this result is consistent with CFCs ages. However in the area ( II ), estimated tritium piston flow groundwater age is different from CFCs ages. When we consider binary mixing flow in this area, the estimated groundwater age by CFCs was approximately 3 years. This longer groundwater age is harmonized with hydrogeochemical characteristics and estimated groundwater flow system. In the area ( III ), the groundwater age was not estimated because of excess addition of urban CFCs. In the area ( IV ), the estimated groundwater age was approximately 2 years by CFCs with piston flow model, however, considering the high dissolved chemical components and the estimated recharge area, the residence time may be longer.