Groundwater age determination using 85Kr and multiple age tracers (SF6, CFCs, and 3H) to elucidate regional groundwater flow systems

The Kumamoto area (945 km2) in the south of Japan, where almost 100% of the drinking water is dependent on groundwater. Simultaneous measurement of groundwater dating tracers (85Kr, chlorofluorocarbons [CFCs], sulphur hexafluoride [SF6], and 3H) was performed in the Kumamoto area, to elucidate the regional groundwater flow system and obtain improved estimates of groundwater ages. The groundwater ages were estimated from the 85Kr concentrations in nine locations from three areas: along two major groundwater flow lines (A–A’ and B–B’); and a high-nitrate-input recharge area (C area). The groundwater ages could not be estimated using CFCs or SF6, particularly in the urban areas because of artificial additions to the concentration over almost the entire study area. However, even in these regional circumstances, apparent ages of approximately 16, 36, and not less than 55 years were obtained for three locations on the A–A’ line (recharge area, discharge area, and stagnant zone of groundwater, respectively) from 85Kr measurements. This trend was also supported by lumped parameter model analysis using a time series of 3H observations. In contrast, along the B–B’ line, the groundwater age of not less than 55 years at three locations, including the recharge to discharge area, where CFCs and SF6 were not detected, implies old groundwater: this is also the area in which denitrification occurs. In the C area, very young groundwater was obtained from shallow water and older groundwater was detected at greater depths, as supported by the long-term fluctuations of the NO3−–N concentration in the groundwater. The results of this study can be effectively used as a “time axis” for sustainable groundwater use and protection of groundwater quality in the study area, where groundwater accounts for almost 100% of the drinking water resources.