Abstract
3H, 3H⁄3He, CFCs, and SF6 have been generally used as age dating tracers for young groundwater in the U.S. and Europe. Each tracer has its own advantages and disadvantages so that multi tracers method is the most reliable approach in estimating groundwater age. However, the multi tracers approach has received very little application in dating groundwater in Japan, due to the paucity of CFCs and SF6 data.In order to validate the effectiveness of using CFCs dating method in Japan, CFCs concentrations of some springs were measured in stratovolcanoes of Mt. Fuji, Mt. Yatsugatake and Mt. Ontake. All the investigated springs have detectable concentrations of CFC-12, CFC-11 and CFC-113, suggesting that these springs were mainly recharged after the 1950s. The comparisons of equivalent air concentrations (EAC) of the springs with the historical atmospheric concentrations suggested that the CFCs in most springs of Mt. Yatsugatake and Mt. Ontake were of atmospheric origin. However, in Mt. Fuji, input of local anthropogenic CFCs (not atmospheric origin) to the springs were observed. Based on the exponential mixing model, the residence time of the groundwater in Mt. Yatsugatake and Mt. Ontake were estimated to be 20 to 30 years and less than 10 years, respectively. The obtained CFCs ages are consistent with those reported using 3H previously. These results suggest that the CFCs method can also be an effective dating tool for young groundwater in Japan, when the input of local anthropogenic CFCs (not atmospheric origin) is negligible.
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