Residence time of submarine fresh groundwater discharge in Rishiri Island, north Japan: Application of groundwater age tracers of Tritium, CFCs and SF6.

Rishiri Island, located on north Japan is a stratovolcano and has the largest submarine"@groundwater discharge (SGD) in Japan. The SGDs distribute around the costal area of Rishiri Island. In this study, we estimate the residence time of the SGD using the multi age tracer method, to understand the mechanism of SGD. Water samples were collected from 2 SGDs, 14 terrestrial springs and 4 terrestrial groundwaters, and were measured in Tritium, CFCs and SF6 concentration. All water samples contain detectable tritium within a range from 4.9TU to 10.6TU, indicating that these waters are recharged at post-bomb period. High tritium concentration appeared in the SGD suggests that the age of SGD is relatively older than the terrestrial spring. CFC-12 and SF6 concentration of water samples show wide variations from 16 to 414 pg/kg and from 0.30 to 2.41 fmol/kg, respectively. High CFC-12 and SF6 values are observed in the terrestrial spring, whereas low values are observed in the SGD. Based on the known solubility relationships and the recharge temperature, the CFC-12 and SF6 concentrations in the water samples were converted to the equivalent air concentrations. Tracer plot of CFC-12 versus SF6 show that most of the terrestrial springs distribute around the line of exponential mixing model and the SGD are plotted on the line of piston-flow model. The mean residence time of these waters estimated by corresponding model range from 5 to 25 years in the terrestrial springs and from 25 to 40 years in the SGD. From these results, we conclude that the SGD in Rishiri Island is generated by relatively deep groundwater flow system with long residence time.