Abstract
Rishiri Island, known for having one of the largest submarine springs in Japan, is a volcanic island located in Hokkaido. We applied multi-transient tracers (3H, CFCs, and SF6) to estimate the residence times of the submarine and terrestrial springs on Rishiri Island. All of the springs contained 3H higher than natural 3H level, indicating that the groundwater on Rishiri Island was primarily recharged during post-bomb periods. CFCs and SF6 concentrations were relatively high in the terrestrial springs and north-eastern submarine spring, and low in the south-western submarine spring. Tracer plots between the 3H, CFCs, and SF6 suggested that the flow pattern of the groundwater on Rishiri Island can be approximated via two flow models: an exponential mixing model (EMM) and a piston flow model (PFM). The terrestrial springs and the north-east submarine spring are formed via a mixture of groundwater with different recharge ages, and the average transit time based on the EMM was approximately 10 to 45 years. The large-scale submarine spring in the south-west may be formed via piston-like groundwater flow at the bottom of a buried valley, and its transit time is estimated to be 40 years. The residence time of spring water was positively correlated with recharge elevation, suggesting that the scale of the groundwater flow system is larger for submarine springs than for terrestrial springs.
