Occurrence of old groundwater in a volcanic island on a continental shelf; an example from Nakano-shima Island, Oki-Dozen, Japan

Groundwater residence times in Nakano-shima Island, Oki-Dozen, Japan, which is situated on the continental shelf, are discussed based on groundwater-age indices (CFCs, 3H, and 14C), stable isotopic ratios of hydrogen (δD), oxygen (δ18O), carbon (δ13C), and noble gases (3He/4He and 4He/20Ne). Samples were taken from springs, shallow wells (up to 5 m deep), water-supply wells (about 10–100 m deep), and the hot spring well (866 m deep). δD and δ18O of all the samples plot along meteoric water lines, indicating a meteoric water-origin. Isotope values of the hot spring water were slightly lower than other samples. Most samples from springs, shallow wells, and the water-supply wells contained higher CFCs and 3H while samples from two water-supply wells (W4 and W7) and the hot spring well showed much lower CFCs concentrations. This result indicates that mixing ratios of old groundwater that is free of CFCs and 3H were higher for these samples. 14C concentrations of samples with lower CFCs were measured, and corrected for addition of 14C-free dissolved inorganic carbon. The corrected 14C concentration of the hot spring water was lower than that of the water-supply well, indicating mixing of much older groundwater in the hot spring water. Because of lower corrected 14C concentration and lower δD and δ18O values, groundwater from the hot spring well is considered to be recharged in a colder climate than present day. Because the island is situated on the continental shelf, the seafloor around the island was most likely widely exposed during the last glacial period. One possible explanation for the existence of groundwater showing lower δD, δ18O, and 14C concentrations, such as the hot spring water, is that fresh groundwater, recharged when sea level was lower, still remains after transgression without being replaced by salt water.