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Chemical and isotopic studies including analyses of noble gases were comprehensively conducted on the groundwater of the entire Kii Peninsula, which is located in the fore-arc region of southwest Japan. Groundwater of Na–Cl–HCO3, Na–HCO3–Cl, and Na–Cl types was shown to be distributed across the whole area. Groundwater in the inland central part of the peninsula shows relatively low salinity, whereas groundwater from the area along the ENE-trending Median Tectonic Line (MTL), on the north side of the peninsula, shows high salinity (up to 18,800 mg/L of Cl−) and the presence of unusual heavy oxygen isotopes. This trend is similar to that documented in saline waters from the Arima region (the so-called “Arima-type thermal water”). High 3He/4He ratios relative to the atmospheric value (up to 6.7 Ra) were recorded throughout the Kii Peninsula, covering a wider area than documented previously. The saline groundwater is also strongly depleted in 20Ne and heavy noble gases. From the wide distribution of high 3He/4He values and the associated 20Ne and Cl− concentrations, we infer that aqueous fluids derived from dehydration of the subducting slab are present at depth beneath almost the entire Kii Peninsula. These aqueous fluids may ascend along the major north-dipping boundary faults. The isotopic composition of groundwater from the southern part of the peninsula suggests that the contribution from these dehydration-derived fluids is relatively small in this region. However, volatile components (e.g., noble gases and CO2) in the groundwater of this area may originate from the dehydration-derived fluids. Upwelling of Arima-type thermal water of the Na–Cl–HCO3 type is expected to undergo a phase separation of volatile species due to decompression as the fluid ascends. The variety of water types documented may be due to this water–gas separation and the subsequent incorporation of gaseous species into shallow meteoric groundwater. The observed high 3He/4He ratios in the absence of a mantle wedge below the southern part of the Kii Peninsula may reflect the oblique ascent of these fluids along north-dipping boundary faults.

Summary 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, 3 H, and 14 C), stable isotopic ratios of hydrogen (δD), oxygen (δ 18 O), carbon (δ 13 C), and noble gases ( 3 He/ 4 He and 4 He/ 20 Ne). 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 δ 18 O 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 3 H 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 3 H were higher for these samples. 14 C concentrations of samples with lower CFCs were measured, and corrected for addition of 14 C-free dissolved inorganic carbon. The corrected 14 C 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 14 C concentration and lower δD and δ 18 O 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, δ 18 O, and 14 C 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.

To understand deep groundwater flow systems and their interaction with CO2 emanated from magma at depth in a volcanic edifice, deep groundwater samples were collected from hot spring wells in the Aso volcanic area for hydrogen, oxygen and carbon isotope analyses and measurements of the stable carbon isotope ratios and concentrations of dissolved inorganic carbon (DIC). Relations between the stable carbon isotope ratio (δ13CDIC) and DIC concentrations of the sampled waters show that magma-derived CO2 mixed into the deep groundwater. Furthermore, groundwaters of deeper areas, except samples from fumarolic areas, show higher δ13CDIC values. The waters' stable hydrogen and oxygen isotope ratios (δD and δ18O) reflect the meteoric-water origin of that region's deep groundwater. A negative correlation was found between the altitude of the well bottom and the altitude of groundwater recharge as calculated using the equation of the recharge-water line and δD value. This applies especially in the Aso-dani area, where deeper groundwater correlates with higher recharge. Groundwater recharged at high altitude has higher δ13CDIC of than groundwater recharged at low altitude, strongly suggesting that magmatic CO2 is present to a much greater degree in deeper groundwater. These results indicate that magmatic CO2 mixes into deeper groundwater flowing nearer the magma conduit or chamber.