Fig. 3. Overview of V, δ¹⁸O and He compositions in springs and groundwater of Fuji catchment.

a,b, Maps of V (a) and δ¹⁸O (b) compositions. c, δ¹⁸O versus V concentrations. Hypothetical recharge elevations (from δ¹⁸O after Yasuhara et al.³⁶) are indicated by dotted vertical lines assuming no mixing of waters from different elevations. d, Average He and ⁴⁰Ar concentrations measured on site with the new portable GE-MIMS instrument¹³³. The black dashed lines represent air-saturated water (ASW) for 20 °C at the three primary recharge elevations after Yasuhara et al.³⁶ for the southwestern basin. The dotted lines indicate hypothetical excess air additions to ASW at 1,700 m ASL and for 0 °C, 5 °C, 10 °C and 15 °C. Error bars indicate ±1 s.d. of the mean of all GE-MIMS measurements taken at each site. The numbers of sampling dates used to quantify average values and standard deviations per site are: A (n = 2), 1 (n = 1), 2 (n = 1), 9 (n = 2), 16 (n = 1), 48 (n = 2), 60 (n = 2), 63 (n = 1). GW, groundwater. e, ³He/⁴He versus ²⁰Ne/⁴He for samples obtained in this study (sites A, 6, 9 and 10a) alongside previous measurements (sites 1a⁹⁶, 1b⁵³, 3a⁹⁶, 3b⁵³, 7⁹⁶ and 10b¹⁰² and data from onsen deep wells^{53,104,107,111}, groundwater wells⁵³ and fumaroles⁵⁷) and end-member isotopic ratios^55,57 (mid-ocean ridge basalt (MORB), continental crust, radiogenic production, ASW at 0 °C and ASW at 20 °C). f, Overview map of ³He/⁴He ratios. TNK, Lake Tanuki. FJM, Fujinomiya. Fum, Fumaroles. For all data and corresponding references, see Supplementary Data 1. Coordinate reference system: WGS 84 / Pseudo-Mercator. Background composite map sources: digital elevation model¹⁶²; red 3D hillshade map^163,164; active tectonic fault locations¹⁶⁵; plate boundaries and major tectonic faults^43,166,167. White dashed lines in a, b and f indicate the FKFZ and KMFZ tectonic zones; red dashed lines in a, b and f indicate active tectonic faults.