Relationship between water temperature at emergence (TE) and recharge altitude (z) as a function difference of elevation between recharge altitude and emergence altitude (Dz) and vertical geothermal gradients at the recharge (GR) and emergence (GE). Dotted and grew square represent respectively the thermal groundwater mixed, and non-mixed, with cold surface waters. For example, the 46 °C springs of Rennes-Les-Bains can come from recharge areas located at 1000 (yellow) or 1500 (blue) m altitude provided that there is a difference in altitude of 600 or 1200m respectively. See Appendix 1 for further details.