Origin and Recharge Altitude of the Thermo-Mineral Waters of the Eastern Pyrenees

The thermo-mineral waters of the axial zone of the Eastern Pyrenees form a geochemically homogeneous group. They emerge in granite or orthogneiss and all have a sodium sulphide chemistry. Principal component analysis of their physico-chemical parameters has distinguished three types of fluid, 1) hot water that has evolved in a closed system and whose chemistry may reflect that of deep water, 2) water that is also of unmixed origin, but whose chemical composition has been modified during cooling by conduction, and 3) water cooled by mixing with surface water.

Stable isotope (¹⁸O, ²H) contents indicate that all the waters are of meteoric origin (from oceanic and/or Mediterranean precipitation). No heavy isotope enrichment has been found that would indicate evaporation or a geothermal effect between water and the host rock.

The differences in isotope contents between surface and thermo-mineral waters are attributed to a difference in recharge altitude; altitude gradients in ¹⁸O and ²H, estimated by two independent methods, are respectively 0.24‰ and 1.84° per 100m. They may, however, be lower when precipitation is in the form of snow. Applying these calculated gradients to thermo-mineral waters, taking mixing effects into account, has given an estimate of the minimum altitude of recharge of 110 springs in the Eastern Pyrenees.