Isotopes and geochemistry to assess shallow/thermal groundwater interaction in a karst/fissured-porous environment (Portugal): a review and reinterpretation

As a distinctive fingerprint of the groundwater sources and water–rock interaction within a karst/fissured-porous environment, isotopic (e.g. δ¹³C, δ¹⁸O, δ³⁴S, ³H, and ¹⁴C) and geochemical data have been used to establish flow paths (diffuse flow) of thermal waters used in Caldas da Rainha Spa (Portuguese mainland). These thermal waters (T ≈ 32 °C) discharge from springs and boreholes located close to an N–S-oriented oblique fault (60°E). This hydrothermal system is dominated by deep karst/fissured-porous Lower Jurassic carbonate formations containing slow flowing groundwater. ¹⁴C determinations in the total dissolved inorganic carbon (TDIC) indicate a mean “age” of about 1600 years BP for the thermal waters. The HCO₃⁻, Ca²⁺, and Mg²⁺ signatures are related to water/calcite–dolomite interaction, whereas Na⁺, Cl⁻, and SO₄²⁻ concentrations are mainly associated with halite and gypsum dissolution. δ¹⁸O values indicate that the hydrothermal aquifer system is depleted in heavy isotopes comparing to the shallow aquifer systems, signifying that the main recharge must be related to the Lower Jurassic carbonate formations of the Candeeiros Mountain. The δ¹³C values measured in the TDIC are typical of carbonate dissolution enhanced by CO₂ soil air dissolution. The δ³⁴S_sulphate and δ¹⁸O_sulphate values of the thermal waters indicate that the sulphate is clearly the result of water–rock interaction with evaporitic layers at depth. Considering a mean geothermal gradient in the region of about 30 °C/km, the silica and K²/Mg geothermometry seems to indicate more reliable circulation depths (1–2 km) for the thermal waters than the SO₄²⁻–H₂O isotope geothermometer (3–5 km depth). The lack of mixing evidences between the thermal and the local shallow cold groundwaters indicates that both water sources are distinct. Furthermore, increasing knowledge on the local/regional hydrogeology is extremely important to achieve the sustainable use of such “invisible” georesources, since most thermal and mineral waters from karst aquifers worldwide are used both as a source of bottled water and a recreational resource (spa facilities, tourism, etc.).