Contributions for the Understanding of the São Pedro do Sul (North of Portugal) Geohydraulic and Thermomineral System: Hydrochemistry and Stable Isotopes Studies

Abstract

São Pedro do Sul thermomineral aquifer system is located in the North of Portugal. Hydrogeochemistry and isotopic studies were conducted in order to improve knowledge of this groundwater system, known since ancient Roman times for their therapeutic properties. One thermomineral spring (NT) and three boreholes (AC1, SDV1 and SDV2) have a mean temperature around 68 C. Currently, these waters are mainly used in thermal spas and for geothermal energy supply. Major cations and anions were used to identify and characterize different water types and sub-systems. Stable isotopes composition, 18O and 2H, have been used to determine the origin and have been used as a first approach to estimate the mean preferential recharge altitude of thermomineral water. The results suggest a single aquifer system with a relative composition of cations and anions and similar mineralization values. 18O and 2H values indicate a meteoric origin with no significant evaporation before infiltration. Besides, the isotopic composition points to recharge areas located at altitudes of about 1000 m a.s.l. This suggests a recharge area in the Freita/Arada mountains located NW of the thermomineral poles.