Environmental hydrogeochemistry assessment as a tool for sustainable hydromineral resources management (Entre-os-Rios thermal baths, NW Portugal)

Portugal is a country rich in hydromineral resources mostly encompassed by alkaline sulphurous groundwater. These waters are mostly confined in the so-called Ancient Massif and result from the infiltration of meteoric waters that may reach great depths giving groundwater special physicochemical characteristics. The main aim of the research was to evaluate the hydrogeochemical evolution of the sulphurous mineral waters of Entre-os-Rios, as well as to achieve a better knowledge of the hydromineral system and to improve its hydrogeological conceptual model. Entre-os-Rios thermal baths are sited in NW Portugal and are recognised for their thermal spa tradition that dates back at least to the middle of sixteenth century. The hydromineral resources of Entre-os-Rios emerge from a confined granitic aquifer, deep seated, and are geotectonically controlled. Hydrochemical data was collected, both from the main mineral spring waters (Torre, Curveira, Arcos Esquerda, and Arcos Direita) and Barbeitos well. Hydrochemical analyses were gathered, including organoleptic characteristics, physicochemical properties, major anions, and cations and minor elements from the 1938 to 2017 periods. In addition, some historical data, from late 19th to early 20th centuries, were included. Moreover, some isotopic hydrology data were discussed (oxygen-18, deuterium, and tritium). Entre-os-Rios groundwater is colourless, with no turbidity, and has a foul odour of rotten eggs, has a low temperature, 17°–21 °C, is alkaline, pH values 8.4–8.8, a total mineralisation lower than 500 mg/L and a high total sulphuration, about 24 mg/L. Major anions follow the tendency HCO₃ > Cl > F > SO₄ > CO₃, with high concentrations of fluoride, 18–21 mg/L and major cations follow the tendency Na > Ca > K > Li > NH₄ > Mg. Among minor elements, boron, caesium, and tungsten have the highest concentrations, 986 µg/L, 247 µg/L, and 186 µg/L, respectively. The dominant hydrogeochemical facies are Na–HCO₃. Isotopic data indicate a meteoric origin for Entre-os-Rios mineral waters, with a long residence time in the aquifer system, and that these waters are, most probably, sub-modern, recharged prior to 1952. The mineral waters of Entre-os-Rios are one of the most important sulphurous groundwater in Portugal and their hydrogeochemical status exhibited a good equilibrium through the last 100 years.