The origin and water quality of spring systems in Monchique, Portugal: A focus on Long-Term sustainability and elevated sodium levels

Spring water systems in Monchique, SW Portugal, not only serve diverse local utilities–from thermal baths to bottled water–but also represent a microcosm of a global concern: elevated sodium levels in spring-sourced bottled waters. This research employs hydrogeochemical and isotopic analyses to investigate the origin, hierarchy and quality of the springs offering a new focus on elevated sodium concentrations, a known global cardiometabolic risk factor. The springs arise from a complex hydrogeological system formed by extensive faulting between argillaceous country-rocks and intruded Cretaceous syenites. Here we show that thermal springs with temperatures of 23–31 °C, have a groundwater age of 5000 years, indicative of extended water–rock interactions and recharge at elevations of 450 to 650 m revealing a hitherto unreported resilience in regional groundwater hydraulics. Conversely, cold springs recharging from 200 m to the area’s peak at 900 m follow shorter paths within the regolith influenced by shallow geological features. The study detects nitrate contamination from surface sources in the cold springs and relatively high sodium levels in the geothermal springs that approach or surpass guideline values. Elevated sodium is attributed, for the first time, to long-term interactions and cation exchange with Na-rich syenites, possibly intensified by heat-induced dissolution at depth. This study informs current perspectives on the long-term sustainability and public health implications of spring water systems. The ion chemistry and isotopic evidence suggest that thermal springs, due to their longer circulation times, are less vulnerable to climatic shifts compared to the more susceptible cold springs. While offering specific insights into the Monchique springs, the research has broad implications to similar high-sodium thermal springs across the Iberian Peninsula and potentially worldwide.