Baseline hydrogeochemical characterisation of a vulnerable pristine highmountain karst aquifer in the southeastern Pyrenees

High mountain karst aquifers located in pristine zones are often an important source of water supply downstream. These hydrological systems with typically short transit times are very vulnerable and there is a general consensus regarding the necessity of their protection. Despite of that, most of these high-mountain hydrogeological systems are not sufficiently characterized neither well understood. From the perspective of climate change impact on groundwater resources, karst aquifers located in the drought-prone Mediterranean areas are currently on the focus of research. The predicted scenarios of both an increasing of temperatures and a decreasing of precipitations may likely impact aquifers recharge. In this regard, a baseline hydrogeochemical characterization of these high mountain aquifers is the first step to understand their hydrological behavior, and therefore adapting to climate change in relation to groundwater resources. This research is focused on the Port del Comte (PC) carbonate massif, which is located in the southernmost part of the Catalan Pyrenees (northeastern Spain). The elevation of the massif ranges from 900 to 2387 m a.s.l, and it covers an area of approximately 110 km2. The PC constitutes an independent structural and regional hydro-geological unit, and contains one of the most important karst aquifers of the Catalan Pyrenees, formed by Lower Eocene – fissured and karstified limestones and dolomites. No previous studies regarding the geochemical and isotopic groundwater baseline characterization of this aquifer are available. This work presents the results of the groundwater and precipitation sampling campaigns conducted during the period Oct 2013 – Oct 2015. A total of 43 springs were visited for groundwater sampling twice per year (i.e. before snowfall and after snowmelt seasons). The hydrogeochemical evolution of groundwater is specifically studied in six karst springs, which were regularly sampled every three to four weeks. In all cases, the "in-situ" physico-chemical parameters (pH, EC, T, redox, alkalinity, TDS) were measured. The geochemical analysis considered major ion and trace elements and isotopic composition (δ2H.H2O, δ18O.H2O, δ34S.SO4, δ18O.SO4, δ15N.NO3, δ18O.NO3) of spring groundwater and precipitation (δ2H.H2O, δ18O.H2O). The hydrogeochemical facies of the sampled waters range from low mineralized Ca-HCO3 waters to Ca- SO4 and Na–Cl water types. Results are discussed in terms of the spatio-temporal variations in the hydrogeochemical characteristics. Acknowledgements: The projects REMEDIATION (CGL2014-57215-C4-1-R) and PACE –ISOTEC (CGL2017-87216-C4-1-R) co-funded by the State Research Agency (AEI) of the Spanish Government and by the European Regional Development Funds (ERDF). The project EFA210/16 PIRAGUA, cofounded by the European Regional Development Fund (ERDF) through the Interreg V Spain-FranceAndorre Programme (POCTEFA 2014-2020) of the European Union. Thanks also to research groups MAG (UB) and GREMS (UPC) funded by the Government of Catalonia and GHUMA by the Government of Andalusia.