Assessment of Groundwater Dynamics and Its Relationship to Alpine Ponds (serra Da Estrela, N Portugal): Coupling Isotopic and Hydrogeochemical Signatures

Serra da Estrela Natural Park is situated on the highest mountain in central Portugal, being the climate of this research region strongly affected by its geographic location and geomorphology. To find out possible relationships between shallow groundwater systems and alpine ponds, groundwater samples from 15 springs were collected between 2010 and 2011 along six sampling campaigns and chemical and isotopic determinations were performed. In the same mountain sector 7 alpine ponds were also sampled and analysed. From the isotopic data of the spring waters, it was possible to establish the Local Meteoric Water Line (ẟ2H = 6.58 ± 0.29 ẟ18O + 1.31 ± 1.23). An isotopic fractionation of the water vapour air masses with the altitude was also identified: 0.13‰/100 m from the oxygen-18 values. The sampled shallow groundwaters are low mineralized and belong to the Cl-Na-type, reflecting small water–rock interaction and short underground flow paths. Based on the chemical signatures and mineralization increase in some shallow groundwaters, it was possible to identify the presence of anthropogenic pollution ascribed to snow melting by de-icing salts in the roads. In addition to precipitation, the isotopic composition of some alpine ponds shows signs of recharge/contribution from shallow groundwater, identified by a depletion in isotopic composition (δ2H and δ18O values); in parallel to this isotopic depletion, the increase in surface water mineralisation is only feasible if a contribution from snowmelt and road de-icing salts to these surface systems is assumed, enhancing the existence of anthropogenic contamination.