Isotopic and Geochemical Signatures of Melgaco CO2 Rich Cold Mineralwaters, NW Portugal

The isotopic and chemical compositions of CO2 rich mineral waters found in the NW of Portugal were investigated. These mineral waters are mainly related to granitic and granodioritic rocks. Based on their chemical composition, two water types are distinguished (Ca-HCO3 and Ca-Na-HCO3), indicating different underground flow paths. Through comparison with local shallow groundwaters, water chemistry indicates that the Melgaco mineral waters evolved through water-rock interaction with the hosted rocks. Stable isotope data indicates the meteoric origin of these CO2 rich mineral waters, being recharged from about 480 up to 730 m a.s.l. Considering the δ18O, δ2H and the hydrochemical data, no indication of mixing seems to occur between the shallow and deep groundwater systems. The δ13C determinations carried out on TDIC of the CO2 rich mineral waters point to the hypothesis of methanogenesis (upper mantle CH4 source) within the system, leading to 13C enrichment. The negligible 14C content (∼ 2 pMC) also indicates a mantle derived carbon source for the groundwater system. (author)