Contribution of Hydrogeochemistry and Isotope Hydrology (δ²H and δ¹⁸O) to Update the Conceptual Hydrogeological Model of the Hyposaline Natural Mineral Waters of Ribeirinho and Fazenda do Arco (Castelo de Vide, Central Portugal)

In this paper, the conceptual hydrogeological circulation model of natural mineral waters from Ribeirinho and Fazenda do Arco hydromineral concession (Castelo de Vide) is updated. These waters are exploited by the Super Bock Group, as bottled waters, and are commercially labelled as Água Vitalis. The physico-chemical data (2004 – 2024) of these waters were processed regarding their joint interpretation with recent isotopic (δ2H and δ18O) data. The study region is dominated by the Castelo de Vide syncline that develops along the southern limit of the Central Iberian Zone. These natural mineral waters have low electrical conductivity (EC) mean values (42.80 < ECmean < 54.45 μS/cm) and a slightly acidic pH (5.14 < pHmean < 5.46), making them hyposaline waters. The recharge area of this aquifer system coincides fundamentally with the outcrops of Lower Ordovician quartzites. The updated conceptual circulation model presented in this work is essentially based on the chloride-sodium signatures of these waters, explained by the preferential recharge of meteoric waters (δ2H and δ18O) and low water-rock interaction temperature. Such isotopic results seem to indicate the non-existence of a flow continuity between the two blocks (NW and SE) of the quartzite ridges, separated by a fault with a local orientation approximately N-S, as indicated by the most enriched isotopic values of the waters from borehole AC22 (δ18O = -5.90 o/oo vs. V-SMOW) located in the SE block, compared to the average isotopic value of the waters from the other boreholes (Vitalis I, II, III, IV, V and VI) located in the NW block (δ18Omean = -6.30 o/oo vs. V-SMOW).