In this work, we analyzed the results of a geochemical analysis aimed to define the origin of pH anomalies (pH > 11) in water samples collected inside a tunnel located in southern Calabria (southern Italy). We also analyzed the precipitates found close to the main drainage pipes. The hydrogeochemical study allowed us to identify a main NaOH water facies for the many samples collected close to the tunnel. In addition, the correlation diagrams highlighted high concentrations of Na, K, and Al, unrelated to simple water-rock interaction. Further evaluation excluded the possibility that interaction between the water and the outcropping lithologies was the only cause of the ongoing processes. This consideration is supported by the high Na and K concentrations, which cannot be accounted for by interaction between water and calcareous marl. Excluding a natural origin and some anthropogenic factors, one possible explanation is an interaction between the groundwater and the mortars used for consolidation during the excavation phase of the tunnel. Mortar and concrete degradation in aqueous environments produces a great increase in pH, initially deriving from interstitial fluids containing strong alkali (NaOH and KOH) and non-negligible K and Na concentrations, such as we observed in the collected samples.