The Cerna River catchment area is located in the south-western part of the Southern Carpathians, along the Cerna Miocene Graben, within which outcrop Cretaceous formations pertaining to either the Danubian Autochthonous or the Getic Nappe. Consequently, secondary structures characteristic to the extensional tectonics have resulted. These structures have been involved in the groundwater flow, leading to the high flow rates recorded for certain karst springs (Pişetori, Seven Cold Springs, Domogled) and to the low flow rates of several cross creeks. On the western slope of the valley, close to the master fault, the karst aquifer complex, developed within the Mesozoic limestone, is strongly influenced by thermal phenomena. Along the valley, on 25 km, to the south, towards the Băile Herculane area, 28 thermal sources (10 wells and 18 springs) have been identified. Structural and tectonic relationships between the deep-seated, brittle granitic bedrock and the limestone cover, capped by Cretaceous argillites, are very important for the dynamics of the thermo-mineral reservoir. There is a clear-cut distinction between the karst springs and the thermal sources, in terms of both hydrochemical facies, and dissolved minerals. The chemical analyses of the groundwater samples collected from the Cerna River catchment area outline clear hydrochemical differences between the karst springs and the thermo-mineral sources. In the karst groundwater, Ca²⁺ and HCO₃^– (up to 200 mg/L) are the prevailing ions. The thermo-mineral water is of Na-Cl type; its mineral content exhibits a markedly increasing trend from the north to the south (from 170 to 8000 mg/L). There is an obvious correlation between Na⁺ and Cl^- concentrations, indicating a mixing process between a common saline end-member and a less mineralized shallow water.