5 Carpathian Journal of Earth and Environmental Sciences, May 2015, Vol. 10, No 2, p. 5 - 17 KARST SYSTEMS WITHIN THE SOUTHERN CARPATHIANS STRUCTURE (ROMANIA) Ioan POVARĂ 1 , Mihai CONOVICI 2 , Cristian-Mihai MUNTEANU 1 , Constantin MARIN 1 & Elena Daniela IONIŢĂ 3 1 "Emil Racoviţă" Institute of Speleology of the Romanian Academy, Frumoasă Street, 31, Bucharest, Romania ipov.iser@gmail.com 2 Geological Society of Romania, Nicolae Bălcescu Avenue, 1, Bucharest, Romania 3 National Mineral Water Society, Water Analysis Laboratory, Iuliu Maniu Avenue, 19, Bucharest, Romania Abstract. 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, Ca 2+ and HCO 3 – (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. Keywords: Southern Carpathians, Mesozoic limestones, graben, karst systems, hydrogeochemistry. 1. INTRODUCTION The Cerna River catchment area extends on over 555 km 2 (Fig. 1), out of which 88.56 km 2 (16%) are covered by Mesozoic limestones. The main landforms are represented by mountains, higher than 2200 m in the north, but only 130 m high in the south. The non-karst rocks (which outcrop on more than 400 km 2 ) allow a permanent surface flow towards the lithological contact with the limestones, where the streams sink in the underground. The groundwater systems are discharged by karst springs with cumulated flow rates of over 10 m 3 /s. According to the classification issued by Mangin (1975), the karst aquifers from the Cerna River catchment area can be assigned to either the type III (e.g. the Cerna Spring, with a stronger karst behavior in the upstream sector), or the type V (e.g. the Pişetori Springs, the Seven Cold Springs and the Domogled Spring, with non-karst behavior). The limestones outcropping within the area of the Băile Herculane Spa play a very important role in the functioning of the thermo-mineral aquifer. 2. GENERAL GEOLOGICAL DATA The area is covered by rock formations pertaining to two main tectonic structures of the Southern Carpathians: (1) The Getic Nappe, which includes meso-metamorphic and Paleozoic and Mesozoic sedimentary formations, that transgressively overlie the crystalline schists of the Godeanu