The complex hydrogeological transboundary catchment of Southern Dalmatia and Western Herzegovina, located in two neighbouring countries, Croatia and Bosnia and Herzegovina, covers an area of 2300 km2. In addition to numerous previous hydrogeological studies, a new research was carried out from September 2013 to September 2020 for the purpose of this thesis. During this period monthly sampling of five spring and surface water locations in Western Herzegovina, 20 locations of spring and surface water in Southern Dalmatia and six locations of spring and surface water at Baćina lakes was performed. Additional research included hydrogeological mapping, a simultaneous tracing experiment (on September 30th and October 2nd, 2014), and stable sulphur δ34S analyses from SO4- ions in water samples, taken during hydrological maximum (April) and minimum (October) of 2019 at ten locations. The study area is composed of sedimentary rocks of Permian, Triassic, Jurassic, Cretaceous, Palaeogene, Neogene and Quaternary ages. The oldest, Upper Permian rocks have been tectonically uplifted to the surface through diapirism and reverse faulting N of the studied catchment area and consist of evaporitic rocks with some carbonate and clastic rocks (sandstones and marls) interbeds. Hydrogeologically they could potentially represent a barrier to groundwater flows, but since they occur outside of the study area they do not influence hydrogeological relationships. However, a very locally restricted diapir in the northern part of the study area as well as extraordinary high sulphur concentrations at some locations indicate probable diapiric uplifts also in the studied area. Triassic deposits, in large part composed of dolomites, form partial barriers to water circulation within the study area. Jurassic rocks, dolomites and limestones. Cretaceous deposits, mostly composed of bedded to massive limestones with rare dolomites prevail in this area. Palaeogene and Neogene deposits consist of a wide range of lithologies, but clastic rocks (mostly sandstones and marls) predominate, especially Eocene flysch deposits which have a diverse impact on groundwater movement. Quaternary deposits cover numerous karst poljes, as well as areas along streams and rivers. The most permeable rocks in the study area are Mesozoic limestones and dolomites, while the least permeable rocks belong to flysch and Quaternary deposits. A complex underground conduit system and extreme karst forms heterogeneity are typical for the study area, characterised by very variable relief from mountains and hilly parts, river valleys and karst poljes to the Adriatic coast. The study area can be divided into five or six hypsometric levels. The first or the lowest level is the discharge zone almost at the sea level. iii This zone extends from Gradac and the nearby coastal karst spring Mandina Mlinica, across the Baćina Lakes and the Klokun 1 spring to the Neretva River valley and its marginal karst springs Prud, Modro oko, Čeveljuša, and several smaller springs. Vrgoračko polje is part of the second level (25 m a.s.l.) and Rastok polje of the third level (70 m a.s.l.). The fourth level includes springs in the Trebižat–Mlade–Tihaljina River valley in Bosnia and Herzegovina (120–180 m a.s.l.), while the fifth level, the highest one, stretches across the Imotski–Bekija polje (260 m a.s.l.). In the nortwesteern part, the fifth level comprises the wider area of the Posuško polje (570–620 m a.s.l.), but the highest part of the area around the Rakitno polje (900 m a.s.l.) could be separated as the sixth level. The study area is abundant in high-quality spring water, of which only a small amount is used for public water supply. Over a hundred temporary and permanent karst springs occur in the area with annual discharge of the largest springs (Klokun 1, Butina, Modro oko, Borak, Vrelo Tihaljine, Prud, Opačac, Klokun 2) ranging from 1.5 to over 10 m3/s. Rivers Ričina, Suvaja, Sija, Vrljika, Tihaljina, Sita, Mlade and Trebižat actually represent the same river characterized by repeated springing, flowing on the surface, sinking, and underground stream sections, along its 106 km long course from the highest spring to its confluence with the Neretva River. Most of the surface waters in the area flow towards the Neretva River, but part of groundwaters flow southwards. The recharge zone of this large catchment area is predominantly in Bosnia and Herzegovina, whereas the final discharge zone is along the Croatian coast and Neretva River valley. During the study for this thesis an integrated hydrogeological map of the whole catchment area has been made by combining four basic geological maps and attributing specific hydrogeological characteristics to all geological units. Furthermore, further delineation of the catchment boundaries and the catchment area was performed, which is an extremely complex task in the karst areas. In order to define external and internal boundaries of such a large and complex catchment area different methods were used, including detailed study of all available geological and hydrogeological data, hydrogeochemical properties of individual springs, hydrological data, and tracing tests data combined with water balance calculated in previous reports. Out of a large number of reviewed studies and published papers on the study area very few of them included the simultaneous monitoring of a large number of springs in both countries. In addition, a simultaneous tracer test was carried out for the purpose of this thesis. iv Since some of the previously determined groundwater connections confirmed by tracing experiments were cutting previously defined groundwater boundaries within some of the subcatchments, it became evident that a new definition of some subcatchments should be introduced. Up to now there were no research of stable isotopes of oxygen 18 (δ18O), deuterium (δ2H) and sulphur 34 (δ34S) within the study area. Those natural stable isotopes are especially important for the determination of the water age and origin, especially in areas where artificial tracing is not feasible at the scale of the regional catchments. Moreover, as evaporite diapirs crop out in the vicinity of the study area and increased levels of sulphur were determined at some studied locations, it was assumed that some diapirs exist in the shallow underground of the research area in contact with groundwater paths, enriching Southern Dalmatia and Western Herzegovina groundwater with sulphate ions. Hence, for the first time in the study area, stable δ34S isotopes from sulphate ions in water samples were analysed to determine their origin. Therefore, the conducted hydrogeological investigations of the studied large catchment area were carried out using several methods to fulfil all the objectives of this thesis. The methods used included: 1) measurements of various water parameters, such as temperature, electrolytic conductivity, and water level measurements with automatic loggers, 2) in situ measurements of physico-chemical parameters, 3) water sampling and laboratory analyses for the determination of the ion composition, 4) analyses of δ18O, δ2H and δ34S stable isotopes, 5) flow measurements, 6) simultaneous tracing with artificial tracers, and 7) collection of available data on precipitation, discharge and microbiological parameters. Delineation of a large and complex karst catchment into six subcatchments from which surface and groundwater flow in different directions was the main result of this thesis. Subcatchments are defined according to their specific geological setting and hydrogeological relations, but their boundaries should not be considered as strict water divides because groundwater or surface water may intersect them during different hydrological conditions. Yet, they are fundamental for water balance calculations and estimating influence on even distant karst springs. Furthermore, new term redefining subcatchments as influential areas was introduced. The proposed term can be used for the description of comparable large karst catchments characterised by interaction between subcatchments. In such subcatchments groundwater fluctuates depending on the water table levels affected by changes in precipitation. Given the hydrogeological conditions during late winter and spring, when the snow melts and v precipitation significantly increases, groundwater often discharges across the defined boundaries. Therefore, the term subcatchment in this case, as in most of the Dinaric karst terrains, can only be used tentatively for easier understanding of the topic, while the term influential area could be far more appropriate. Results of stable δ34S isotopes composition from the measured sulphate ions were compared with the Permian evaporites δ34S isotope curves from the literature, which supports the possible sulphate ions origin from permian evaporites in Western Herzegovian and Southern Dalmatian groundwaters. Since the main recharge area of the studied large system is located in Western Herzegovina and the final discharge zone in Southern Dalmatia, the groundwater mostly passes through karstified carbonate rocks prevailing in the entire area, but is probably partly influenced by underground deposits of evaporite rocks that were probably uplifted close to the surface in the form of diapirs. Results of stable isotops analyses of sulphur from the sulphate ion in different hydrological conditions indicate a rapid reaction of the system to precipitation and mixing of dissolved sulphate from evaporites (mostly gypsum) with rainwater and possibly polluted water (sewage water). The presented comprehensive hydrogeological study has shown that the studied system has a zone of underground and surface runoff (sometimes intertwined) between the the Neretva River valley and hydrogeological barrier of flysch deposits along the Adriatic coast. Furthermore, research indicated that most of the studied major springs recharge from the same aquifer in their hinterland. Higher concentrations of sulphate ions in several springs of the Southern Dalmatia also indicate recharging from more distant areas – mainly W and N of Trebižat–Mlade–Tihaljina River, where evaporite rocks are cropping out and where the domination of sulphate ions in water was documented. Seawater impact was recorded in some coastal springs and surface waters, indicating fresh–seawater interaction. Such springs are located closer to the flysch barrier edge, in contact with permeable carbonates. Further to the east, the effect of underground penetration of the seawater decreases. In addition, multivariate mathematical cluster analysis of hydrogeochemical data shows two main clusters within the system, lithogeochemical cluster and marine cluster, while multivariate mathematical factor analysis separated three factors – seawater influence factor, carbonates and evaporates dissolution factor, and biological/chemical reactions factor. Determined three main hydrochemical facies within the investigated area (calcium–hydrogen carbonate facies, calcium–sulphate/hydrogen carbonate facies, and sodium–chloride facies) reflect the chemical response of lithology and groundwater flow pattern. vi Physico-chemical parameters of spring waters indicate shorter water retention time and domination of conduit flow, whereas recession curve analysis indicates rapid drainage of the aquifer due to immense karstification. Time-series data (cross-correlation analysis) indicates a rapid response to precipitation. All aforementioned analyses provided insight into the main aquifer characteristics and represented a basis for the interpretation of δ34S stable isotope data. Obtained results could improve the quality of the establishment of sanitary protection zones, as the new tracing experiment proved that the groundwater has much higher apparent velocity than considered previously, as well as better management of the, studied and similar transboundary catchments, which can often be challenging due to differences in development and policies between different countries. Thesis could be of great importance for the future management of this transboundary catchment because ensuring high-quality groundwater is crucial for sustainable development and protecting the ecosystem integrity of each country. In conclusion, the local and regionally important scientific contribution of the Ph.D. Thesis is manifested in: (1) Detailed determination of Southern Dalmatia and Western Herzegovina transboundary catchment area hydrogeological properties; (2) Newly proposed delineation of the studied large and complex karst catchment; (3) Establishing a new classification of subcatchments as influential areas applicable to other complex karst terrains; (4) Introducing sulphur δ34S stable isotope analysis as an important research methodology fa or complex karst catchment area of the Outer Dinarides.