## 1 Introduction  
Groundwater is a priceless resource for high-quality freshwater in Croatia and all around the world. Dinaric karst in Croatia is one of the most known karst localities in the world and several terms connected with karst originate from it [1]. Events during the Quaternary are interesting for the Dinarides area as they directly influenced the development of water systems in today's coastal area. During the ice ages, the level of the Adriatic Sea was up to 150 m lower than today [2]. In the conditions of low sea level, at the beginning of the Quaternary, the processes of karstification began. As the sea level rose, the base of the karstification also rose, filling previously fracture-cavernous systems with water. In this way, the sea penetrated the marginal mountains and, due to the different specific weights of salt and fresh water, filled the deep parts of karst aquifers. Thus, in the coastal parts of the aquifer we can expect karstified carbonate rocks up to about 150 m deep, which, if the geological structures are without lower permeable deposits or rocks and opened to the sea, allows direct contact of salt sea and freshwater of coastal aquifers, forming a labile dynamic balance of salt and fresh water in the aquifer. This balance of salt and fresh water can be disturbed during extremely dry periods, by reduced aquifer recharge, climate change or excessive groundwater exploitation, and lead to the displacement of the salt and freshwater mixing zone towards the mainland. This salinized part of the coastal aquifer and prevents its use for water supply or irrigation. As groundwater moves through rocks, its chemical composition normally changes, and the longer groundwater remains in contact with aquifer rocks the greater the amount of material it will take into the solution [3]. There are many obstacles in water supply systems that use groundwater in coastal aquifers, and seawater intrusion is a major one [4, 5, 6, 7], referring to varying degrees of mixing saltwater with groundwater. Seawater intrusion is higher in summer months when water consumption rises due to the duration of the tourist season, and at the same time recharge of the aquifer is decreased during the dry summer season [4]. In addition to the exploitation of groundwater from coastal aquifers,climate change also contributes to salinization. This mostly refers to the higher air and water temperatures, as well as the increase in the intensity of precipitation, which enhances the surface runoff and reduces infiltration and recharge of the aquifers [8]. The quality of groundwater depends on chemical and isotopic parameters and is the subject of research in many scientific and professional studies [9; 10; 11; 12]. The most widely used salinity indicators of coastal aquifers are chloride and sulfate concentrations and the value of electrical conductivity. Concentrations of chloride and sulfate greater than 150 mg/L, and values of electrical conductivity greater than 1 mS/cm, are usually taken as limits when coastal aquifer being under the influence of salinization [13]. Elevated values of these indicators compared to the average values indicate salinization, and their advantage is that they are very easily and cheaply measured. Mediterranean seawater is characterized by the salinity of 40 g/L, while seawater, in general, has uniform chemistry due to a long residence time of major constituents with the predominance of Cl\({}^{-}\) and Na\({}^{+}\)[14]. In the carbonate aquifers, the interpretation of the chemical content of the groundwater is represented as a complex image of geochemical evolution due to the mineralization and salinization [15]. The ion composition of fresh groundwater in coastal carbonate aquifers is typically dominated by Ca\({}^{2+}\) and HCO\({}_{3}\)\({}^{-}\) which arise from the dissolution of calcite, and the cation exchanger is also dominated by adsorbed Ca\({}^{2+}\)[16]. Small increases in sodium chloride in groundwater near the coast can be detected and may indicate a landward movement of the mentioned mixing zone.