## 2 Case Study Bokanjac-Policnik  
Bokanjac-Policnik is the coastal karstic aquifer located in the area called Ravni kotari, which is part of the northern Dalmatia in Croatia (Figure 1). Bokanjac-Policnik covers an area of 302 km\({}^{2}\)[20]. The Bokanjac-Policnik aquifer is formed in the carbonate rocks of the Adriatic carbonate platform with a characteristic scalp structure, in which water-permeable carbonate rocks and water-impermeable clats alternate (Figure 2) [21]. The core of the anticlinal parts is built of limestones of the Upper Cretaceous age, and the synclinal flysch of the Paleogene age. The spread of scalp forms is northwest-southeast, and it is also the direction of spreading of rocks of different hydrogeological characteristics [22]. In the area of the Bokanjac-Policnik catchment area, several tracing tests were performed for the purpose of determining sanitary protection zones, determining the catchment area of water supply facilities, but also assessing the impact of individual facilities on water pumping stations and water supply springs/wells (Figure 2). Groundwater in the Bokanjac-Policnik catchment area flows parallel or subparallel to geological structures in the southeast-northwest direction in two separate branches. The first branch is on the northeast side of the flysch barrier that stretches from Smokovic to the northwest following the provision of structures. The majority of groundwater of this branch feeds the coastal well Golubinka and the water pumping station Boljkovac. Only after the zone where the flysch barrier emerges does the groundwater move perpendicular to the spread of the structures, towards the Jezerce water pumping station. The second branch drains towards Bokanjacko blade and further towards Boljkovac and the northwestern part of the structure where groundwater diffusely infiltrates into the sea near Zaton and Nin. Figure 1: Position of Bokanjac-Policnik catchment area in Croatia. The main surface watercourse in the catchment area is Miljasic jaruga and, in addition to its natural immediate basin, surface waters also characterize its most important tributary, Bokanjacko blato, with which it is artificially connected by a hydrotechnical tunnel. The natural conditions of surface and groundwater dynamics have been partially changed by the construction of a tunnel which, since 1963, drains the floodwaters of Bokanjacko blato towards Miljusic jaruga, which prevents the formation of temporary lake in Bokanjacko blato [20]. Prior to the construction of this facility, the area of Bokanjacko blato was occasionally flooded and slowly swallowed into temporary swallow-holes in the northwestern part of the field. With the construction of the tunnel, the area of Bokanjacko blato is no longer flooded, thus enabling permanent agricultural activity in this area. Additionally, irrigation and drainage canal systems have been built for this purpose. However, this intervention also changed the conditions of groundwater recharge, i.e., reduced the positive impact of infiltrated surface waters on the hydrogeological conditions in the aquifer. Drainage of high waters of Bokanjacko blato towards Miljasic jaruga by the hydrotechnical tunnel affected the condition of groundwater in the area of Bokanjacko blato. A decrease in the pumping capacity of wells in the area of Bokanjacko blato was observed due to reduced infiltration of aquifers with flood waters. With reduced recharge of the aquifer, caused by drainage of surface water towards the Miljasic jaruga, the problem of the occasional impact of the sea on the Jezerce well began to appear, and even impact, although minimally, the Bokanjac pumping station. To prevent this, in the mid-1970s a grout curtain about 40 m deep was built downstream from Bokanjacko blato [20]. The main function of this facility was to cut fracture systems in the aquifer, i.e., to prevent salinization of water supply facilities in Bokanjacko blato area (Jezerce, Bokanjack), as well as a deeper intrusion of salt seawater into the aquifer.