Abstract:Lička Jesenica is a sinking river situated in the Dinaric karst environment of the Lika region of Croatia. The two main karst springs, Veliko Vrelo and Malo Vrelo, provide the majority of Lička Jesenica's water. Because of the quality and abundance of the water they contain, these springs are strategically important to the public water supply. Previous knowledge regarding the springs`karst system has been negligible. Therefore, a bulk hydrogeological research program was conducted with the purposes of establishing protection zones and assessing the springs' water-supply potential. The research work comprised the following methods: hydrogeological mapping, setting up of monitoring stations for evaluating discharge and water quality, time series analyses (hydrograph-chemograph), hydrochemical analyses, and geophysical surveys. The main results and conclusions of the research include the following: (i) confirmation that both springs drain a common aquifer; (ii) determination of the highly vulnerable nature of the karst systems; (iii) identification of the Veliko Vrelo Spring's more direct connection to the aquiferdrainage system in comparison to the Malo Vrelo; (iv) estimation of the positions and depths of the springs`main feeding conduits; (v) confirmation of the extent of the geologically presumed catchment area; and (vi) finally, establishment of sanitary protection zones.

Monitoring of various naturally present substances or physical properties of the water, commonly called environmental tracers, can provide valuable insight in characteristics of groundwater flow systems and intrinsic processes. Analysis of individual tracer data most often gives an ambiguous interpretation, but employment of multiple diverse tracers can greatly increase interpretation reliability. We monitored multiple natural tracers including spring water electrical conductivity, temperature, loads of major anions and cations, stable isotopes of water, and total organic carbon together with discharge dynamics on Krbavica springs located in the Croatian part of Dinaric Karst region. We also monitored dissolved oxygen concentration as an indicator of “excess air” dynamics in the spring water, which is a very rarely recognized and not properly understood phenomenon in the karst groundwater studies. Analysis of the monitoring data revealed main characteristics of the karst system, among which following can be emphasized: (1) oxygen concentration together with conductivity and temperature (parameters monitored with high temporal resolution) were strongly related to discharge dynamics, while seasonal patterns were absent; (2) supersaturation with oxygen confirmed presence of “excess air”, most pronounced in high water conditions following the hydrograph peaks, indicating prevalence of closed flow conditions within the system; (3) electrical conductivity showed “anomalous” gradual decrease during the hydrograph recessions, attributed both to CO2 dynamics and mobilization of water from tiny fissures during high recharge conditions; and (4) stable isotope measurements confirmed good mixing of water within the system with mean residence time of a few years. Simultaneous monitoring of diverse tracer dynamics enabled detailed characterization of the karst system without excessive ambiguity.

Journal of the Croatian Geological Survey and the Croatian Geological Society heterogeneous hydraulic properties manifested by rapid groundwater flows concentrated in conduits and disolutionally widened fractures, usually sparsely distributed, while the storage of groundwater is mainly related to surrounding fractured rock matrix and surficial epikarst (or subcutaneous) zone (GOLDS-CHEIDER et al., 2007). Because of its complexity, karst aquifer research necessitates an interdisciplinary approach (DREYBRODT, 1988; COVING-TON et al., 2011). In addition to conventional methods, the use of environmental stable isotope tracers supplemented by statistical analysis, contributes to a better understanding of karst hydrogeology (CRISS et al., 2007; PALMER, 2010; GRIMMEISEN et al., 2017). Stable isotopes of water (2 H and 18 O) are analysed for several reasons, e.g. to explore recharge processes and reservoir mixing (LONG & PUTNAM, 2004; MALOSZEWSKI et al., 2002), to explore vadose water mixing, residence times and storage in non-conduit parts of karst systems (PALMER, 2010; MO-ERMAN et al., 2014; DÓŠA et al., 2011), to estimate mean recharge area elevations (BARBIERI et al., 2005; CHARIDEH & RAHMAN, 2007), etc. Detailed knowledge of the various processes within a karst hydrological system is crucial for the sustainable management of water resources, their allocation, and protection against pollution (MANCE et al., 2018 a, b). The black-box approach is often used

Crystalline rocks are generally characterized by negligible porosity and permeability in terms of groundwater exploitability. However, alteration processes can greatly increase their fracture permeability and induce formation of modest, but locally important aquifers. Therefore, subsurface characteristics of alteration zones are of major importance for hydrogeological evaluation of crystalline terrains. Alteration processes greatly affect rock total porosity and water content, causing contrasting electrical resistivity of rocks affected by varying degrees of weathering. This makes electrical resistivity tomography (ERT) a preferable geophysical method for the exploration of alteration zones in crystalline rocks. In our research, we used an integrated approach, combining the ERT method with monitoring of spring discharge and hydrochemistry to characterize metamorphic aquifers on slopes of the Medvednica Mountain (Croatia). Significant fracture flow aquifers are found to be formed in intensely fractured but not highly weathered rock masses (medium to high resistivity values), while highly weathered masses (low resistivity values) form local barriers for fracture flows. Subsurface structure of the alteration zone proved to be highly irregular, with sharp contacts between more and less weathered rocks. Decrease of permeability below the alteration zone keeps the water level near the surface and enables spring occurrence on the mountain slopes. Studied aquifers have relatively limited extent, resulting in typical capacity of major springs of a few l/s. More frequent but less productive springs are attributed to the draining of the shallow part of the alteration zone (mostly saprolite). Combination of the ERT method with spring monitoring proved to be very effective as a first and relatively inexpensive methodology for hydrogeological characterization of crystalline terrains, both in local and catchment scales.

Extreme heterogeneity of karst systems makes them very challenging to study. Various processes within the system affect its global response, usually measured at karst springs. Research conducted in caves provides a unique opportunity for in situ analysis of separate processes in karst underground. The aim of the present study was to research the water and air dynamics within a deep karst system. Air and water basic physical parameters across the Lukina jama-Trojama cave system (−1,431 m) were continuously monitored during a 1-year period. Recorded hydrograph of the siphon lake at the bottom of the cave was used to interpret the characteristics of an unexplored phreatic/epiphreatic conduit network. Water origin in the siphon was determined based on temperature and electrical conductivity. Air temperature and humidity monitoring revealed a strong inflow of air of sub-zero temperature into the upper portion of the cave during winter. Cave passage morphology was interpreted as the main determinant of air dynamics, which caused ice to accumulate extensively in the upper portions of the cave and caused the temperature on the top of the homothermic zone to be significantly below the mean outside temperature. Air dynamics also lowered the temperature of water flowing through the cave vadose zone and feeding the phreatic zone of the massif. The pronounced temperature difference between the phreatic zone and the top of the homothermic zone probably contributed to the thermal gradient observed in the cave, which is steeper than in ice-free caves in the area. Our results enabled the development of a conceptual model that describes coupling between air and water dynamics in the cave system and its surroundings.