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.

Managed aquifer recharge (MAR) refers to a suite of methods by which excess surface water or non-conventional water is stored underground for subsequent recovery or environmental purposes. MAR solutions have been largely used in unconsolidated aquifers, while their application in karst aquifers is rare. This research presents the first results of a MAR viability study on the island of Vis, a small karstic island in the Adriatic Sea. Favorable geological and hydrogeological conditions enable the formation of karst aquifers, making the island autonomous in terms of water supply. The island's main aquifer, exploited in the Korita well field, is protected from seawater intrusion by several hydrogeological barriers. However, climate change and high seasonal pressures related to tourism pose a threat to the future availability of freshwater. Multidisciplinary field and laboratory investigations were carried out to detail the geological and hydrogeological setting of the island and its groundwater resource. Field analyses consisted of groundwater monitoring and sampling, geophysical investigations (i.e., electrical resistivity tomography), and structural measurements. Laboratory analyses included measurements of principal cations and anions and tritium activity. Despite low precipitation during the observation period (September 2019 - December 2020), the groundwater resource at the Korita site showed stable trends of physico-chemical parameters with a good storage potential and a long-term reserve. Geophysical investigations evidenced a relatively homogeneous sequence of the rock mass at a larger scale, while structural analyses indicated the occurrence of E-W karstified and open fractures that could represent a preferential flow path in the carbonate aquifer. A MAR solution for the Vis island was proposed combining an infiltration pond scheme with the direct injection of the accumulated waters into the aquifer using available wells. The potential water source could be represented by the runoff collected in an old artificial channel and the associated pond system in Korita.

Topusko is the second warmest natural thermal water spring area in Croatia, located at the southwest edge of the Pannonian Basin System. Due to favourable geothermal properties, these waters have been used for heating and health and recreational tourism since the 1980s. Thermal springs with temperatures up to 50 °C are the final part of an intermediate-scale hydrothermal system. However, systematic research on the Topusko spring area has not been conducted to lay the foundation for sustainable resource utilisation. Multidisciplinary research including the hydrogeochemical characterisation of naturally emerging thermal water, an electrical resistivity tomography (ERT) investigation conducted to reconstruct the subsurface geology, and hydrogeological parametrisation of the geothermal aquifer was carried out to refine the existing local conceptual model. The results show Ca-HCO₃ facies of Topusko thermal waters, which get heated in a Mesozoic carbonate aquifer. The water equilibrium temperature in the geothermal aquifer is estimated to be 78 °C based on the SiO₂-quartz geothermometer. The fault damage zone, which enables the upwelling of thermal water, was identified by ERT investigations. The transmissivity values of the aquifer derived from the results of step-drawdown tests range from 1.8 × 10⁻² to 2.3 × 10⁻² m²/s. Further multidisciplinary research is necessary to improve the existing conceptual model of the Topusko hydrothermal system.

The sustainable utilization of geothermal energy mostly depends on the characteristics of the geothermal resource from which it is extracted. Among others, detailed geological modeling is a key factor for estimating the potential of a geothermal resource. This research focuses on the modeling and reconstruction of the geological setting of the Daruvar thermal spring area using geophysical techniques. An integrated geophysical approach based on electrical resistivity tomography (ERT) and both active and passive seismic (MASW and HVSR) methods was used. Based on ERT results and the stratigraphic logs of the wells in Daruvar, three resistivity layers/geological units were identified. The deepest layer with resistivity < 150 Ωm is the Triassic carbonate that constitutes the thermal aquifer. Sharp lateral variations in the resistivity distributions within the bedrock were interpreted as fault damage zones saturated with thermal waters. Integrating the results of the seismic methods, the thickness of the first seismic layer that corresponds to the Quaternary cover was estimated from 5 to 20 m. Here, results of the geophysical investigations were combined into a 3D geological model highlighting the occurrence of subvertical N-S and E-W trending faults in the Daruvar spring area. The N-S-trending fault was interpreted as a fault plane parallel to the regionally mapped Daruvar fault. This fault juxtaposes the Triassic carbonate complex of the thermal aquifer with a Neogene sedimentary sequence of significantly lower permeability. Neogene–Quaternary tectonic activity further increased the fracturing and the permeability field in the Daruvar spring area, as proven by the smaller scale E-W faults and the well logs. This fracture network permits a quick upwelling of thermal fluids resulting in thermal springs with temperatures up to 50 °C. This work proves that the construction of a detailed geological model is crucial for assessing the reservoir and fault geometries in thermal systems hosted in fractured carbonate rocks.

During the pumping of wells, the groundwater level drawdown, as measured in the pumped well, is increased by non-linear losses caused by the water flow velocity through the well screens. This undermines the adequacy of the direct use of the measured drawdown data in the well for the purpose of the realistic identification of the effective well radius and aquifer parameters. This anomaly is avoided by reshaping the drawdown function into a function of the specific drawdown $s_w/Q$ of the pumped well. This reshaping simplifies the exclusion of non-linear losses from the sequence of measured data of the water level in the well at the position of the effective radius of the pumped well. Combining the data of linear losses and the respective pumping rate of the pumped well, a function of the specific drawdown of the radial flow $s_w/Q$ was formed. This function describes the aquifer parameter relations during the respective test pumping. A consistent sequence of the function of the specific drawdown $s_w/Q$ of the pumped well reveals the actual value of the coefficient of nonlinear losses. Moreover, the specific drawdown function enables the reliable estimation of aquifer transmissivity using only the pumped well drawdown data.

The Gajevo landslide is located in a hilly area of northern Croatia, where numerous landslides endanger and damage houses, roads, water systems, and power lines. Nevertheless, available landslide data are relatively scarce. Therefore, the Gajevo landslide location was chosen for detailed research and the development of a typical landslide model for this area. During initial research, the geographical and geological settings were reviewed and historical orthophotos were analysed. Due to the complexity and vulnerability of the area, the location required detailed investigations and the integration of multi-level data: remote (based on high-resolution LiDAR data) and field landslide mapping were performed and a map of the landslide area was developed. Precipitation data were reviewed, while shallow boreholes with material sampling and geophysical measurements provided information on material characteristics and 3D (depth) insight. As a result, knowledge was gained about material resistivity and composition along with the depth of sliding surfaces, and an engineering geological map of the Gajevo landslide area with the landslide and directly endangered areas marked was developed to be used by the local community in landslide risk assessment. As it is reasonable to expect that an extreme rainfall event will occur in combination with snowmelt in the coming years, resulting in the reactivation of Gajevo landslide, further research and continuous landslide monitoring are recommended.