Tracer methods are useful for investigating groundwater travel times and recharge rates and analysing impacts on groundwater quality. The most frequently used tracers are stable isotopes and tritium. Stable isotopes of oxygen (δ¹⁸O) and hydrogen (δ²H) are mainly used as indicators of the recharge condition. Tritium (³H) is used to estimate an approximate mean groundwater age. This paper presents the results of an analysis of stable isotope data and tritium activity in Croatian groundwater samples that were collected between 1997 and 2014 at approximately 100 sites. The composition of the stable isotopes of groundwater in Croatia originates from recent precipitation and is described using two regional groundwater lines. One of them is applied to groundwater accumulated in the aquifers in the Pannonian part of Croatia and the other is for groundwater accumulated in the Dinaric karst of Croatia. The isotope content shows that the studied groundwater is mainly modern water. A mix of sub-modern and modern water is mostly accumulated in semi-confined porous aquifers in northern Croatia, deep carbonate aquifers, and (sub)thermal springs.

Global warming has been monitored for many years. The increase in air temperature and changes in the distribution and frequency of high temperatures are recorded continually. Lakes are one of the important water resources for aquatic ecosystems and water supply, which are significantly affected by global warming. The increase in lake water temperature increases the evaporation from the free lake surface, lowering the lake level, and changes the water quality. In the last few decades, analysis of changes in lake water temperature has been increasing. In situ measurements of water temperature in Vrana Lake on Cres island (Croatia), the largest freshwater lake on the Mediterranean islands, were analysed over 43 years. The results showed that the mean annual lake surface water temperature (LSWT) increased by 0.47 °C decade-1 (p < 0.0001). The increase in the mean annual lake summer surface water temperature (July–September) was 0.44 °C decade-1 (p < 0.0001), and the maximum annual LSWT was 0.56 °C decade-1 (p < 0.0001). All these amounts are in accordance with the published data on the increase in water temperature in the investigated European lakes. The number of days with LSWT higher than 25 °C increased by almost 9 d decade-1. An increase in the minimum LSWT (0.17 °C decade-1) corresponding to isothermal conditions was also determined but was not statistically significant. The minimum mean monthly LSWT increased by 0.36 °C decade-1 (p < 0.0001). Because the increase in water temperature can negatively affect the lake's ecosystem, and become a threat to safe water supply; LSWT, thermal stratification and evaporation should be continuously monitored. The impacts of climate warming on the lake stratification and aquatic ecosystems need to be further investigated.

Background: Croatia, as a Member State of the European Union, has to shape its water policy in accordance with the European Union Water Framework Directive. One of the tasks is to determine whether groundwater are susceptible to anthropogenic changes that would result in a significant surface water status reduction as well as terrestrial and aquatic ecosystems that directly depend on the groundwater.Methods: Quality and quantity data on groundwater and surface water, as well as ecosystems data were collected from multiple sources. Data were analyzed for the selection groundwater associated aquatic and groundwater-dependent ecosystems. Chemical time series data were analyzed and compared with the national guideline limits set by the Croatian water quality standard. The impact of the groundwater abstraction on the groundwater-dependent ecosystem was considered by analyzing the aquifer hydrogeological characteristics, as well as location of the pumping site in relation of the ecosystem, the pumping rate and groundwater level or discharge.Results: In karst areas of Croatia, groundwater-dependent aquatic ecosystems are predominant; while in the Pannonian area of Croatia, typically, there are groundwater-dependent terrestrial ecosystems. The status of groundwater bodies was defined as good. The analyzed chemical parameters are at acceptable concentration levels. Groundwater abstraction does not adversely affect most ecosystems. However, many springs that are captured for the water supply dry up in the summer. Given that this is also a common phenomenon at springs not captured, it has to be expected that such environments will be inhabited by organisms that have already adjusted to these conditions. Many different bioindicators have been found to indicate a good chemical and quantitative state of groundwater.Conclusions: To increase the degree of reliability, research monitoring has been proposed. The monitoring should reduce knowledge gaps including an increase of knowledge of the status of the ecosystems, the definition boundary conditions and threshold values of a good quantitative and qualitative status for individual ecosystems. In future prospects, unavoidable focus has to be climate change that may compromise the availability of groundwater resources.

Vrana Lake on the karst island of Cres (Croatia) is the largest freshwater lake in the Mediterranean islands. The lake cryptodepression, filled with 220 million m³ of fresh drinking water, represents a specific karst phenomenon. To better understand the impact of water level change drivers, the occurrence of meteorological and hydrological droughts was analysed. Basic machine learning methods (ML) such as the multiple linear regression (MLR), multiple nonlinear regression (MNLR), and artificial neural network (ANN) were used to simulate water levels. Modelling was carried out considering annual inputs of precipitation, air temperature, and abstraction rate as well as their influential lags which were determined by auto-correlation and cross-correlation techniques. Hydrological droughts have been recorded since 1986, and after 2006 a series of mostly mild hot to moderate hot years was recorded. All three ML models have been trained to recognize extreme conditions in the form of less precipitation, high abstraction rate, and, consequently, low water levels in the testing (predicting) period. The best statistical indicators were achieved with the MNLR model. The methodologies applied in the study were found to be useful tools for the analysis of changes in water levels. Extended monitoring of water balance elements should precede any future increase in the abstraction rate.

This paper presents a hydrogeological map of the western part of the Drava Basin in northern Croatia. An area of approximately 660 km² is included in the hydrogeological map at a scale of 1:100,000. The map was made using ESRI ArcGIS 10 and ArcMap 10.2.1. This map describes the geometry of the Quaternary aquifer system and its hydrogeological characteristics as well as the distribution of groundwater flow. According to the values of the hydrogeological conductivity, the deposits are classified into a single hydrogeological unit. Approximately 400 boreholes and wells are identified within the map. Five 2D lithological cross-sections and five 3D hydrogeological cross-sections are displayed on the map. The distribution of the covering deposits thickness and the lithological composition of the upper part of the covering deposits are also displayed. According to its hydrogeological characteristics, the Quaternary aquifer system in the study area represents a strategic groundwater resource in Croatia.