Waste management in Europe has improved in recent years, reducing the amount of waste disposed at landfills. However, there are still many landfills in the countries. It is well known that landfills that do not have measures in place to control leachate entering groundwater can contaminate groundwater long after the landfill is closed. Collecting monitoring results from all landfills allows permitting and management agencies to improve action plans. This relies on a synoptic risk assessment that allows prioritization and milestones to be set for required actions. The developed method of synoptic risk assessment is based on a conceptual model of the landfill and the results of chemical groundwater monitoring tested at 69 landfills in Slovenia. The study confirms that most landfills have a direct or indirect impact on groundwater quality. All landfills were classified into three priority classes on the basis of the synoptic risk assessment. The results show that a total of 24 landfills have a clearly pronounced impact on groundwater. A total of 31 landfills have a less pronounced impact due to the favorable natural attenuation capacity of the soil or the technically appropriate design of the landfill itself. A total of 14 landfills have a less pronounced or negligible impact on groundwater.

Slope stability strongly depends on the prevailing hydrological and hydrogeological conditions. The amount and intensity of precipitation and changing groundwater levels are important landslide triggering factors. Environmental tracers, including the chemical and stable isotope compositions of precipitation and groundwater, were used to gain insight into the groundwater dynamics of the Urbas landslide. The landslide is situated in a mountainous area with steep slopes and high precipitation amount and poses a high risk for the safety of the Koroška Bela settlement that lies downstream. The stable isotope analyses of oxygen-18 (¹⁸O) and deuterium (²H) in the precipitation and groundwater were used to estimate the groundwater mean residence time and the average altitude of the landslide recharge area. This information will help to plan and prioritize remedial landslide measures aiming to reduce the recharge of the landslide body and, thus, lower the risk of transformation of the sliding material into debris flow. The results of the chemical analysis of samples taken from springs and a piezometer show a Ca–HCO₃ water type. This indicates low water–rock interaction in a landslide area composed of Upper Carboniferous and Permian clastic rocks and points to upper laying carbonate rocks and scree deposits as the main recharge area. Water samples for stable isotope analyses of δ¹⁸O and δ²H were collected from a rain gauge, springs, and a piezometer over a two-year period (2018–2020). The estimated mean recharge altitude of the groundwater at sampling points was from approximately 1700 to 1800 m a.s.l. with a mean residence time of 2–5 months.

Within the framework of »Water Management Plan for the Danube River Basin and the Adriatic Sea 2015-2021 (NUV II)«, a study on the groundwater status of karst aquifers which are habitats of groundwater dependent ecosystems (GDEs) was carried out. Among them are the habitats of olms (Proteus anguinus), the endemic species of Dinaric karst. These habitats have been included in the Slovenian part of the Natura 2000 network, which aims at protecting and preserving rare, endangered and endemic animal and plant species and habitat types. However, the conservation status of some olm habitats is unfavourable given that the number of specimens is declining. For this purpose we prepared hydrogeological conceptual models, where recharge areas were delineated and quantitative as well as qualitative status of surface and ground-waters were studied. Knowing the basic characteristics of olms, their habitats and critical parameters that adversely affect their conservation status, we identified locations where critical parameters could exceed the threshold values in groundwater. We also identified the areas where and what are the trends of the observed parameters that exceed the natural background (As, Zn) or are undoubtedly of anthropogenic origin (PCB).

Monitoring of stable isotopes in throughfall (δ¹⁸O, δ²H) and meteorological parameters is a valuable tool for researching forest hydrology, particularly during extreme events like droughts and floods. This study presents the first systematic analysis of air temperature and precipitation changes over the past 65 years in two Slovenian lowland forests: Murska šuma and Krakovski gozd, in combination with isotopic composition research of throughfall. The observed rising air temperatures and altered precipitation patterns are reflected in the isotopic composition of throughfall. Over the last 65 years, air temperature has increased by approximately 2.5 °C. Although total annual precipitation amounts have remained relatively stable, in the last 35 years there is a notable decrease in precipitation in growing season and an increase during the dormant season, influenced by air masses of Mediterranean origin. Extreme drought in 2022 and flood in 2023 are confirmed by the Standardized Precipitation Index and isotopic variations in throughfall due to fractionation processes. Annual variability appears as seasonal changes, with sine-curve amplitudes of 3.71‰ in Krakovski gozd and 3.61‰ in Murska šuma. Together with the Local Meteoric Water Lines, these patterns support estimates of groundwater mean residence time and the origin of water used by trees.

In times of climate change, when drinking water resources and the water needed for the survival of groundwater-dependent ecosystems, such as lowland forests depend on the availability of water, the sustainable use of groundwater requires strategic planning based on an assessment of the amount of groundwater available. To properly assess and plan the amount of available and accessible water resources, it is first necessary to understand the processes of the water cycle.About 25 years ago (in 2000), a high mortality rate was observed in the groundwater-dependent ecosystem (GDE) of several lowland oak forests in Slovenia and Europe. Understanding the processes of the hydrological cycle is becoming increasingly important, mainly because of the high mortality of these forests, whose main problem is probably the lowering of the groundwater table and the reduced accessibility of water to these trees as a result of changes in weather patterns or the regime and amount of precipitation recharging the aquifers in the area of these forests.By short-term monitoring of the composition of stable water isotopes (δ18O and δ2H) in precipitation in the area of lowland oak forests Murska šuma (northeast Slovenia) and Krakovski gozd (southeast Slovenia), we have obtained important results on the variability of the precipitation signal, which will be compared with the results of the neighbouring long-term stations (e.g. Murska Sobota, Ljubljana, Vienna, Graz and Zagreb). In the future, we will expand our research and also monitor the isotope signal of stable water isotopes in soil, groundwater and xylem water of adult trees. This will allow us to assess the origin of water sources used by trees and to identify physiological responses to changing climatic conditions such as drought stress or altered precipitation regimes. By integrating water isotope data into ecological and hydrological models, we can better predict how trees will respond to current and future climate change, providing information for conservation strategies and sustainable management practices.Activities for this presentation were funded in last decade by the Slovenian Research Agency (Grants P1-0143, N1-0054, N1-0309 and P1-0020), IAEA (CRP F31006, TC RER7013, TC RER7017, SLO7001) and projects Mladi forum and FREYA which are co-funded by the Slovenian Research and Innovation Agency within the framework of Developmental funding pillar.