The hummocky Balaton Highland is located in western Hungary and is part of the Transdanubian Mountains, the most extensive carbonate aquifer system in Hungary. The study region also encompasses Lake Balaton, the biggest lake in central Europe, which is to the south of Balaton Highland. The surface water–groundwater interaction in the Balaton Highland–Lake Balaton region and the groundwater contribution to Lake Balaton are revisited in this paper. Hydrostratigraphic classification was performed first; then, groundwater flow directions by hydraulic head distribution were analysed, and baseflow indices of surface watercourses were calculated. Regarding hydrochemical characterisation, general hydrochemical facies were identified, natural tracers of temperature, chloride and uranium were applied, and the stable isotopic composition of oxygen and hydrogen was determined. Finally, groundwater flow and heat transport were simulated in a 2D numerical model. A high level of hydraulic interaction was evidenced between surface water and groundwater and the sub-regions of Bakony Mountains, Balaton Highland and Lake Balaton by physical and chemical parameters, numerical simulation and groundwater-flow-related natural manifestations, revealing hydraulic continuity in the study region. Based on the results, the division of legislative water bodies can be reconsidered, especially that surface water and groundwater should be regarded as interconnected, and Lake Balaton can be considered a groundwater-dependent ecosystem in any water-use planning in the region.

This study investigated the hydrogeochemistry of a shallow Quaternary sedimentary aquifer in an esker deposition in western Finland, where distinct spatial and temporal variability in groundwater hydrogeochemistry has been observed. Field investigation and hydrogeochemical data were obtained from autumn 2010 to autumn 2013. The data were analyzed using the multivariate statistical methods principal component analysis (PCA) and hierarchical cluster analysis (HCA), in conjunction with groundwater classification based on the main ionic composition. The stable isotope ratios of δ¹⁸O and δD were used to determine the origin of the groundwater and its connection to surface water bodies. The groundwater geochemistry is characterized by distinct redox zones caused by the influence of organic matter, pyrite oxidation, and preferential flow pathways due to different hydrogeological conditions. The groundwater is of the Ca-HCO₃ type and locally of the Ca-HCO₃-SO₄ type, with low TDS, alkalinity, and pH, but elevated Fe and Mn concentrations, KMnO₄ consumption, and, occasionally, Ni concentrations. The decomposition of organic matter adds CO₂ to the groundwater, and in this study, the dissolution of CO₂ was found to increase the pH and enhance the buffering capacity of the groundwater. The mobility of redox-sensitive elements and trace metals is controlled by pH and redox conditions, which are affected by the pumping rate, precipitation, and temperature. With the expected future increases in precipitation and temperature, the buffering capacity of the aquifer system will enhance the balance between alkalinity from bioactivity and acidity from recharge and pyrite oxidation.

Lake Velence is a shallow soda lake whose water level and water quality show a severely deteriorating tendency in recent years. Until recently, the groundwater component in the lake’s water budget has not been taken into consideration. To integrate the lake into the groundwater flow system at the regional scale, methods of “basin hydraulics” were applied. In addition, 17 water samples were collected for δ²H and δ¹⁸O, and for ΣU, ²²⁶Ra and ²²²Rn activity measurements to use these parameters as environmental tracers of groundwater contribution. Groundwater mapping revealed that groundwater recharges in Velence Hills and the local elevations south of the lake, whereas discharge occurs by the lake’s shoreline and along surface watercourses. The results indicated that Lake Velence is the discharge point of local groundwater flow systems known to be more sensitive to climate changes and anthropogenic activities (e.g., contamination, overexploitation). Groundwater and lake water have similar uranium activity concentrations serving as another sign of groundwater inflow into the lake. Therefore, it is necessary to consider both the groundwater component in the lake’s water management and its vulnerability regarding local and short-term changes in the catchment area.

This research is based on information about approximately a hundred dug wells in five different areas in Finland. The fallout of sulphur compounds has determined the choice of the areas and the goal is to find out the degree of acidification of ground waters. The current situation has been compared to that 30 years ago as there was an investigation concerning the same wells at that time. The pH-value and degree of alkalinity of well water have systematically diminished in all the areas - most of all in areas of major sulphur compounds fallout. The observations and areas being few in number, no decisive conclusions should be drawn concerning the whole country.

Groundwater–surface water (GW–SW) interactions play a critical role in carbon cycling and storage but remain understudied. This study investigates the aquifer-river system within the Oulankajoki River Valley in northeastern Finland to understand how hydrogeological structure governs spatiotemporal variability in groundwater hydrogeochemistry and carbon dynamics. Using a combination of a geological model, hydrogeochemical data, and multivariate statistical analysis, we delineate the mechanisms and timescales of carbon exchange across hydrological phases. Results reveal that carbon cycling in the Oulanka aquifer is primarily controlled by hydrostratigraphy and recharge dynamics, with redox evolution governing the transformation between organic and inorganic carbon. Groundwater is DOC-poor (1.2 ± 0.6 mg L−1) and DIC-rich (20.3 ± 6.7 mg L−1) relative to surface waters (DOC = 8.3 ± 1.9 mg L−1; DIC = 4.5 ± 0.2 mg L−1), indicating efficient subsurface processing of organic carbon. Multivariate analyses (PCA, HCA) distinguish recharge-dominated shallow groundwater from carbonate-buffered, redox-controlled deep groundwater, revealing strong vertical decoupling between transient DOC inputs and persistent DIC storage. Interannual variability shows a shift from recharge-driven DOC mobilization toward internally regulated DOC-to-DIC transformation under longer residence times. Our results suggest that the aquifer alternates between acting as a carbon sink during recharge and a carbon source during baseflow, with its role modulated by recharge pathways and subsurface geochemical conditions. These findings advance the understanding of GW–SW carbon coupling in boreal systems and underscore the importance of subsurface processes in regional carbon budgets under climate change.