Managed aquifer recharge (MAR) is an increasingly popular technique; however, the significance of groundwater flow dynamics is rarely examined in detail regarding MAR systems. In general, a high hydraulic gradient is not favoured for MAR implementation, as it causes higher water loss and mixing of recharge water with native groundwater. However, during groundwater-dependent ecosystem (GDE) rehabilitation, these hydraulic gradient-driven flow processes can be taken advantage of. The aim of this research is to test this hypothesis by evaluating the effect of groundwater table inclination, topography, and other local characteristics on MAR efficiency from the perspective of GDE restoration. MAR efficiency was examined from recharge to discharge area in a simple half-basin based on theoretical flow simulations, using GeoStudio SEEP/W software. Different scenarios were compared to analyse the groundwater level increase and the infiltrated water volumes and to assess the efficiency of MAR based on these parameters in each scenario. The theoretical results were applied to a close-to-real situation of Lake Kondor, a GDE of the Danube-Tisza Interfluve (Hungary), which dried up in the past decades due to groundwater decline in the area. Based on the results, initial hydraulic head difference, model length, and hydraulic conductivity are the most critical parameters regarding water level increase at the discharge area. The water amount needed for increasing the water table is mainly influenced by the thickness of the unsaturated zone and the material properties of the aquifer. The findings can help better understand MAR efficiency in light of local groundwater flow processes and contribute to optimising MAR systems. The results of the study suggest that, if water is infiltrated at the local recharge area, the water table will also increase at the corresponding discharge area, which positively effects the connected GDEs. This approach can serve as a nature-based solution (NBS) to sustain sensitive ecosystems in changing climatic conditions.

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.

Climate change and increasing human impacts are more emphasised in recharge regions, where the main flow direction is downward, resulting in negative water balance. Two wetlands located in the recharge position of regional groundwater flow systems were investigated in the Nyírség region, Hungary, as pilot areas for representing wetlands in similar hydraulic positions. Hydraulic data processes, chemical data evaluations, and numerical simulations revealed that the wetlands are fed via local flow systems, superimposing regional-scale recharge conditions in the area. The wetlands are discharge and flow-through types in connection with local flow systems. Nevertheless, in the case of significant regional water table decline—due to the high vulnerability of recharge areas to climate change—local flows are degraded, so they are not able to sustain the wetlands. To preserve the groundwater-dependent ecosystems in the areas, water retention at the local recharge areas of the wetlands may help in the mitigation of water level decline under present-day conditions. If the regional water table continues to decline, comprehensive water retention solutions are needed in the whole region. The results highlight that understanding the natural wetland–groundwater interactions at different scales is crucial for the preservation of wetlands and for successful water retention planning.

Numerical simulations focusing on groundwater age have not yet been carried out in the Buda Thermal Karst system (BTK) (Hungary), although isotopic and chemical data from thermal springs [e.g. Fórizs et al., 2019] are available to compare calculated and measured results. The main objective of this study was to improve understanding of regional-scale heat transport processes and groundwater flow associated with ageing in complex hydrogeological system, such as the BTK including deep carbonate sequences and adjoining sedimentary basins (“DC&SB”).A comprehensive sensitivity analysis was completed to validate the numerical method [Zimmermann, 2006] implemented with ‘age mass’ concept [Good, 1996], and to reveal the influence of crucial hydrogeological parameters on the groundwater age distribution in 2D and 3D synthetic models. It was established that the average (τav) and the maximum (τmax) groundwater mean age correlate with the permeability anisotropy, heterogeneity (exponential permeability decrease with depth), the model depth, while the values anti-correlate with the amplitude of water table and the permeability.For general investigation of the “DC&SB” type groundwater flow systems, two “half-basins” [Wang et al., 2017] were combined into a single asymmetric basin. The “DC&SB” model was characterised by (i) a water table configuration with higher amplitude and a homogeneous (unconfined) aquifer on the left-hand side; (ii) a lower water table amplitude and a three-layered (unconfined aquifer – aquitard – confined aquifer) domain on the right-hand side. As a result of the forced convection, decreased temperatures and reduced groundwater mean ages are noted in unconfined parts of the model, while heat accumulation and increased ages were calculated in the aquitard and the confined aquifer. Using experiences from synthetic tests, the groundwater age calculation was integrated into the preliminary 3D hydrogeological model of the BTK system. The results showed that the measured and the calculated mean groundwater age values sampled the different parts of the hierarchically nested flow system are of the same order of magnitude.The results are very important to uncover the groundwater flow in complex hydrogeological systems which is unavoidable both in regional-scale (e.g. drinking water management, geothermal exploration and geothermal energy utilisation) and local-scale explorations (e.g. managed aquifer recharge, environmental remediation). The research was supported by the National Research, Development and Innovation Office in the framework of project No. PD 142660; and by the National Multidisciplinary Laboratory for Climate Change, RRF-2.3.1-21-2022-00014 project.ReferencesFórizs, I., Szabó, V.R., Deák, J., Halas, S., Pelc, A., Trembaczowski, A., Lorberer, Á. (2019). The Origin of Dissolved Sulphate in the Thermal Waters of Budapest Inferred from Stable S and O Isotopes. Geosciences 9(10), 433, p. 13.Good, D.J. (1996). Direct simulation of groundwater age. Water Resources Research 32, pp. 289-296.Wang, J.Z., Jiang, X,W., Zhang, Z.Y., Wan, L., Wang, X.S., Li., H. (2017). An analytical study on three-dimensional versus two-dimensional water table-induced flow patterns in a Tóthian basin. Hydrological Processes 31, pp. 4006-4018.Zimmermann, W.B.J. (2006). Multiphysics modeling with finite element methods. Singapore: World Scientific Publishing Company, p. 422.