DOC and nitrate in farmland represent key chemical species that determine the water quality in the Karst Critical Zone (KCZ). The work reported here focuses on quantifying fluxes of these species in an experimental farm site (University of Leeds Farm, UK) overlying a dolomitic karst aquifer of Permian age. In this research, the Transect Method was applied for the first time to farmland by combining hydrochemical data from soil and groundwater for computation of mass fluxes. The Transect Method, developed for management of industrially contaminated sites, was applied to a farm source due to the presence of localised contamination from application of pig slurry.Required inputs for our approach include concentrations of nitrate and DOC in soil water and groundwater, net recharge flux (here derived from a MODFLOW-2005 model) and local hydraulic gradient and conductivity measurements. Key outputs are fluxes and downstream groundwater concentrations of DOC and nitrate. Downstream concentrations were validated against direct groundwater measurements, demonstrating the veracity of the approach. The approach shows that the localised contamination has a significant impact on both concentrations of nitrate and DOC in groundwater, although the DOC impact is greater, because the upstream land uses also produce nitrate as a result of agricultural practices that are widespread in the region.The results of the study also constrain the zone vulnerable to contamination to the upper ~40 m below the ground surface. Future modelling efforts on solute contaminant transport should focus on this shallow vulnerability zone (0-40 mBGL) and the Transect Method applied in this work can be used to define boundary conditions. Hence, following this research, we envisage to export a generic approach that combines physical flow parameters and hydrochemical analyses for computation of subsurface mass fluxes using the Transect Method, to identify the degree of impact of specific point sources and to support conceptualization and modelling of contaminant transport in the KCZ of farm areas.

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.

In Hungary the drinking water supply is mainly based on groundwater, in which radionuclides are common components. Since the mobility of the most common radionuclides, uranium and radium, is strongly influenced by the geochemical conditions, knowledge on the geochemical parameters of water is required. This depends on the flow system and the flow regime. Therefore, hydrogeology has a crucial role in revealing the origin of elevated activity concentrations. This research presents a case study in Hungary where the drinking water supply is provided by bank filtered and karst wells. In most of the wells of the research area the gross alpha values are above the screening level, 0.1 Bq L 1. The aim of this study is to determine which radionuclides may cause the elevated radioactivity and explain their occurrence using the hydrogeological approach. All samples of the study were analysed for (U-238þU-234), Ra-226, Rn-222. Alpha spectrometry applied on Nucfilm discs was used to measure the uranium and radium activity while radon activity was determined by liquid scintillation. The study revealed the correlation between the river water level fluctuation and the uranium content of the wells. The results of this study highlighted the transient nature of river bank filtered systems, which should be taken into account in the monitoring and water supply strategy.

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<p>Our study aimed to understand the origin of elevated (>100 mBqL<sup>–1</sup>) gross alpha activity measured in groundwater-derived drinking water in the vicinity of the Sopron Mountains and Lake Fertő (Neusiedl). Water samples from 10 springs and 7 water wells were analyzed for major ions and trace elements. Total U and <sup>226</sup>Ra activity concentrations were determined by alpha spectrometry using Nucfilm discs, and <sup>222</sup>Rn activity was measured by liquid scintillation counting. <sup>234</sup>U/<sup>238</sup>U ratio was determined by ICP-MS and alpha spectrometry. Additionally, δ<sup>2</sup>H and δ<sup>18</sup>O measurements were performed. To get an insight into the dynamics of the groundwater flow system and to better understand the radionuclide mobilization and transport processes, the geochemical results were evaluated in the groundwater flow system context.</p> <p>Uranium activity was measured up to 540 mBqL<sup>–1</sup>, thus it can be concluded that dissolved uranium causes the previously measured elevated gross alpha values, though no health risk arises from drinking water consumption. The occurrence of dissolved uranium can be explained by oxidizing conditions that are prevalent along local flow systems and in recharge areas. The relatively short residence time of water, thus the presence of local flow systems is indicated by δ<sup>1</sup>8O (-11.96 to -7.17‰) and δ<sup>2</sup>H values (-83.4 to -52.6‰). Spring samples have lower uranium activity (up to 93 mBqL<sup>–1</sup>) than groundwater samples (up to 540 mBqL<sup>–1</sup>) which can be explained by the longer residence time of water. Uranium is transported along flow paths under oxidizing conditions and the longer the flow route the higher the uranium concentration.</p> <p>This topic is part of a project that has received funding from the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement No. 810980. Besides, the research was funded by the National Multidisciplinary Laboratory for Climate Change, RRF-2.3.1-21-2022-00014 project project. Some radioactivity measurements were supported by the European Commission’s Joint Research Centre (JRC) – Research Infrastructure Access Agreement No. 36227-1/2021-1-RD-EUFRAT-RADMET.</p>

<p>Riverbank filtered drinking water supply systems are strongly dependent on the river stage. Climate change-induced extremely low or high river stage may cause water quantity and quality problems. In this study, a riverbank-filtered drinking water supply system along the Danube River was investigated from a radioactivity point of view: we aimed to understand the origin of elevated (>100 mBq L<sup>–1</sup>) gross alpha activity measured in some wells and the variation in water quality with river level fluctuation.</p> <p>10 producing, 2 monitoring wells, and the Danube were sampled at lower and higher river stages. The water samples were analyzed for major ions and trace components. Total U (<sup>234</sup>U+<sup>235</sup>U+<sup>238</sup>U) and <sup>226</sup>Ra activity concentration were determined by alpha spectrometry using Nucfilm discs, and <sup>222</sup>Rn activity was measured by liquid scintillation counting.</p> <p>Total uranium activity was measured in the highest concentration (up to 334 mBq L<sup>–1</sup>). Radium and radon activities were barely above the detection limit. Based on our results the previously measured elevated gross alpha activity is most likely caused by dissolved uranium in the groundwater. Uranium activity concentrations show increasing values from N to S which corresponds well to the occurrence of organic matter-rich, clayey floodplain deposits underlying the aquifer.</p> <p>Besides spatial variation, a temporal change can also be observed: lower uranium activity was measured at a lower river stage (32–248 mBq L<sup>–1</sup>) compared to a higher river stage (26–334 mBq L<sup>–1</sup>). This phenomenon could be explained by the dynamic relationship between the groundwater and the river. At the low river stage, oxygen-rich (ground)water flows from the river toward the inland and may cause the remobilization of uranium from the clayey basement layers. This process will be more and more dominant by extremely low river stages during long-lasting drought periods in the future causing water quality problems.</p> <p>The research was funded by the National Multidisciplinary Laboratory for Climate Change, RRF-2.3.1-21-2022-00014 project.</p>

<p>In Hungary the drinking water supply is mainly based on groundwater from aquifers characterized by different lithology. Riverbank filtered systems represent 40 % of drinking water supply. According to the EURATOM drinking water directive, there are recent regulations in Hungary regarding the natural radioactivity of drinking waters. Based on this, if gross alpha or gross beta radiation exceeds the limit, nuclide-specific measurements are required to be performed by the relevant waterworks. Since the mobility of uranium and radium is strongly influenced by the geochemical conditions, knowledge on the geochemical parameters of water is required. Therefore hydrogeology has a crucial role in revealing the origin of elevated activity concentrations. This research presents a case study in Hungary where the drinking water supply is provided by bank filtered and karst wells. The main aim of this study is to determine which radionuclides may cause the elevated radioactivity and explain their occurrence using hydrogeological approach, considering also the temporal variation of groundwater/surface water ratio. In most of the wells of the research area the gross alpha values are above the screening level, 0.1 Bq L<sup>-1</sup>. The study revealed the correlation between the river water level fluctuation and the uranium content of the wells. Among the investigated radionuclides, the uranium activity concentrations responded the most to the water level changes of the river and showed systematically higher values during low water conditions. In addition the karst wells showed low activity concentrations. This suggests, that uranium is transported by the groundwater component, and possibly sourced from the fluviatile sediments. The results of this study highlighted the transient nature of river bank filtered systems, which should be taken into account in the monitoring and water supply strategy. Nevertheless, the study emphasizes the importance of considering the dynamics of groundwater and associated geochemical environment in addition to geological factors, when investigating the radioactivity of groundwater or other potential contaminants.</p><p>This study was supported by the ÚNKP-19-3 New National Excellence Program of the Ministry of Human Capacities. This study is part of a project that has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 810980.</p>