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Lake Velence is a shallow soda lake, the third largest natural lake in Hungary. It is a valuable protected aquatic ecosystem but at the same time it is exposed to various anthropogenic pressures such as commercial and recreation activities, including water sports and fishing. Its shallow depth makes it susceptible to droughts and evaporation. In recent years, the water level of the lake has decreased dramatically, resulting in changes in water management, declining water quality and conflicts over its multiple uses. Climate change effects in Hungary will likely stress the lake’s water resources and ecosystems even further in the future.Despite groundwater mapping in the area proving that the lake is at the discharge point of local groundwater flow systems, only the surface water components and precipitation are considered in the lake’s water budget. Numerical models showed that groundwater contributes an annual average of 5% (up to 12%) of Lake Velence's inflows. Considering that the watercourses flowing into the lake are groundwater fed as well, the share of groundwater in the lake’s inflow can be as high as 56%. Revisiting water management practices in the area is thus necessary for the local ecosystem and tourism industry. We assessed surface water-groundwater interactions in the catchment area using natural tracers to collect further evidence on the importance of this connection. Water samples were collected from different water sources: from the lake, inflow streams, an artificial reservoir, and groundwater wells. The samples were analysed for stable isotopes of oxygen and hydrogen, and natural radioactive isotopes of uranium, radium, and radon. Additionally, these geochemical tracers were mapped across the lake, providing a spatially specific signal of groundwater inputs throughout the entire water body. Radon activity concentration was measured by liquid scintillation (LSC) technique and by a radon-in-air monitor with RAD-AQUA attachment (RAD7, Durridge). Uranium and radium were measured using selectively absorbing NucFilm discs and alpha spectroscopy.The results provided not just physical proof of groundwater inflow into Lake Velence and its inflowing streams but a detailed spatial distribution of these inputs. For example, radon concentrations were significantly higher than expected from just in-situ production alone and was detected even by liquid scintillations technique (1-6 Bq/L) both in the lake and in the streams. Measurements with the RAD7 showed values between 0.017 and 4.72 Bq/L. Uranium values were between 96 and 498 mBq/L. All isotopes combined provide unequivocal evidence that groundwater contribution to lake water budgets is important and that groundwater management has to be reconsidered in order to improve lake water levels and water quality.The research was supported by the János Bolyai Research Scholarship of the Hungarian Academy of Sciences. This work has been implemented by the National Multidisciplinary Laboratory for Climate Change (RRF-2.3.1-21-2022-00014) project within the framework of Hungary's National Recovery and Resilience Plan supported by the Recovery and Resilience Facility of the European Union.