Chloride and sodium budgets of a shallow freshwater lake – Current status and the impact of climate change

The large, shallow Lake Balaton (Hungary) has experienced rapid salinization since the 1970s. This study investigated the causes of salinization and aimed at predicting the effects of climate change. Monthly mass balance models for chloride and sodium were calibrated using water balance and water quality monitoring data (1976-2022) to analyze the effects of climate change (2022-2100) through ensemble modeling under the IPCC RCP 4.5 scenario. Current (2016-2020) emission inventories were developed for both chloride and sodium. The long-term (1921-2022) emission inventory of chloride was used to build a simplified chloride balance model for the catchment. Historical salinization occurred with almost constant external loading, in parallel with the increasing water residence times. According to the mass balance model, 18-28 % of the cumulative chloride and sodium loads has been accumulated in lake sediments, potentially slowing recovery of the lake from salinization. Climate change was predicted to aggravate salinization by further reducing the water balance surplus. Even the extremely high chloride concentrations of the future will remain well below the drinking water limit, but they may adversely affect the aquatic ecosystem. Both agriculture and road deicing contributed about one-third of current chloride emissions. Wastewater accounted for <20 % due to significant wastewater diversion to adjacent catchments. The rapid intensification of Hungarian agriculture from the mid-1960s, followed by a sudden economic collapse in 1990, resulted in a large emission peak of chloride in the 1970-80s, providing a unique opportunity to estimate the long-term retention of chloride in the catchment. We estimated that 30 % of the chloride emitted since 1921 may still be present in groundwater/soils.