Modelling PFAS Transport in Lake Ekoln: Implications for Drinking Water Safety in the Stockholm Region.

Per- and polyfluoroalkyl substances (PFAS) are found frequently in both groundwater and surface water sources across Sweden posing challenges to drinking water supply. Lake Ekoln is located south of Uppsala and is the basin of Lake Mälaren; Lake Mälaren is the third largest lake in Sweden and is the drinking water source for more than two million people. The aim of this study was to simulate the fate and transport of PFAS in Lake Ekoln during the period 2017 - 2020 using three-dimensional hydrodynamic modelling. The simulated water temperatures were in agreement with the observed water temperatures. The simulated PFAS concentrations were generally in agreement with the available measurements, but the lack of measurements made the comparison uncertain. The modelling results described the seasonal variations of PFAS in Lake Ekoln informing the operation of the drinking water treatment plants located downstream. The modelling results confirmed that the main inflow to the lake - the river Fyrisån - is the main source of PFAS to Lake Ekoln, highlighting the importance of mitigating this source in the context of ensuring safe drinking water supply in the Stockholm region. Regular monitoring of PFAS in the river Fyrisån is needed, and additional measurements in Lake Ekoln would facilitate further model development.