Modelling PFAS contamination in Lake Mälaren to assess the impact on the drinking water supply

Per- and polyfluoroalkyl substances (PFAS) are environmentally persistent contaminants that resist degradation and pose potential risks to human health. This study investigates the distribution and transport of the sum of the four key PFAS compounds PFOA, PFNA, PFHxS and PFOS concentrations in Lake Mälaren, Sweden’s third-largest freshwater lake and a critical drinking water source for over two million people. Using the three-dimensional hydrodynamic model PyGETM, the spread of PFAS from major tributaries such as Fyrisån, Örsundaån, Lövstaån, Märstaån, Öxundaån, Brobäcken and Bällstaån was simulated from 2019 to 2024. The model outputs highlight both spatial and temporal variations in PFAS spread in Lake Mälaren. The results show that PFAS levels at key drinking water intakes in the lake can at times exceed the Swedish drinking water guideline of 4 ng/L. The model identifies critical sources of PFAS, underscoring the need for targeted source control and monitoring to protect water quality.