Prognostisering av NOM (naturligt organiskt material) i en råvattentäkt: Mälaren

Natural Organic Matter (NOM) poses increasing challenges for drinking water management, particularly in lakes impacted by both climate variability and land-use change. This study investigates NOM dynamics in lake Mälaren, Sweden, by analysing long-term trends in Total Organic Carbon (TOC), absorbance at 420 nm, and colour, across inflowing rivers and lake basins. Using daily hydrological and meteorological datasets (1980-2025) from four main tributaries, Fyrisån, Örsundaån, Märstaån, and Oxundaån, combined with lake chemistry records, the study explores seasonal variability, lag effects, and the impact of extreme events.TOC concentrations showed consistent seasonal peaks in spring and autumn, with the strongest signals in forested and agricultural catchments like Fyrisån and Märstaån. However, only 5-10% of extreme discharge events (>95th percentile) resulted in elevated TOC levels (>75th percentile), suggesting that antecedent conditions, not peak flow alone, drive NOM mobilisation. Spatial analysis revealed a gradient of declining TOC from Ekoln through Skarven to Görväln, indicating in-lake attenuation through sedimentation, microbial degradation, and photodegradation. Lag correlation analyses revealed rapid catchment responses at river outlets, but delayed peaks at the Görväln intake, reflecting travel time and in-lake processing. A simplified mass balance model was used to estimate TOC at Görväln based on upstream inflows. Although the model captured general trends (R² = 0.60), it tended to overestimate concentrations, reinforcing the role of internal lake functions. Oxundaån was examined as a conceptual proxy for broader lake processes. Its nested structure showed strong NOM attenuation between inlet and outlet, emphasising the role of internal buffering even in small systems. Overall, the study underscores that effective prediction and management of NOM in drinking water resources requires integrating catchment characteristics, cumulative hydrometeorological inputs, and in-lake processes. The findings provide a foundation for adaptive water treatment strategies in the face of a changing climate.