Simulating Tracer Transport Times in Mälaren : Quantifying Tracer Transport Times, Assessing Sub-Basin Discharge and Seasonal Dynamics

Sammanfattning

Understanding contaminant transport times in complex freshwater systems is critical for safeguarding drinking water infrastructure. Lake Mälaren in Sweden serves as a vital water source but features complex morphology and multi-basin dynamics that complicate risk assessments. This study explored simulated passive tracer behaviour over a five-year period (2010-2014) from six primary inflows to three major drinking water intakes in eastern Mälaren to quantify transport times, identify seasonal patterns, and determine the influence of sub-basin discharge. Data from the 3D hydrodynamic model pyGETM were analysed using single-variable correlations, multi-variable linear regressions, and seasonal analysis. The findings reveal a clear geographically driven gradient, establishing a strict transport hierarchy constrained by physical proximity. Small, nearby catchments exhibit rapid, stable transport across all seasons whereas transport from distant inflows showed more variability and seasonal effect. Seasonally, the spring flood triggers a highly synchronized compression of transport times lake-wide, while summer acts as a stagnant hydraulic phase. Multi-variable regressions revealed that the combined North-Central basin discharge acts as the dominant regional accelerator, triggering substantial velocity increases across most routes. The Western basin mostly causes localized acceleration. Overall, higher flows from all sub-basins tend to accelerate water transport throughout the lake. These findings underscore that Mälaren cannot be managed as a uniform system. For drinking water producers, combining current flow data with the transport times calculated in this study can provide an early-warning framework to anticipate rapid contaminant arrivals.