A hydraulic and physically based transport model for the catchment of a well field was set up. With the study area situated in a region strongly influenced by surrounding well galleries the boundary conditions had to be worked out during modelling and partially had to be transient. Two important processes were clarified: Bank filtrate extracted at the investigated transect is composed of 3 water qualities from horizontal layers, each with a different age and infiltration area. Sampled wells containing the different water types were identified, providing information for correct chemical interpretation. Secondly, the lake sediments show a pronounced seasonal fluctuation in their leakage coefficient, with its winter values doubling in summer, and lagging 2–4 months behind water temperature.