Numerical models are not only applied for predictions but also with the aim to improve the understanding of certain geological and hydrogeologie conditions. In this case of a three dimensional flow pattern to a small brook in the catchment area of the Raszyn Ponds nearby Warsaw the numerical groundwater flow model MODFLOW was used to: 1. Verify the conceptional understanding of the area in order to prove if a hydrogeologie window exists which is feeding the present aquifer. 2. Calculate travelling times of groundwater from the upper agriculturally used part of the aquifer to an experimental field in the lower part of the basin. The realisation of the hydrogeologie window in MODFLOW is described as well as the verification of the assumed conception, and results from travelling time calculations are shown.