This study presents results from a ground water monitoring and groundwater modeling program carried out in the framework of the construction of the Lötschberg Base Tunnel in karstified limestones and marls. The Lötschberg Base Tunnel (LBT) is a double tube railway tunnel in central Switzerland with a total length of 34.5 km that was successfully completed in fall 2007. Within the framework of the study, it could be shown that the impact on the ground water flow system of a shallow tunnel, located 400 m above the base tunnel, is massive and hydraulic heads within the limestone aquifers are lowered by up to 180 meter. The additional impact caused by the excavation of the deep LBT is significantly smaller (up to 50 m of head drawdown in the limestone aquifers). Measured steady state pressure reductions monitored in a deep borehole interval below a gravel filled valley amount to about 26 m of head.