Crossing hydrogeological and ecological sensitive aquifers, water ingress can occur, which can lead to irreversible damage to hydrogeological and ecological systems. In order to avoid this, the rock mass surrounding these hydrogeological sensitive tunnel sections is sealed by injections to reduce the hydraulic permeability, the water inflows and consequently the regional water level drawdowns. In deep tunnels, complete sealing is not possible due to the high water pressures. Residual water volumes will continue to enter the tunnel. The magnitude of these water inflows must be such that irreversible damage to the systems does not occur. These allowed water inputs inflows are called sustainable yield. The aim of the article is to explain the concept and determination of sustainable yield for a sensitive aquifer, the Hochstegen marble, in the Brenner Base Tunnel project. After determining the water resources and their complex relationships, calibrated, regional and local hydrogeological numerical models are used. The analysis shows that water inlets in order of 35 l/s lead to regional water level drawdowns in the shallow aquifers hydrogeological and ecological still acceptable.

Keywords: tunnel, water inflow, hydrogeological numerical model, cone of depression, impact.