When building tunnels in rock, an inflow of groundwater is likely induced. Depending on the hydraulic properties of the rock and the surrounding soils, the inflow may cause a decline in the groundwater level above the tunnel line with possible consequences for nearby well facilities or groundwater dependent environments. Discharge of the inflowing groundwater represents an operation of water (vattenverksamhet in Swedish) according to the Swedish environmental law, meaning that permission must be applied for at the Environmental Court. In the application, the operator presents an area of influence for groundwater, which defines the area where the groundwater levels could change due to the planned operation. In this work, the area of influence has been defined as the area where the groundwater level is lowered by more than five centimeters. As part of the project Tvärförbindelse Södertörn in southern Stockholm, the Swedish Transport Administration is planning for the construction of a tunnel in rock adjacent to Glömstadalen in Huddinge municipality. The aim of this master thesis was to investigate the extension of the area of influence of the tunnel through two-dimensional groundwater modelling. A site-specific model was created in the program SEEP/W by establishing a cross-section orthogonal to the tunnel and through examination of the hydrogeological conditions in the area. Steady-state modelling of both unsaturated and saturated flow was then carried out including and excluding the tunnel. Through this, the change in groundwater levels due to the tunnel could be reviewed. Since the development of the model required simplifications and assumptions of the site-specific conditions, a simple sensitivity analysis was also performed where a few model parameters were altered to examine how the area of influence changed. The modelling results showed that the tunnel, when sealed suitably, at most changed the groundwater levels 680 meters north and 840 meters south along the studied cross-section. The smallest effect on the groundwater levels was observed 400 meters north and 560 meters south of the tunnel. Differences in the extension of the area of influence were noticed depending on how the outflow of groundwater in Glömstadalen was represented, and which sealing properties around the tunnel were used. Furthermore, the modelling showed that the hydraulic conductivity of the rock is of major importance for the location of the groundwater table, and therefore it is considered relevant to perform hydraulic tests in the rock to increase the reliability of the model.