## 1 Introduction  
The world population has been increasing at an alarming rate. A majority of this growth is occurring in urban areas [1]. By 2030, urban dwellers are projected to constitute nearly 60% of the global population [2]. The impact of two main constraints, anthropogenic pressure and property economics, has led to the vertical development of urban areas [3]. Underground structures below groundwater level often modify natural groundwater flow, especially in the aquifers of urban regions [4]. This phenomenon was reviewed in [5], which presented the state-of-the-art on the impacts of disturbances caused by underground structures (tunnels, basements of high-rise buildings, deep foundations, etc.) on groundwater flow in urban aquifers. Several case study examples from all over the world provide proof for previous claims, for example, the subway construction in Barcelona, Spain [6], where the tunnel cut a large section of the Llobregat Delta Shallow Aquifer. Major modifications in the distribution of the groundwater head can be observed when underground structures transversally cut an aquifer, as has been analyzed in Milan and Turin, Italy [7, 8]. The same behavior problem was investigated between the construction of the mechanized twin tunnels of the Shiraz metro line and the groundwater [9] in Tehran, Iran. The effect of pre-existing underground structures on groundwater flow induced by dewatering and excavation was examined based on the actual excavation of a metro station in Tianjin, China [10]. According to [11, 12], the lack of understanding of the interactions between different structures and groundwater flow remains an obstacle to improving urban planning. In particular, understanding the flow of groundwater in an urban context is the first step in improving quantitative and qualitative groundwater resources. All the mentioned case studies have one common denominator: they have analyzed the impact of underground structures on groundwater flow under urban conditions using 3D numerical modeling (either the finite element (FEM) or the finite difference method (FDM)). Even with this common denominator, it is quite complicated to compare the results of the analyzed studies due to their different hydrogeological conditions (hydraulic conductivity and thickness of aquifer) and construction processes. The planned TEN-T track project is part of the Trans-European Network for Transport (TEN-T). Transport networks (roads, railways, airways, and waterways) have been planned or constructed within the TEN-T framework in order to make them usable for all European countries, without any limitations [13]. The first part of the planned project in Bratislava city is the connection between the airport and the railway network. The second part, which is the technically and economically most demanding section of the planned project, will be the design of the alignment of the rail line between the Bratislava Slovany-Bratislava Filialka and Bratislava Petrzalka railway stations (RWS) (red lines in Figure 1). This section should be built in an underground tunnel along the Carpathian Mountains and will significantly affect the groundwater flow regime. This was the reason for creating a quasi-3D finite element numerical model to evaluate the impacts of the railway tunnel on the groundwater level regime, whereby we assumed that this model would serve to simulate the groundwater regime after implementing the technical measures proposed by us [14]. This section of the route passes through a densely developed area of the central urban zone, crosses the Danube riverbed, and connects an existing rail line. Large development projects comprising multi-story buildings are under preparation for a part of the area that is in direct contact with the route. Therefore, the underground route of the rail line is unavoidable. With respect to the sensitive character of the urban area, any negative effect on the surface must be minimized as much as possible or eliminated. Figure 1: (**a**) Map showing the location of Bratislava in Slovakia [15].