## 2 Materials and Methods
### Modeling Process  
The engineering-geological survey results, derived from available geological documents [19] and augmented with field drilling and survey data gathered for the TEN-T project, in conjunction with supplementary information from the SHMI database, were utilized to perform parameterization of the filtration area. Figure 7: Area of interest with the considered boundaries and indications of the average groundwater levels (m a.s.l.) at the SHMI boreholes. (blue part expressed the Dirichlet boundary condition the red part expressed the Neumann boundary). Parametrization of the individual input values meant assigning the investigated parameter to each node of the finite element simulation network. The aforementioned transformation of spatially orientated data could be performed in several ways. In our case study, we tried three different methods that could be recommended for processing input parameters: the TinInt, InvDist, and Kriging methods [29]. The evaluated background materials were used to create a digital terrain model (Figure 3), a hydraulic conductivity map (Figure 8), a cover layer thickness map, and an aquifer thickness map or a Quaternary-Neogene base interface map (Figure 9). These parameters entered the model as important input values to solve the groundwater level regime. From the evaluation of the geological conditions of the territory, it followed that the thickness of the aquifer increased in the direction from west-northwest to east and from a value of 10-18 m to 50-100 m in the south of the territory. The cover layers of soil throughout the territory were mainly created by clay, the thickness of which is relatively low, ranging from 0.5 to 1.2 m and, occasionally, up to 4 m. The hydraulic conductivity values in the area of the future TEN-T railway track range from 100 to 1000 m\(\cdot\)d\({}^{-1}\) (1 \(\times\) 10\({}^{-3}\)-1 \(\times\) 10\({}^{-2}\) m\(\cdot\)s\({}^{-1}\)). According to these values, the aquifer horizon could be evaluated as a rock environment, with very high permeability. To simulate the groundwater flow, TRIWACO [29] was used. TRIWACO is a simulation software system for the quasi-three-dimensional simulation of continuous groundwater flow based on the finite element method (FEM). The general equation for porous media--partial differential Equation (1) that was solved (by approximation) in the program TRIWACO Flairs--followed from [PERSON]'s law and the equation of continuity. In the derivation of the equation, the Dupuit-Forchheimer assumption was used so that the partial differential equation could be written in terms of the potential 'h' (or groundwater head), as follows:  
\[\frac{\partial}{\partial x}\left[T\frac{\partial h}{\partial x}\right]+\frac{ \partial}{\partial y}\left[T\frac{\partial h}{\partial y}\right]+q=S\frac{ \partial h}{\partial t}, \tag{1}\]  
where \(T\) is the transmissivity (m\({}^{2}\cdot\)s\({}^{-1}\)), \(h\) refers to the piezometric (groundwater) head (m a.s.l), \(S\) refers to the storativity (-), and \(q\) refers to the recharge (m\({}^{3}\cdot\)s\({}^{-1}\)/m\({}^{2}\)). For a multi-aquifer system, Equation (1) holds for each aquifer, and the top aquifer may be phreatic. The aquifers are coupled through the recharge \(q\), where the components of the recharge are recharged into the aquifers due to precipitation and infiltration from rivers and from point sources due to leakage between aquifers. In this case, transmissivity is a function of the groundwater head and the hydraulic conductivity of the aquifer, which can spatially vary, while the storativity \(S\) changes if the aquifer changes from a confined to a phreatic condition [29]. It is possible to approximate the 3D flow to a quasi-3D flow by dividing the aquifer into several aquifers, and the program uses FEM to carry out a simulation of the groundwater flow or the head (for a quasi-three-dimensional modelling system). The boundary, necessary to delimit the model, was specified as the boundary of the Bratislava self-governing region.