## 2 Materials and Methods
### Hydrogeological Study  
During the project stage, a detailed hydrogeological characterization of the massif was carried out by the company EPTISA, determining the conceptual model of the aquifer's functioning, quantifying its water uses, and establishing the relationship between groundwater and the planned works before their execution. The hydrogeological study and its monitoring during the works were identified as critical elements in ensuring the final success of the project [25].  
Figure 4: Location of the executed tunnel within the influence zone of the Autzagane aquifer. The figure shows the alternation of more or less permeable layers that explain its nature.  
Figure 5: Calacreous sandstone facies with isolated intercalations of marl material, recognized in the advance of Axis 1.  
In order to achieve these objectives and establish the effects that tunnel drilling could have on the natural functioning of groundwater and its uses, various networks of piezoelectric and forometric control were installed.  
Finally, the project included the creation of a series of numerical models of groundwater flow that have allowed validating the conceptual model of natural groundwater functioning and thus predict the effects of the tunnel on the aquifer's functioning and its water uses.  
In order to carry out the hydrogeological study, existing information was analyzed first, including the construction projects that the promoter had drawn up in the work area, as well as general geological and hydrogeological studies that encompass the study area [26]. A total of 16 boreholes were drilled in which the piezometric level could be measured, with drilling depths ranging from 17 to 91 m.  
In order to obtain the hydraulic parameters of the aquifer, several pumping tests were conducted. In this test, water is extracted in a controlled manner for a significant period of time, which is one of the most widely used methods for obtaining the aforementioned data [27]. In the research campaign carried out for the study, once the piezometric wells and pumping wells were drilled, three pumping tests were carried out in the wells called B-1, B-2, and B-3. Each test consisted of extracting water from the well and observing the variations in the water level depth produced in the pumping well itself and in all the existing perforations in the surrounding area that have been conditioned as piezometers. In each well, a stepped pumping stage, a constant flow stage, and a recovery stage were carried out (Figure 6). Mean permeability values of \(3.5\times 10^{-6}\) m/s and a storage coefficient of \(1.2\times 10^{-3}\) were obtained.  
During the hydrological study, 41 water points were identified. The main use of groundwater resources in the area closest to the south portal of the Urdinbide tunnel occurred in the headwaters of the Ategorri stream. In this sector, there were a total of 4 groundwater intakes, 3 surface water intakes, 1 distribution chamber, and a reservoir, connected by a complex network of pipes (Figure 7), all belonging to the water supply system of the Amorebieta-Etxano municipality.  
Arometric network was also installed in order to allow continuous monitoring of the flow rates of the springs and intakes in the study area. To implement this control, the following infrastructure was established to determine the circulating flow rates: 3 electromagnetic flow meters with a diameter of 50 millimeters (two of them in the pipes entering the distribution chamber and the third in the pipe that goes from the Ategorri-A weir to the reservoir) and a 3-inch Parshall flume in the Ategorri stream upstream of the weir.  
Figure 6: Graphical representation of pumping test data in well B-3.