Fernández Lagunas, Albert

Master thesis supervised by Hermans, Thomas; Van Noten, Koen (UGent)

Abstract: The border faults of the Roer Valley Graben (RVG) have been reported to act as barriers to groundwater flow and altering its regional behaviour. At the location of these faults, high hydraulic gradients have been observed due to the low permeability of the fault zone. This study focuses on the Grote Brogel Fault (GBF), which is a major WNW-ESE striking normal fault in Belgium that diverges from the main NW-SE striking border fault system of the RVG. Deckers et al. (2018) conducted a study at two sites along the GBF, Maarlo and Bree, in which these authors integrated Electrical Resistivity Tomography (ERT), Cone Penetration Test (CPT) and borehole data. Monitoring data of the groundwater levels, which showed a large hydraulic head difference of up to 13 m as a result of the GBF acting as a flow barrier, is available for both sites. In one of the sites (Bree) a local stepover in the shallow subsurface was observed, affecting the groundwater levels in the different fault blocks (Deckers et al., 2018). The aim of the present study is to investigate the role of the GBF on the local hydrogeological conditions, considering local fault complexities observed along this border fault. To achieve the main objective, a couple numerical groundwater models were conducted. A preliminary sensitivity analysis was performed to determine the parameters controlling the fault zone hydrogeological behaviour. With the obtained knowledge, the monitoring data from Maarlo was used for calibration and further validation of the model. Finally, a model was set up for the Bree site to test different fault linking scenarios and observe their response to groundwater flow. The results of the sensitivity analysis showed that the thickness of the fault zone is the most crucial parameter controlling its behaviour. The hydraulic conductivity (K) is also a determining parameter as the ratio of the thickness and the K of the fault define the resistance to groundwater flow. Moreover, the ratio between the formation K and the fault K controls the steepness of the hydraulic gradient across the fault. Once calibration was achieved, the model was able to simulate the trends of the hydraulic head in the footwall and the hangingwall. The calibrated parameters are coherent with those discussed by Lapperre et al. (2019) for already existing groundwater models in the RVG. For the Bree site, the defined scenarios for the stepover pointed out differences in groundwater flow, specially between the soft-link and the hard-link scenarios. These differences could be observed in the field by installing a monitoring network with several piezometers. Some guidelines will be given for the installation of a larger monitoring network.

Keyword(s): faults ; hydrogeology ; Roer Valley Graben