Fernández Lagunas, Albert ; Hermans, Thomas ; Van Noten, Koen ; Deckers, Jef

Talk presented at 48th IAH Congress on 2021-09-06

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 affects the flow patterns and travel time and is of high importance for characterizing the protection zones around groundwater extraction wells. This study focuses on the Grote Brogel Fault (GBF), which is a major WNW‐ESE striking normal fault in Belgium. Previous studies at two sites along the GBF, Maarlo and Bree, have integrated Electrical Resistivity Tomography (ERT), Cone Penetration Test (CPT) and borehole data. Monitoring data of the groundwater levels, showing large hydraulic head differences 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. 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, numerical groundwater models were built. A preliminary sensitivity analysis was performed to determine the parameters controlling the fault zone hydrogeological behaviour. 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 and hydraulic conductivity (K) of the fault zone are the most crucial parameter controlling its behaviour. Moreover, the ratio between the formation K and the fault K and the dip of the fault control the steepness of the hydraulic gradient across the fault. After calibration, the model was able to simulate the trends of the hydraulic head in the footwall and the hangingwall which are largely controlled by the recharge rates. For the Bree site, the defined scenarios for the stepover pointed out differences in groundwater flow, specially between soft‐link and hard‐link scenarios. These differences could be observed in the field by installing a monitoring network with several piezometers. Some guidelines are given for the installation of a larger monitoring network.

Keyword(s): Ground water flow ; Grote Brogel Fault ; Roermond Valley Graben