This post is part of the GeoConnect³d blog.
The influence of faults on groundwater movements is well-known. In the Cenozoic Roer Valley Rift System in northwestern Europe, nearly all faults in the shallow subsurface act as barriers for groundwater flow (Bense et al., 2003). In this way they prevent groundwater flow from the topographic high footwall areas towards the topographic low hangingwall areas. This results in relatively high groundwater tables in the footwall areas (Fig. 1). Locally this groundwater even reaches the surface, leading to seepage. The seeped groundwater typically shows temperature anomalies and iron-enrichments, as it travelled from the deeper subsurface upwards along the fault zone. As the dissolved iron reaches the surface, it oxidizes and precipitates. The precipitated iron – such as those in fault zones – was used as a renewable source for iron ore production since Roman times. The peculiar topographic high, wet, iron ore enriched areas are known as Wijstgronden in the Netherlands (subscript Fig. 1). They are associated with a particular fauna and flora (Ettema, 2010). The Wijstgronden all developed along the footwall of the Peel Boundary fault system, a major fault delimiting the Roer Valley Rift System in the northeast.
The influence of the Peel Boundary fault on hydrological conditions in the Netherlands was intensely investigated since the mid-20th century. Along the major faults at the southwestern flank/side of the rift system, located in Belgium, little research was performed on the influence of major faults on groundwater levels. To fill this knowledge-gap, Deckers et al. (2018) analyzed the influence of the major Grote Brogel fault (GBF) on local hydrological conditions. The GBF delimits the topographic high Campine Plateau in its footwall (> 60 m reference level) from the topographic low plains in its hangingwall (< 50 m reference level). Deckers et al. (2018) selected two investigation sites, one in Bree near the eastern tip of the GBF and one in Maarlo near the western tip of the same fault. First, they localized the GBF in the shallow subsurface by combining topographic analyses, borehole (B1 and B2 on Fig. 2) and Cone Penetration Test interpretations as well as Electrical Resistivity Tomography analyses. Once the fault traces were localized, groundwater measurements were performed within the boreholes on each side of the GBF for the next months.
The groundwater measurements revealed a consistent, major jump in groundwater table across the GBF, on average 12.7 m at the Bree site and 6 m at the Maarlo site (Fig. 2). This major hydraulic head difference shows that the GBF – like many other faults in the Roer Valley Rift System – forms an important barrier for groundwater flow. The amounts of natural hydraulic head differences at the GBF are markedly higher than across other faults in the Roer Valley Rift System, which are generally in the order of 1-3 m (Lapperre et al., 2019). The large hydraulic heads at the GBF can be related to the high topographic gradients at the investigation sites. As the vertical throw of the GBF decreases from east to west, its topographic gradient decreases in the same direction, together with the resulting hydraulic head from the Bree site towards the Maarlo site. The reason for the low permeability along the GBF and other faults in the Roer Valley Rift System is still a matter of debate. It is notable, however, that the fault barrier is formed by a dynamic process, since according to farmers, even after deep ploughing of fault zones to improve water drainage, their agricultural land suffers from renewed wetness within a period of only a few years (Lapperre et al., 2019).
VITO, Vlaams Instituut voor Technologisch Onderzoek – Belgium
Bense, V.F., Van Balen, R.T., and De Vries, J.J., 2003. The impact of faults on the hydrogeological conditions in the Roer Valley Rift System: an overview. Netherlands Journal of Geosciences, 82, 41-53.
Deckers, J., Van Noten, K., Schiltz, M., Lecocq, T., and Vanneste, K., 2018. Integrated study on the topographic and shallow subsurface expression of the Grote Brogel Fault at the boundary of the Roer Valley Graben, Belgium. Tectonophysics, 722, 486–506.
Ettema, N., 2010. Vijf Wijstreservaten in Noord-Brabant. Stuurgroep DeMaashorst: 63 pp.
Lapperre, R.E., Kasse, C., Bense, V.F., Woolderink, H.A.G., and Van Balen R.T., 2019; An overview of fault zone permeabilities and groundwater level steps in the Roer Valley Rift System. Netherlands Journal of Geosciences, 98, e5, 1-12.
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