Fault-zone hydraulic conductivity distribution in unconsolidated sediments: insights from a trench study across the Peel Boundary Fault Zone, the Netherlands

Rimbaud E. Lapperre; Victor F. Bense; Cornelis Kasse; Jelle T. Buma; Ronald Harting; Ronald T. van Balen

doi:10.70712/njg.v105.12686

Rimbaud E. Lapperre Department of Earth Sciences, VU University Amsterdam, Amsterdam, the Netherlands https://orcid.org/0000-0003-3954-266X
Victor F. Bense Wageningen University & Research, Wageningen, the Netherlands https://orcid.org/0000-0002-3675-5232
Cornelis Kasse Department of Earth Sciences, VU University Amsterdam, Amsterdam, the Netherlands https://orcid.org/0000-0002-1466-1016
Jelle T. Buma TNO – Geological Survey of the Netherlands, Utrecht, the Netherlands https://orcid.org/0009-0001-7090-0157
Ronald Harting TNO – Geological Survey of the Netherlands, Utrecht, the Netherlands https://orcid.org/0000-0002-7228-3375
Ronald T. van Balen Department of Earth Sciences, VU University Amsterdam, Amsterdam, the Netherlands; and TNO – Geological Survey of the Netherlands, Utrecht, the Netherlands https://orcid.org/0000-0002-8019-7426

DOI: https://doi.org/10.70712/njg.v105.12686

Keywords: anisotropy, fault hydrogeology, grain size distribution, permeability, rift system

Abstract

In the subsurface, the faults of the Peel Boundary Fault Zone, part of the Roer Valley Rift System in the Netherlands, Germany, and Belgium, often act as fault-parallel conduits and fault-perpendicular barriers for groundwater flow. However, our understanding of the conduit–barrier architecture of these faults remains limited. This study addresses this gap by presenting a detailed overview of the spatial variation in hydraulic conductivity and anisotropy, based on 161 collected samples, resulting in 150 measurements of saturated hydraulic conductivity (K_sat) and total porosity (P_total), and 151 measurements of median grain size (GS) from a trench site near Uden, the Netherlands. The results indicate that K_sat values across the fault range by almost four orders of magnitude, from 0.004 to 32.1 m day⁻¹. Within this range, the fault exhibits the lowest horizontal (0.01–3.6) and vertical (0.004–7.1) K_sat values. Compared to the fault, values in the foot-wall damage zone (horizontal: 1.4–22.6 and vertical: 0.08–18.4) are significantly higher for both orientations, whereas in the hanging-wall damage zone (horizontal: 2.0–22.7 and vertical: 1.8–9.5), all values are higher, but only the horizontal values reach statistical significance. These findings indicate that the conduit–barrier effect of faults in unconsolidated sediments extends to the near surface. Hydraulic conductivity largely correlates positively with median GS but shows little to no overall correlation with total porosity (P_total). This trench study also reveals that the near-surface conduit–barrier architecture and hydraulic conductivity distribution is likely to evolve over geological timescales due to ongoing hydrogeological processes and fault activity. Such insight has the potential to contribute to future approaches for protecting and restoring fault-related seepage areas known as ‘wijstgronden’.