Temporal and spatial variability of cross-fault groundwater-level differences: the impact of fault-induced permeability reduction, precipitation and evapotranspiration

Abstract

Faults in the Roer Valley Rift System (Netherlands, Belgium, and Germany) act as barriers to lateral groundwater flow in unconsolidated sedimentary aquifers. This causes a cross-fault groundwater-level step of up to several metres. Using a dataset obtained through 5 years of high-frequency monitoring, the effect of fault-zone permeability, precipitation and evapotranspiration on cross-fault groundwater-level steps is studied at two sites situated across the Peel Boundary Fault. Hydraulic conductivity values at the fault are 1–3 orders of magnitude lower than that of similar lithologies away from the fault, indicating that fault displacement has a significant impact on groundwater flow. The influence of precipitation and evapotranspiration on fault-zone hydrology is inferred from water-table fluctuations over short distances across the fault. On the foot wall, the water table is nearer to the surface and displays a shorter level range with a spiky temporal variability. On the hanging wall, a deeper water table is sloping away from the fault and shows a wider level range with a smoother temporal variability. The observed groundwater level fluctuations are attributed mainly to precipitation and evapotranspiration dynamics. At a larger spatial scale, the 5-year-average cross-fault groundwater-level steps at the two sites are 1.59 and 1.39 m. At a smaller scale, the cross-fault groundwater-level step is much less because of the rising water table towards the fault on the hanging wall. At the smallest scale, just across the fault zone, the groundwater level step is around 0.2 m, indicating that the fault is semi-impermeable.