Implications of stress field evolution on groundwater flow at the northern Variscan front and its relevance for the Einstein Telescope site selection (Euregion Meuse-Rhine)

The Einstein Telescope (ET), a proposed third-generation underground gravitational-waves observatory requires an acoustically quiet subsurface environment to minimize the effect of seismic ambient noise. A detailed site characterization is currently underway in the Euregion Meuse-Rhine (Germany, Belgium, Netherlands), one of the potential locations for the ET. The site is wedged between the northern margin of the Variscan deformation front and the subsiding Lower Rhine Graben. Subsurface fluid flow, including natural groundwater circulation and drainage constitute sources of induced seismic and gravity-gradient variations. Pre-existing faults and fractures act as preferential flow paths and, critically control potential water inflow into the infrastructure and influence related water management strategies. Consequently, characterizing the relationship between the tectonic stress field and hydraulic characteristics of the host-rock formations is essential for a resilient ET design. In this study, we investigate the evolution of the tectonic stress field (i.e., from paleo- to recent in-situ stresses) and its control on fracture permeability, using an integrated dataset of boreholes drilled in the study area from 2021 to 2025 and down to 250 to 420 m. Paleo-stress conditions are reconstructed from fracture orientations and kinematic indicators observed on drill core material and borehole-televiewer data. The present-day stress-state is evaluated using hydraulic fracturing and hydraulic testing of pre-existing fractures tests. Fracture architectures are characterized using televiewer imagery and core samples, while their hydraulic relevance is assessed through in-situ methods such as impeller flow-meter measurements, temperature logs, and hydraulic packer tests. Slip versus dilation tendency analysis is applied to evaluate deformation modes and associated permeability anisotropies. These results are compared to independent hydraulic indicators to distinguish between hydraulically active and inactive discontinuities. Our findings demonstrate how the complex tectonic history governs the present-day fracture network and associated groundwater pathways, providing key constraints on groundwater management and suitability assessments for the site selection of the ET project in naturally fractured sedimentary host-rocks.