Structural-permeability favorability in crystalline rocks and implications for groundwater flow paths: a case study from the Aar Massif (central Switzerland)

Abstract

Groundwater flow in granitic bedrock is of major interest for underground projects such as radioactive waste disposal. It is generally accepted that granitic rocks of the upper crust are characterized as faulted low-porosity rocks showing fault-related permeability. In this study, the influence of existing faults on the present-day water flow in the Grimsel Test Site (Switzerland), an underground rock laboratory situated in granitoid rocks, was investigated by mapping water discharges. As a result, the link between water flow and faults considering slip-tendency analysis and fault intersections is evaluated. Water-conducting features were combined in a structural-permeability favorability map. Faults and dykes occur as three orientation groups, NE–SW, E–W, and NW–SE trending, all steeply dipping southwards with fault intersections also steeply plunging southwards. In total, 100 water discharges were mapped in summer 2014 and 85 in winter 2016, which are located along faults or fault intersections. A comparison of water discharges with structures showed that high-slip-tendency metabasic dykes and fault or dyke intersections represent the dominant flow paths. Further, it could be demonstrated that higher slip-tendency tends to lead to enhanced average hydraulic conductivity and therefore more constant water flow. Based on water fluxes, fault intersections are inferred to represent first-order locations of water percolation followed by high-slip-tendency metabasic dykes. The combination of all water-conducting features into a structural-permeability map results in covering all water discharges. Therefore, the structural-permeability favorability map can serve as suitable representation for constraining water inflow in fractured granitoid host rocks.