A comprehensive worldwide permeability data set has been compiled consisting of 29,000 in situ permeabilities from 221 publications and reports and delineating the permeability distribution in crystalline rocks into depths of 2000 meters below ground surface (mbgs). We analyze the influence of technical factors (measurement method, scale effects, preferential sampling, and hydraulic anisotropy) and geological factors (lithology, current stress regime, current seismotectonic activity, and long‐term tectonogeological history) on the permeability distribution with depth, by using regression analysis and k‐means clustering. The influence of preferential sampling and hydraulic anisotropy are negligible. A scale dependency is observed based on calculated rock test volumes equaling 0.6 orders of magnitude of permeability change per order of magnitude of rock volume tested. Based on the entire data set, permeability decreases as log(k) = −1.5 × log(z) − 16.3 with permeability k (m2) and positively increasing depth z (km), and depth is the main factor driving the permeability distribution. The permeability variance is about 2 orders of magnitude at all depths, presumably representing permeability variations around brittle fault zones. Permeability and specific yield/storage exhibit similar depth trends. While in the upper 200 mbgs fracture flow varies between confined and unconfined, we observe confined fracture and matrix flow below about 600 mbgs depth. The most important geological factors are current seismotectonic activity (determined by peak ground acceleration) and long‐term tectonogeological history (determined by geological province). The impact of lithology is less important. Based on the regression coefficients derived for all the geological key factors, permeability ranges of crystalline rocks at site scale can be predicted. First tests with independent data sets are promising.

An outstanding legacy data set has been compiled from underground excavations mostly prospected and mined by the former Soviet (German) Stock Company Wismut describing the hydrology of faulted basement rocks in the Ore Mountains (SE Germany). It consists of more than 5000 detailed descriptions of groundwater inflows to about 660 km of tunnels and 57 km of drillings measured during or shortly after excavation. Inflow measurements (recorded between 1E−8 and 4E−2 m3/s) have been converted to fracture transmissivities using a simplified analytical solution. Discarding site specific effects, the median log transmissivity decreases from 1E−7 to 1E−10 m2/s within the studied depth interval of 0–2000 meters below ground surface (mbgs), and the spacing of conductive fracture increases from 0.1 to 2500 m. This general trend is overprinted at three mining sites by a clear reversal of fracture transmissivity which correlates with contact metamorphic aureoles around Variscan granite intrusions (327–295 Ma). We hypothesize that this transmissivity increase is caused by processes accompanying granite intrusion and contact metamorphism. The thickness of these hydraulically active aureoles is greater in lower‐grade metamorphic schist than in higher‐grade metamorphic gneisses. Rock mass equivalent continuum conductivities have been estimated by arithmetic averaging of fracture and matrix transmissivities over 100 m intervals and have been converted to permeabilities. The median equivalent continuum permeability decreases with depth according to log(k) = − 1.7 * log(z) − 17.3 (k in m2 and increasing depth z in kilometer being positive). Matrix conductivity controls the bulk conductivity below about 1000 mbgs and is less sensitive to the occurrence of contact metamorphic aureoles.

A comprehensive dataset for discrete groundwater inflows to mines in the Poehla‐Tellerhaeuser Ore Field and the mining scale fault zones has been compiled from unpublished data recorded by eastern German and Soviet hydrogeologists at the Soviet‐German stock company (SDAG) Wismut. This dataset has been analyzed to provide novel insights into the 3D distribution of preferential groundwater pathways and the impacts of faulting on the distribution of hydraulic parameters in crystalline rocks at site scale. The sampled 1030 discrete inflows include flow rates ranging from 1.7E‐8 to 3.7E‐2 m3 sec−1, which were transformed into mesoscale fracture transmissivity values ranging between 3E‐13 and 2E‐4 m2 sec−1. These mesoscale fracture transmissivities were spatially correlated with fault zones exhibiting trace lengths between 0.3 and 30 km, which were mainly formed during and reactivated several times since Variscan orogeny. The statistical correlations are based on a 3D geological model composed of 14 litho‐stratigraphic units and 131 mining scale faults, separated into five main strike directions. These fault zones strongly overlap and cover about 90% of the investigated rock mass volume with a decreasing percentage of overlap in the investigated depth range (0–900 mbgs). 97% of all inflows are located within fault damage zones, and most of the flow occurs within the overlap of multiple fault damage zones. A dimensionless hydraulic model for the distribution of flow Q as a function of the position x within mining scale fault zones has been derived as Q = 1.1e−4.5x (where x decreases from the fault core to the protolith and the exponent varies as a function of fault orientation). 75–95% of the flow occurs within the inner 50% of the damage zone, and mainly NW‐SE and NE‐SW striking mining scale faults are transmissive. The orientations of conductive mesoscale fractures within these damage zones show a larger variability than the corresponding mining scale faults.

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