Spatial Distribution of Preferential Groundwater Pathways in the Poehla Mine (Ore Mountains, Saxony, Germany)

Abstract:

Groundwater flow in fractured basement rocks on aquifer scale and processes involved in the creation of fracture network permeability are poorly understood even though they have been studied for decades. A unique hydrogeological dataset from mining activities of the uranium producer in former East-Germany SDAG Wismut has been compiled and quantitatively interpreted. In this presentation we focus on data from the Poehla ore mine where a total of about 800 preferential groundwater inflows to about 115 km of drifts and 12 km of boreholes could be analysed down to a depth of 900 m below ground surface. The dataset consists of localized inflow rates, petrography of the host rock, mineralogy of the fracture infill, hydrochemical measurements, structural information and periodic cumulative inflow measurements to larger mine sections. Transmissivities in range of 3E-4 m²/s and 4E-13 m²/s were calculated from the inflow measurements using simplified analytical approximations and show no distinct depth-dependency. Highest inflows correlate with steeply dipping and preferentially NW-SE-oriented fractures, which is a direction prone for shearing. Most transmissive fractures show calcareous infill material or no mineralization. Equivalent block permeabilities were calculated by integration of inflow over 100 m long drift sections. Rock-matrix hydraulic conductivities were estimated from humidity transfer by ventilation for intervals without discrete inflows. The 100 m-block permeability shows a decreasing trend with depth due to increasing number of intervals not exhibiting distinct inflows. The derived permeabilities were successfully verified for 2 mine levels (~300 m and ~550 m below surface) with cumulative inflow data and a simplified axisymmetric, numerical, steady-state model. As the structural and mineralogical mine inventory is also very well documented, a GoCad model has been set up and multivariate statistical analyses in 3D will be conducted to investigate spatial correlations between fracture transmissivity and local mineralogic-tectonic conditions. These correlations shall aid in the understanding of processes creating and destructing permeability of fractured rocks at various depth.