Summary:

Fractures constitute some of the most permeable groundwater migration pathways. The directional regularity of fracture sets largely determines the hydraulic anisotropy (single- or multi-directional) of fractured rocks. Knowledge of the preferential directions of possible water flow and pollutant migration paths along fracture-controlled azimuths is useful for understanding the flow patterns and spread of pollutants in a given aquifer. Yet systematic regional fracture mapping is still rare in hydrogeological studies. To assist in ongoing predictive modelling of mine water pollution plume migration within the Magnesian Limestone Aquifer (Permian) in County Durham, U.K., a survey was undertaken of basic fracture characteristics, including angles of dip, strike directions, aperture and frequency. These parameters were measured both in natural exposures and working quarries across the outcrop of the aquifer in the study area. Scrutiny of the measurements reveals the presence of two major fracture sets, one oriented WNW to NNW and the other oriented NNE to ENE. These are likely to be extensional joints, with perhaps a few minor faults, that form reasonably consistent conjugate sets distributed across the Permian outcrop, and suggest that the permeability tensor in the aquifer is also likely to have axes oriented with maxima trending NNE to ENE. This is only slightly oblique to the general eastward trend of groundwater flow towards the coast, suggesting a mild tendency to sinistral preference in transverse dispersivity.