DEPTH EVOLUTION OF FRACTURED ROCK PERMEABILITY IN SHALLOW CRYSTALLINE FRACTURED ROCK AQUIFERS

Fluid movement and availability in crystalline and heavily metamorphosed rocks is dominated by the secondary porosity generated through fracturing. The distributions of fractures and fracture zones determine permeable pathways and the overall productiveness of these rocks as aquifers. Fractured bedrock in many parts of the world are now being used as sources of water for both drinking and industrial uses. Understanding the sustainability and dynamics of these aquifer systems is crucial for future planning and regulating their use. In this presentation I present the results of a subsurface study of such a region experiencing expanded use of fractured bedrock for aquifers. This field-based study visited 17 wells and logged the distribution of fractures, identified flowing fractures, and hydraulically characterized the rock mass intersecting the borehole. Wells with total depths ranging from 30m to 300m showed trends of decreasing fracture frequency with depth, with hydraulically active fractures showing a similar trend. Of all the fractures encountered, only 4 were deemed to be hydraulically active. Decreases in the number of hydraulically active with depth would have the effect of restricting topographically drive flow systems to near surface regions. Observations of borehole temperature profiles suggest that this is indeed the case with little effects of hydrologically altered profiles below 100m. Mechanisms responsible for permeability alteration include: stress related fracture aperture reduction, reduced frequency of unloading related features, reduction in total fractures creating lower connectivity. Results from this study suggest that active flow systems in these geologic settings are shallow and that fracture permeability outside of the influence of large-scale structures should generally follow a quantifiable decreasing trend with depth.