Linking regional‐scale lineaments to local‐scale fracturing and groundwater inflow into the Päijänne water‐conveyance tunnel, Finland

Abstract

ABSTRACTPotential structural or lithological controls of groundwater flow into the mostly rock‐surfaced Päijänne water‐conveyance tunnel in southern Finland were investigated by integrating geological, hydrogeological and aerogeophysical data. Fracture‐zone geometry and distribution, interpreted from processed aeromagnetic images, and topographic and geological information are compared for three different sections of the Päijänne Tunnel. Documented observations on fracturing and ground‐water inflow from inside the tunnel provide subsurface information on the fractures and their hydraulic properties. Topographic and aeromagnetic lineaments both exhibit general NE–SW and NW–SE orientations at a regional scale. Based on the aeromagnetic interpretation, bedrock fractures intersecting the tunnel are part of a regional fracture network and their evolution can be connected to the main tectonic stages in southern Finland. Their distribution is lithologically controlled and reflects the rock type and its structure. For more detailed scales, a superficial deposit relief map is a powerful tool for obtaining a more accurate location of fracture zones and for evaluating their connection to superficial deposits as possible groundwater reservoirs. Depending on the scale, integrated interpretation using both topographic and aeromagnetic data gives the best results, particularly when supported by independent verification such as the observed fracturing inside the tunnel.NW–SE appears to be the most common strike trend among the measured fractures linked with water inflow. In the magnetic data, this orientation is displayed (1) as swarms of short, faint signatures indicating brittle, shallow features, and (2) as extended, broad, linear magnetic gradients denoting block boundaries. Their lengths, of tens to hundreds of kilometres, suggest that they may also be deep‐reaching. The coincidence and the parallel orientation of these two magnetic features of different scales suggest their genetic relationship: the faulted block boundaries were reactivated at later tectonic stages, resulting in brittle fracturing along the same earlier structural weakness zones. Many of the locations where water‐conducting fracturing occurs, or where groundwater inflow has been measured at a larger scale, are associated with intersecting or individual topographically interpreted fracture zones, especially NW–SE trending ones.