Impact of gently dipping discontinuities on basement aquifer recharge: An example from Ploemeur (Brittany, France) S. Ruelleu a , F. Moreau a , O. Bour a, ⁎, D. Gapais a , G. Martelet b a Géosciences Rennes, UMR 6118 CNRS, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France b BRGM, 3 av. Claude Guillemin, BP6009, 45060 Orléans Cedex 2, France abstract article info Article history: Received 16 April 2009 Accepted 17 December 2009 Keywords: Fractured rocks Gravity Gravity modeling Crystalline rocks Groundwater Classically, geological surveys of water resources in hard-rock aquifers are made from aerial photographs or geophysical techniques that basically permit to detect vertical features. On the other hand, aquifers only linked to vertical structures are very limited in space and offer in most cases very poor water resources. In the present case, we do show that an outstanding aquifer in Brittany is linked to a geological context associated with low-angle faults possibly directly connected with vertical feeders. In particular, we show through a high-resolution gravimetric survey that the highly fractured contact between granite and micaschists, which constitutes the main path for groundwater flow, is a gently dipping structure. Combined gravimetric, magnetic and geological data allowed us to establish the overall aquifer geometry by running a three-dimensional forward model. In addition, details about the shape of the contact have been obtained using an iterative scheme based on the method of Oldenburg (1974). The gravimetric model confirms the presence of sub-vertical faults that may constitute important drains for the aquifer recharge. Thus, the geological context associated with exceptional water resources for this crystalline aquifer is characterized by a sub-horizontal to gently dipping contact between granite and micaschists. Only such a geological context can allow sufficient recharge to provide the main water supply for a town of 18,000 inhabitants at an average rate of about a billion of cubic meter per year since 1991. Thus, instead of looking for possible vertical structures like in classical hard-rock hydrogeology, it appears much more efficient to detect sub-horizontal permeable fractures and faults for providing consistent water resources. © 2010 Elsevier B.V. All rights reserved. 1. Introduction At the global scale, the distribution of water resources depends highly on climate and geological environment. The climate governs the hydrologic cycle and controls the amount of renewable water in the watersheds. On the other hand, the geological conditions control the storage of part of the renewable water in the aquifers. Due to their low permeability and porosity, crystalline or more generally base- ment aquifers are often considered as poor water resources. Nevertheless, increasing interest is given to such areas because they are widespread and because of the increasing need for water supplies. Particular interest is given to rock fractures that may enhance the rock permeability by three to four orders of magnitude (Kiraly, 1975; Clauser, 1992; Aquilina et al., 2004). For instance, the main water resources in Africa often come from groundwater resources located in fractures within crystalline rocks (Wright et al., 1994). A major problem in water resource prospecting in crystalline rocks is the detection of the permeable fractures where the flow is localized. Hence, many boreholes drilled in crystalline rocks are often unproductive because most of them have not crossed water-bearing fractures (Boeckh, 1992). Hydrogeological surveys in crystalline terrains are classically based on large-scale satellite imaging or aerial mapping of fractures (Sander, 2007), and on local-scale geophysical and geological prospecting (Telford et al., 1990). Large-scale satellite or aerial imagery may be useful to localize lineaments and shallow weathered areas that may locally provide some valuable water resources. It may be also very helpful to detect potential water drains as well as to image the variability of fracture density (Sander, 2007). Such information may be critical to identify a potential prospecting site. The power of such analysis is nevertheless highly dependent on vegetation cover and degree of weathering. Moreover, even when correct conditions are encountered, satellite or aerial mapping in zones of limited relief is more efficient for detecting sub-vertical fractures than gently dipping ones at a low-angle to the mapping surface. On the other hand, a complementary prospecting tool is provided by geological maps that show contacts between different geological units and associated structural discontinuities. Results obtained through these methods are often combined with those of classical geophysical methods. Traditionally, electrical and electromagnetic methods are the most popular geophysical tools for Journal of Applied Geophysics 70 (2010) 161–168 ⁎ Corresponding author. E-mail address: olivier.bour@univ-rennes1.fr (O. Bour). 0926-9851/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2009.12.007 Contents lists available at ScienceDirect Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo