Origin of fracturingin hard‐rock aquifers: what are the factors controlling the properties of the fractured layer?

Robert Wyns (1) , Benoît Dewandel (1) , Patrick Lachassagne (2)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
2 Evian-Volvic-Sources

The discovery,during the last 1990s, of a thick fractured layer below the saprolite in lateritic profiles developed on crystalline rocks has resulted in a redefinition of hardrock aquifers. During the first 2000s, numerous researches were conducted in different continents in order to validate thisnew concept of stratiform hardrock aquifer linked to weathering processes.Commonly, the term of " lateritic profile " is used for subtractive weathering profiles developed on metamorphic, plutonic or volcanic rocks. They are the only rocks that are able to develop a fractured layer atdepth. Figure 1–Structure of a lateritic profilepartially eroded by the present topography A lateritic profile usually shows, from top to bottom (Figure 1):-A ferraliticduricrust, 1 to 10 m thick, resulting from recrystallization of goethite (iron hydroxide) to hematite (iron oxide) due to seasonal desiccation of the top of the profile. Where preserved from later erosion and recharged by heavy rainfall this duricrust can give rise to seasonal perched aquifers, particularly in tropical humid areas.-Loose alterites (saprolite), made up of a mixing of clays, hydroxides, oxides and residual minerals (quartz). At the top of saprolite, mottled clay (meter-thick) is a transition horizon to iron crust. In granular rocks (granitoids, gabbros), the lower part of the saprolite shows a characteristic laminated texture (" laminated layer "), due to high density of tension microcracks (millimetric spacing).The saprolite is of rather low hydraulic conductivity, about 10-6 m/s in granites, lower in their laminated layer and at the top (more clayey) of the saprolite (10-7-10-8 m/s). This layer plays the capacitive role of the hard rock aquifer.-A fractured layer, characterized by a high density of cracks in the hard rock that plays the permeable role of the hard rock aquifer. The density andconnectivity of

Mots clés: Hardrock aquifer; weathering; hydrogeology

Domaines: Hydrologie; Géologie appliquée

Robert Wyns, Benoît Dewandel, Patrick Lachassagne. Origin of fracturingin hard‐rock aquifers: what are the factors controlling the properties of the fractured layer?. Flowpath 2017, IAH Italian Chapter, Jun 2017, Cagliari, Italy. pp.14 - 16. ⟨hal-01507252⟩