A generalized 3-D geological and hydrogeological conceptual model of granite aquifers controlled by single or multiphase weathering
Résumé
The weathered layers of crystalline rocks form aquifers that are of prime interest for water supply in hard rock areas. These weathering profiles generally develop under both stable geodynamic conditions (weathering rate ≫ erosion rate) and a hydrolysing climate. They are composed of thick stratiform layers that follow the paleo-landscape (paleotopography) and thus present a gently dipping sequence at a regional scale.
The structure and the hydrodynamic properties of the weathering profile of a granitic area (53 km2 Maheshwaram catchment, state of Andhra Pradesh, India) were characterized in detail and mapped from observations on outcrops, 80 vertical electric soundings and lithologs from 45 borewells in which flowmeter measurements and injection tests were also performed to characterize the hydraulic conductivities of the conductive fissure zones.
The structure of the weathering profile results from a multiphase process: an ancient weathering profile was partly eroded, down to its fissured layer. It was later re-weathered more or less parallel to the current topographic surface. This peculiar structure is linked to the geodynamic history of the Indian Peninsula that underwent alternate weathering and erosion-dominated phases. The profile is thus composed, from top to bottom, up to a total depth of 35 m, of sandy regolith saprolite (1–3 m), 10–15 m of laminated saprolite containing unusual preserved fissures and a 15–20 m thick fissured layer.
By comparing various case studies in similar terrain, a generalized 3-D geological and hydrogeological conceptual model of granite-type aquifers (granites, gneisses, etc.) is proposed. In granite-type rocks, single weathering or multiphase weathering and erosion processes induce similar geological structures. A few main geological differences arise from the comparison of single phase and multiphase weathering profiles: (i) in the later, the respective thicknesses of the various layers can be deeply modified, (ii) the ancient fissured layer can be re-weathered, and the resulting new laminated layer may contain quite well preserved and conductive ancient fissures, and (iii) the upper part of the fissured layer is more densely fissured. As a result of the same weathering processes, the fissures from the fissured layer of single phase or multi-phase profiles exhibit very similar hydraulic conductivities, and show both a higher density of conductive fissures at the top of the layer. The preserved ancient fissures within the laminated layer of a multiphase profile are partly obliterated by the recent weathering; their hydraulic conductivity is thus significantly reduced. Nevertheless, they do significantly contribute to borewell yield. These weathering-induced fissures provide most of the aquifer permeability.
The use of such a conceptual model for the precise geological mapping of the weathering structure appears to be a prerequisite for groundwater development and management in hard-rock areas as it answers to several key issues.