Abstract
Hard rocks or crystalline rocks (i.e., plutonic and metamorphic rocks) constitute the basement of all continents, and are particularly exposed at the surface in the large shields of Africa, India, North and South America, Australia and Europe. They were, and are still in some cases, exposed to deep weathering processes. The storativity and hydraulic conductivity of hard rocks, and thus their groundwater resources, are controlled by these weathering processes, which created weathering profiles. Hard-rock aquifers then develop mainly within the first 100 m below ground surface, within these weathering profiles. Where partially or noneroded, these weathering profiles comprise: (1) a capacitive but generally low-permeability unconsolidated layer (the saprolite), located immediately above (2) the permeable stratiform fractured layer (SFL). The development of the SFL’s fracture network is the consequence of the stress induced by the swelling of some minerals, notably biotite. To a much lesser extent, further weathering, and thus hydraulic conductivity, also develops deeper below the SFL, at the periphery of or within preexisting geological discontinuities (joints, dykes, veins, lithological contacts, etc.). The demonstration and recognition of this conceptual model have enabled understanding of the functioning of such aquifers. Moreover, this conceptual model has facilitated a comprehensive corpus of applied methodologies in hydrogeology and geology, which are described in this review paper such as water-well siting, mapping hydrogeological potentialities from local to country scale, quantitative management, hydrodynamical modeling, protection of hard-rock groundwater resources (even in thermal and mineral aquifers), computing the drainage discharge of tunnels, quarrying, etc.
Highlights
Crystalline rocks, or hard rocks (HR), are plutonic and metamorphic rocks (Michel and Fairbridge 1992); marbles, cannot be categorized as HR aquifers as they often constitute karstic aquifers
This paper aims to describe the role of weathering in hard-rock aquifers (HRA) hydrodynamic properties and to present a corpus of efficient operational applications that have been deduced from these genetic concepts
The demonstration that HRA are constituted by rather homogeneous layers at least at the scale of a few hundreds of meters allows their deterministic mathematical modelling with “classical models” similar to those used for sedimentary aquifers modelling
Summary
Crystalline rocks, or hard rocks (HR), are plutonic and metamorphic rocks (Michel and Fairbridge 1992); marbles, cannot be categorized as HR aquifers as they often constitute karstic aquifers. Lachassagne et al (2011) and 2014b describe in detail that several authors, in various regions of the world (Africa, India, Australia, Europe, etc.), have acknowledged for several decades the role of weathering in the creation of the unconsolidated part (the saprolite) of the HRA: see for instance Davies et al (2014), who describe case studies in Africa (Tanzania, Zimbabwe, Nigeria, SouthAfrica), Madagascar, Asia (Hong-Kong (China), India), and Brazil, and in Europe, in the UK It is only in the last 15 years that the creation of the fractured layer was demonstrated to be linked to the same weathering processes (Dewandel et al 2006, 2011; Lachassagne et al 2011). The main outcome is that the stratiform fractured layer is considered to belong to the weathering profile as well as the above-lying unconsolidated layer (the saprolite; Fig. 1). Worthington et al (2016) confirm this conceptual model, which is today more and more accepted within the hydrogeological community and used by several authors to explain their observations (see for instance Allé et al 2017; Baiocchi et al 2014, 2016; Guihéneuf et al 2017; Soro et al 2017; Vouillamoz et al 2014, 2015; Wubda et al 2017, etc.)
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