Review: Hydrogeology of weathered crystalline/hard-rock aquifers—guidelines for the operational survey and management of their groundwater resources

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.

Improving our ability to model crystalline aquifers using field data combined with a regionalized approach for estimating the hydraulic conductivity field

“…Maréchal et al, 2004;Dewandel et al, 2012). Furthermore, it confirms that where granitic rocks are exposed to deep weathering processes, aquifers are characterized by similar ranges of hydrogeological properties (Dewandel et al, 2006;Lachassagne et al, 2011Lachassagne et al, , 2021, which facilitates, and justifies, the use of training data in the absence of sufficient local data, as it was done here. Although the method does not claim to be perfectly accurate, the produced map, a first-order result, describes the spatial heterogeneity and shows that sectors of similar hydraulic conductivity cover areas of the order of few km 2 .…”

“…However, in such a granitic weathering profile, the hydraulic conductivity of the fractured layer, due to a denser horizontal network, is about 10 times higher horizontally than vertically (Maréchal et al 2004;Lachassagne et al, 2021), which thus promotes horizontal flows. In addition, as the method uses a basin to sub-basin scale approach and that the aquifer thickness is small (a few ten meters) compared to the sub-basin scale (km scale), then at this scale Published in Journal of Hydrology (2021); https://doi.org/10.1016/j.jhydrol.2021.126652 horizontal flows dominate (e.g.…”

“…Moreover, where exposed to deep weathering processes, such rocks develop several stratiform layers, parallel to the weathering surface. Layers are mainly a saprolites layer (highly weathered parent rocks of low-permeability) and an overlying fractured layer (most permeable), in which hydrogeological properties are closely related to the degree of weathering (Taylor and Howard, 2000;Wyns et al, 2004;Dewandel et al, 2006;Lachassagne et al, 2011Lachassagne et al, , 2021.…”

Improving the Methods for Processing Hard Rock Aquifers Boreholes’ Databases. Application to the Hydrodynamic Characterization of Metamorphic Aquifers from Western Côte d’Ivoire

“…They are required for domestic, industrial, and agricultural needs (human food). Both for reasons of quantity, particularly during periods of low water levels when surface water may not be available, and for reasons of quality (lower treatment costs than surface water), groundwater is being used more and more frequently throughout the world [1]. It is now almost systematically utilised in African countries [2] and frequently in hard rock contexts.…”

“…It is now almost systematically utilised in African countries [2] and frequently in hard rock contexts. Hard rocks (HR) are plutonic and metamorphic rocks, from which we exclude marbles, as they can be karstified, and obviously limestones and non-metamorphosed volcanic rocks [1]. Hard rock aquifers generally occupy the first tens of metres below ground surface [3].…”