A conceptual hydrodynamic model of a geological discontinuity in hard rock aquifers: Example of a quartz reef in granitic terrain in South India

Abstract

The structure and hydrodynamic properties of geological discontinuities and of a deeply weathered granite aquifer near these structures are described on the basis of geological, geophysical and hydrodynamic investigations in two sites of South India located along a 20-40-metre-wide quartz reef intruding a weathered Archean biotite granite. One of the two sites also comprises a metre-wide dolerite dyke. Weathering processes appear to be at the origin of fissures development and of a related enhanced local hydraulic conductivity, both in the quartz reef and in the surrounding granite. The weathering profile in the granite (saprolite and fissured layer) is characterized by an abrupt deepening of the weathered layers in the granite near the contact and in the quartz reef itself. Therefore, the weathering profile shows a ‗U'-shape geometry with, among others, the verticalization of the granite's fissured layer. The hydraulic conductivity of this verticalized Dewandel et al. A conceptual hydrodynamic model of a geological discontinuity in hard rock aquifers: example of a quartz reef in granitic terrain in South India 2 layer is on average 5 x-6 m/s and storativity about 10-3 (-). The hydraulic conductivity of the fissured quartz is 4 to 6 x 10-6 m/s and its storativity about 3 to 5 x 10-4 (-). Both media are also characterized by a matrix hydraulic conductivity (10-7 to 10-9 m/s) and by a significant heterogeneity in hydrodynamic properties that generates preferential flow paths along the subvertical fissures parallel to the reef axis. A special attention has been paid for characterizing this heterogeneity. The weathering of the dolerite dyke, however, results in a local low hydraulic conductivity, which consequently does not enhance either the thickness of weathered granite layers or its hydraulic conductivity. The obtained results complete the conceptual hydrogeological model developed for weathered granite aquifers in characterizing the relationships between weathering processes and hydrodynamic properties near geological discontinuities.

Caractérisation du fonctionnement des aquifères de socle dans la région de Ouagadougou à partir d’une étude de la qualité chimique des eaux

“…Les forages profonds qui captent cet aquifère permettent de prélever des débits conséquents puisque 102 forages sur 205 répertoriés dépassent les 10 m 3 •h -1 (débit au soufflage) produits entre 40 et 70 m de profondeur (Figure 2). Ces indications sont cohérentes avec le modèle conceptuel proposé par LACHASSAGNE et WYNS (DEWANDEL et al, 2006;DEWANDEL et al, 2011;LACHASSAGNE et WYNS, 2005;LACHASSAGNE et al, 2011) qui montre que c'est à l'« horizon fissuré stratiforme », de 50 à 100 m d'épaisseur que l'aquifère du socle doit l'essentiel de sa perméabilité. L'altération du socle granitique est bien connue dans ses principes et a fait l'objet de recherches actives sur ses ressources en eau au Burkina-Faso depuis plusieurs décennies par SAVADOGO (1984), FAILLAT (1986) et COMPAORE (1997.…”

Analyse des propriétés hydrodynamiques des aquifères fissurés de la région de San-Pedro (Sud-Ouest de la Côte d’Ivoire)

“…La bonne productivité enregistrée dans certaines roches du socle de San-Pedro serait attribuable à l'importance de l'altération . Le processus d'altération en région tropicale peut conduire à la formation de couches d'altération pouvant atteindre jusqu'à 100 m d'épaisseur sur les schistes et 50 m sur les granites (Faillat, 1985) et d'un horizon fissuré développé (Maréchal et al, 2003(Maréchal et al, , 2004Dewandel et al, 2006Dewandel et al, , 2011Lachassagne et al, 2011). La genèse des fractures secondaires est due principalement à l'altération des minéraux phylliteux notamment la biotite, dont le gonflement provoque la fissuration (Hill, 1996;Maréchal et al, 2003;.…”

Groundwater isotopic variations in a uranium mining site: subsidies for contamination studies

“…In this type of aquifer, the occurrence and pattern of water rely on the existence and degree of fracturing, as the water is stored and transmitted through crevices, joints and fractures [11,12,13]. Groundwater flow and storage in this area have always been a major issue due to its abundant heterogeneity and anisotropy, with an irregular distribution of groundwater pathways.…”