Hydrological behavior of a deep sub-vertical fault in crystalline basement and relationships with surrounding reservoirs

Abstract

International audienceCrystalline-rock aquifers generally yield limited groundwater resources. However, some highly productive aquifers may be encountered, typically near tectonic discontinuities. In this study, we used a multidisciplinary experimental field approach to investigate the hydrogeological behavior of a sub-vertical permeable fault zone identified by lineament mapping. We particularly focused our investigations on the hydrogeological interactions with neighboring reservoirs. The geometry of the permeable domains was identified from geological information and hydraulic test interpretations. The system was characterized under natural conditions and during a 9-week large-scale pumping test. We used a combination of piezometric analysis, flow logs, groundwater dating and tracer tests to describe the interactions between permeable domains and the general hydrodynamical behaviors. A clear vertical compartmentalization and a strong spatial heterogeneity of permeability are highlighted. Under ambient conditions, the vertical permeable fault zone allows discharge of deep groundwater flows within the superficial permeable domain. The estimated flow across the total length of the fault zone ranged from 170 to 200 m3/day. Under pumping conditions, hydrological data and groundwater dating clearly indicated a flow inversion. The fault zone appears to be highly dependent on the surrounding reservoirs which mainly ensure its recharge. Groundwater fluxes were estimated from tracer tests interpretation. This study demonstrates the hydrogeological capacities of a sub-vertical fault aquifer in a crystalline context. By describing the hydrological behavior of a fault zone, this study provides important constrain about groundwater management and protection of such resources

Timescales of regional circulation of saline fluids in continental crystalline rock aquifers (Armorican Massif, western France)

“…2). Moreover, previous studies in the Armorican basement have identified a compartmentalization of these crystalline rock aquifers from hydrological, geochemical, geological and geophysical data (Ayraud et al, 2008;Roques et al, 2014), and from regional-scale numerical modeling (Goderniaux et al, 2013). Geochemical studies also showed specific chemical compositions of the water in the deep fractured aquifer, potentially including saline groundwaters Ayraud et al, 2008;Pauwels et al, 2013).…”

“…Conversely, high groundwater productivities are associated with fault zones below the weathered zone. Indeed, groundwater resources at significant depth have been found in the Armorican bedrock (Le Borgne et al, 2006;Roques et al, 2014), and it can be considered that the fractured system constitutes a water reservoir in the conti- nental basement. Free groundwater circulation in crystalline environments has been reported from several studies (Banks et al, 1996;Larsson, 1987).…”

“…Hydrodynamic properties of crystalline rock aquifers of the Armorican basement have been intensively investigated at several research sites (Ploemeur and St-Brice-enCogles sites, Fig. 2) (Le Borgne et al, 2004Borgne et al, , 2006Dorn et al, 2012;Leray et al, 2013;Roques et al, 2014;Touchard, 1999) (Fig. 2).…”

“…The crystalline basement of the Armorican Massif presents aquifers with a high transmissivity associated with fault zones (Roques et al, 2014). Considering the current topography of the Armorican Massif, including three domains of elevations higher than 200 m, the groundwater flow system is driven gravitationally, which leads to both local and regional circulation loops (Toth, 1963).…”

Groundwater recharge and flow processes as revealed by stable isotopes and geochemistry in fractured Hornblende-biotite-gneiss, Rivirivi Catchment, Malawi

“…Gleeson and Novakowski (2009) and de Vries and Simmers (2002) state that groundwater recharge and groundwater flow processes are generally controlled by climate, morphology and geological conditions. To this day, several researchers have looked at groundwater recharge and groundwater flow processes in various climatic regions and geological media, particularly those with a combination of permeable rock matrices and conductive fractures (Verbovsek and Kanduc, 2016;Roques et al, 2014;Monjerezi et al, 2011;Apaydin, 2010;Chilton and Foster, 1995). It was shown in these studies that basic assumptions regarding groundwater recharge and flow processes that are applicable to porous media are not necessarily relevant in fractured rock environments, especially at field scale.…”