Respective roles of the weathering profile and the tectonic fractures in the structure and functioning of crystalline thermo-mineral carbo-gaseous aquifers

• Published in: Journal of hydrology (Amsterdam) 2017-04, Vol.547, p.690-707
• Main Authors: Dewandel, B., Alazard, M., Lachassagne, P., Bailly-Comte, V., Couëffé, R., Grataloup, S., Ladouche, B., Lanini, S., Maréchal, J.-C., Wyns, R.
• Format: Article
• Language: English
• Subjects: Crystalline rocks thermo-mineral and carbo-gaseous aquifers; Deep fractures; Earth Sciences; Geology; Hydraulic tests; Hydrochemistry; Hydrology; Sciences of the Universe; Weathering profile
• ISSN: 0022-1694; 1879-2707
• DOI: 10.1016/j.jhydrol.2017.02.028

•Multidisciplinary approach to understand a complex thermo-mineral hydrosystem.•The weathered fissured layer of the granite aquifer plays a major hydrodynamic role.•Identification and location of deep fractures providing mineralised water.•Deep fluids’ hydrochemical spatial variability according to various deep fractures. Crystalline thermo-mineral and carbo-gaseous (CTMCG) hydrosystems are well known for their economic importance in fields such as thermal, spa activities and natural mineral water (NMW) bottling. Such systems are usually associated with strong structural complexity, which is rarely characterised in detail or robustly. This research focuses on a CTMCG hydrosystem associated with a peri-alpine graben. A multidisciplinary approach with a very large set of data and methods – geological modelling with geophysics and geological data from outcrops and several boreholes, hydrodynamic data, hydrochemistry, hydrogeological and geochemical modelling – reveals very novel results and allows a robust conceptual model to be constructed. The aquifer at the origin of the carbo-gaseous natural mineral water is the 100–125m-thick fractured stratiform layer of the weathering profile of the crystalline rock (granite). It forms a rather large and thick inertial aquifer that can be numerically modelled, in a similar fashion to a porous medium. The majority of tectonic faults length act as impervious boundaries that divide this aquifer into around ten elongated compartments that were precisely delineated. These tectonic faults are permeable only along two small areas that were also precisely located. These permeable zones feed some aquifer compartments with deep, highly mineralised carbo-gaseous water, which mixes with “fresher” water and forms the exploited NMW. These results can be generalised and in particular show a strong opposition between low-inertia CTMCG hydrosystems without a subsurface reservoir, as the weathering profile was eroded, and high-inertia hydrosystems such as the one studied.