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RECENT PROGRESS OF THE HYDROGEOLOGICAL CONCEPTUAL MODEL OF HARD ROCK AQUIFERS

Author(s): APPLICATIONS FOR THEIR SURVEY, MANAGEMENT, MODELLING AND PROTECTION
Congress: 2008
Author(s): P. Lachassagne(1), Sh. Ahmed(2), B. Dewandel(1), N. Courtois(1), F. Lacquement(3), J.P. Maréchal(1), J. Perrin(4), R. Wyns(3)
- 1- BRGM, Water Division, Hard Rock Aquifers Unit, 1039, Rue Pinville 34000 Montpellier, France; p.lachassagne@brgm.fr (* Corresponding author);  2- National Geophysical Research Institute, Indo-French Centre for Groundwater Research, Uppal Road, 500 007 Hyderabad, India, shakeelahmed@ngri.res.in  3- BRGM, Geology Division, BP 36009, 45060 Orléans, Cedex, France, r.wyns@brgm.fr  4- BRGM, Water Division, Resource Assessment, Discontinuous Aquifers Unit, Indo- French Centre for Groundwater Research, Uppal Road, 500 007 Hyderabad, India. ; perrin@ngri.res.in

Keyword(s): aquifers, hard rocks, groundwater, management, modeling, survey, protection
Article:
AbstractHard rocks (granites, metamorphic rocks) constitute the basement of the continents and, as such, occupy large areas throughout the World (Africa, South and North America, India, several areas in Europe, Asia, Australia, etc.). Hard rock groundwater resources are modest in terms of available discharge per well (from 2-3 to 20 m3/h for high yield bore wells), compared to those from other aquifer types (porous, karstic, volcanic). These aquifers are, however, geographically widespread and therefore well suited for the supply of scattered populations and small to medium size cities, or suburbs of larger cities. These resources largely contribute to the economical development of such regions, especially for those exposed to arid and semiarid climatic conditions where the surface water resource is limited. In such regions, they constitute the only available perennial water resource, supplying the population, and also the agricultural and industrial sectors. These aquifers also ensure environmental functions (low stage discharge of the streams, wetlands sustainment, etc.). Significant advances have recently emerged on the genesis, geometry and functioning of hard rock aquifers. The hydrodynamic properties of these aquifers appear to be mainly related to past weathering processes. The superficial stratiform weathering profile, up to more than 100 m thick where not eroded, is mainly composed of two superposed layers, the saprolite layer (unconsolidated, mainly constituted of clay and sand), ensuring a capacitive role only, and the underlying fissured layer that mainly assumes the transmissive function of the aquifer. All together and where saturated, these two layers constitute a composite aquifer that plays the major role as a hydrogeological point of view. Locally, the superficial weathering processes can also reach up to a few hundreds of metres below the topographic surface but only in the near vicinity of existing discontinuities (e.g. lithological contacts, veins, ancient fractures, etc.). As a consequence, such discontinuities are permeable. The bore wells, mines, or tunnels intersecting them thus show similar water bearing structures as those encountered in the stratiform fissured layer. The geometry (X, Y, Z) of such weathering profiles, or their remains after erosion, can now be mapped at the catchment scale. Thus, the spatial distribution of the hydrodynamic properties (namely hydraulic conductivity and storage) of the hard rock aquifers can also be mapped. These newly developed geological and hydrogeological concepts, that enable to regionalize hard rock aquifers properties, find numerous practical applications, and particularly hydrogeological applications that will be presented in this paper: - in the field of water resources development, management and protection: from the mapping of groundwater potential at a regional scale, through well sitting techniques and methods increasing the success rate in terms of exploitable discharge, to water resources management at the watershed scale, crucial in areas where groundwater is heavily exploited (e.g. in India), and delineation of Groundwater Protection Zones. These new concepts thus allow the development of Decision Support Tools at the basin scale either based on the global modeling of the aquifer or on the use of multilayer mathematical modeling tools to simulate their functioning, taking into account the geometry and hydrodynamic properties of their different layers, similarly to methods dedicated to more common aquifers (porous for instance), - in the field of the evaluation of the environmental impacts of various civil works, among which tunnels at shallow depth (<300 m below ground surface) bored in such hard rocks, on rivers, springs, piezometric levels, as well as for the prediction of tunnels water inflows.
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