From 2015 June 11 to 2015 June 13

La-Roche-sur-Yon, Vendée, FRANCE

Hard rock aquifers LA ROCHE S/ YON

The 20th scientific congress of the French Hydrogeology Chapter of the International Association of Hydrogeologists (IAH) took place from 11 to 13 June 2015 in La Roche-sur-Yon (Vendée). The international congress focused on groundwater in so-called “hard rock” regions (granite, schist, gneiss, etc.). The two-day congress, followed by a one-day technical visit, was attended by 180 participants from 20 different countries, both in Europe and also from major hard rock regions throughout the world (the USA, Africa, Australia, etc.). This congress brought together many specialists from around the world: researchers, teachers, Public Service hydrogeologists, and also consultancy companies, water resource managers and decision-makers. The most recent results were shared, scientific and technical points of view compared and applications made available to operational players. With simultaneous English-French and French-English interpreting, the congress was organised in 3 main sessions, presented below.

Design template of hard rock aquifers: process at the origin of their permeability

It is now well known and accepted that the hydrodynamic properties of hard rock aquifers, throughout the world, are linked to the weathering of rocks. In regions/sectors where loose weathering (saprolite) is protected from erosion, it provides the aquifer's storage capacity. The underlying, permeable fissured horizon, several tens of metres thick, also results from the weathering process and ensures the productivity of the boreholes. Discontinuities, such as lithological contacts, veins, dykes, ancient fractures or joints, etc. constitute prime locations for the spread of weathering and can also have a certain permeability, in particular on their walls. Consequently when the saprolite and the stratiform fissured horizon are totally eroded, these subvertical structures can be a target for the placing of boreholes, although with a reduced storage capacity. In some cases, these deep-seated structures can give rise to significant exchanges of groundwater and surface water.

Methods of placing boreholes and mapping of potential groundwater sites in hard rock: from regional to local level

Numerous examples from around the world were presented, from local methods (geophysics) to the mapping of areas suitable for the exploitation of these aquifers at regional level (examples in Brittany) or country level (Benin, Burkina Faso). Although geophysical methods do not yet allow a permeable fissured horizon to be distinguished (and permeable fractures to be located in it) from a less permeable or clogged formation, scientists and users (consultancy companies, managers) provided proof, during the round-table discussions, that 2D geophysical methods should be favoured, 1D being subject to numerous artefacts.

Functioning, management and modelling of hard rock aquifers

Presentations and discussions showed that hard rock aquifers can, due in particular to the design model presented above, and should be managed and modelled like other types of aquifer, such as porous environments. The chemical quality of their water was also discussed, in particular with the denitrification processes, time transfer and isotopic tracers, etc., with applications on the protection zones and in the catchment areas, for the prevention or mitigation of NPS (non-point source) pollution. The new design of these aquifers is now totally accepted by the hydrogeologist community and allows greatly improved predictability of the properties of these rocks and their groundwater. Overall, the congress presented many scientific publications of international standing, but also and most importantly, new operational applications. The 2-day congress was followed by a one-day technical visit to discover various outcrops presenting, in a very educational way, the stratiform fissured horizon of granite and also a thermal spring whose source is still to be interpreted.

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