2015 June 11
20th Conference of the French Chapter of the International Association of Hydrogeologists, Hard rock Aquifers: up to date concepts and practical applications, La Roche-sur-Yon, Vendée, France, 11-14th June 2015
The 20th Conference of the French Chapter of the International Association of Hydrogeologists, dedicated to the topic “Hard rock Aquifers: up to date concepts and practical applications”, was organised in La Roche-sur-Yon, Vendée, France, between 11 and 14 June 2015. In this two days conference, followed by two days of field trips, 180 participants were present in the two days conference,
90 on the first day in the field trip, and 32 on the geological and touristic tour of the fourth second day.
Participants were coming from 20 different countries (France, UK, Switzerland, USA, Canada, Portugal, Benin, Burkina Faso, Algeria, Ivory Coast, Australia, Germany, Italy, Cameroun, Morocco, Nigeria, Ireland, Netherlands, Uganda, Belgium), with a total of 39 oral presentations
and 42 posters, and were a well balanced mix of researchers, hydrogeologists from public services (geological surveys, water agencies, local and governmental administrations), and from consultancy firms and the industry.
The oral and posters presentations and the roundtables and debates were organized in 3 main sessions, during two days, with simultaneous English-French-English translation.
These sessions were dealing respectively with:
1. Hard Rock Aquifers (HRA) conceptual models, and the processes at the origin of their hydraulic conductivity. It was there acknowledged that the hydrodynamic properties (hydraulic conductivity, storativity) of most HRA worldwide, from Norway to South Africa for instance, depends most from the total depth of the weathering layers. In regions where a decametre thick saprolite is totally preserved from erosion, it
can ensure the storage in the aquifer, whereas the underlying transmissive stratiform fissured layer ensure the hydraulic conductivity and thus the productivity of the aquifer. This last layer is decametric in thickness, and the fracturing decreases with depth. It was also shown that most
discontinuities in HR, such as lithological contacts, veins, dykes, ancient faults, etc. can be weathered at their periphery, giving origin to some
permeable fractures. Where the saprolite and the stratiform fissured layer has been totally eroded or is unsaturated, such deeper discontinuities may be the target for drillings, but the storativity can be strongly reduced. In some cases, such deep fractures can be connected to the surface, with, local groundwater-surface water relationships.
2. Methods for water well siting and for mapping HRA potentialities, with several examples in various areas of the
world, from the local scale (field geophysics) to mapping at the scale of a region or a whole country, such as Britany, Benin or Burkina Faso for instance. Even if geophysical methods cannot yet differentiate a permeable fissured horizon (with several permeable fractures) from a non-permeable or clogged one (this issue must consequently be a research topic for the coming years), it was clearly demonstrated that 2-D methods must be encouraged. In fact, for instance, 1-D electrical profiles interpretation may be ambiguous: a surface heretogeneity can induce a similar signal to a deep structure.