The contribution of water geochemistry to the understanding of the regional hydrogeological system

The main results of hydrogeochemical investigations around the Andra Underground Research Laboratory (URL) located in eastern part of the Paris Basin are described hereafter. Groundwater data (chemistry and isotopic composition) have been acquired all around the URL to determine the origin and residence time of water circulations in limestones of Oxfordian and Dogger age, surrounding the Callovo-Oxfordian argillites, and to determine the hydrogeologic role of the argillites regarding aquifers. The main geochemical result is that deep groundwaters from Oxfordian and Dogger limestones are significantly dissimilar to waters of superficial formations. Chemical and isotopic compositions indicate differences between these two deep-aquifer groundwaters. Groundwaters from the Oxfordian limestone are typically meteoric while some groundwater samples from Dogger limestone may correspond to a mixture of meteoric-brine water. Geochemical and hydrogeologic evidence indicates that these deep samples are old groundwaters. Water from Oxfordian limestone may have residence times of the order of 10 4 -10 5 year, while the age of water in the Dogger is probably older. Natural tracer studies in the overall sedimentary series indicate that the argillites allow chlorine transfer by diffusion from the Dogger to Oxfordian aquifer. For the porewater of argillites, the approach is totally different: geochemical modelling has been developed to define the chemistry which relies entirely on a complete set of measured physical and chemical parameters.