1. Centre de Recherches Pétrographiques et Géochimiques (CRPG), 15 Rue Notre-Dame des Pauvres, BP 40, 54501 Vandoeuvre lès Nancy Cedex, France
2. Ecole Nationale Supérieure de Géologie, Rue du Doyen Marcel Roubault, BP 40, 54501 Vandoeuvre lès Nancy Cedex, France
3. Present address: ANDRA, BP 9, 55290 Bure, France

Correspondence to: Bernard Marty^1,2 Email: bmarty@crpg.cnrs-nancy.fr

Abstract

Aquitards—layers of rock having low permeability—have been suggested as potential long-term reservoirs for toxic materials such as nuclear or chemical waste. But information about the isolation properties of aquitard layers is essential to evaluate whether they can indeed be used safely as reservoirs. Here we investigate the long-term mobility of groundwaters between two aquifers surrounding an aquitard layer in the eastern recharge area of the Paris basin, France, using helium isotopes as a geochemical tracer. The deeper Trias sandstone aquifer, which lies above the crystalline basement, accumulates radiogenic ⁴He and primordial ³He from large regions of the crust and mantle at rates comparable to the degassing of the whole crust¹ and of mid-ocean ridges². We show that the overlying carbonate Dogger aquifer, which is separated from the Trias aquifer by an aquitard layer consisting of a 600 m succession of shales and clays, is stagnant and has been extremely well isolated from the Trias over the past several million years. This finding, together with previous studies at the centre of the Paris basin^{3, 4}, shows that diffusive mass transfer across aquitards is negligible and that cross-formational flow in basins takes place preferentially in faulted areas.