Bernard Marty and colleagues at the Centre de Recherches Pétrographiques et Géochimiques (CRPG) and the Ecole Nationale Supérieure de Géologie performed their studies on the eastern Paris basin near Nancy in France. This natural site contains a layer of low-permeability 'aquitard' rock sandwiched between two water-bearing layers of rock called 'aquifers'. The upper aquifer layer (Dogger) is separated from the lower layer (Trias) by about 600 metres of aquitard rock made of shales and clays (see figure).

The researchers analyzed the movement of groundwater between the two aquifer layers by monitoring the presence of helium-3 isotopes. Helium-3, which is chemically inert and has a small atomic radius, is one of the most mobile isotopes in nature. The presence or absence of the tracer would indicate how water has moved over the ages.

Using samples taken from drill holes made by the French Nuclear Waste Agency, Marty and co-workers found, as expected, the presence of helium-3 in the Trias aquifer. It exists in the Trias aquifer because it has accumulated the isotope from the Earth’s crust and mantle. However, the Dogger layer was found to contain no helium-3, despite there being water in the two aquifer layers for several million years. The team concludes that helium-3 has not flowed into it due to the "the isolating properties of the intermediate aquitard".

The researchers calculated that the mass transfer of isotopes through aquitards is negligible on geological time scales. Any flow in the basin takes place only where there are naturally occurring faults or fractures. These would inevitably allow some substances to seep through, they say.

“This work has implications for potential waste storage in these impermeable layers, but of course our results do not provide a definite answer to this,” says Marty.