Role of porous and permeable layers in an argillaceous formation: Preliminary results on groundwater residence time based – an isotopic approach

Argillaceous layers are often considered as impermeable formations preventing migration of contaminants. Nevertheless, porous and permeable layers in such formation may exist and play a role in contaminant dispersion. The objective here is to investigate the residence time of such porous and permeable layers based on a geochemical and isotopic approach.

The study site is located on the Narbonne sedimentary basin, south of France. The Oligo-Miocene argillaceous sedimentary series overlays the cretaceous limestone substratum. A Quaternary deposit partially covers the Tertiary formations. On the western margin the Oligo-Miocene series abuts on the limestone formation. The Oligo-Miocene series is crossed by permeable to semi-permeable layers of conglomeratic deposits, from a few centimetres to a few meters thick, which may represent hydraulic short-circuit.  

Groundwater samples from the 3 formations were collected for analyses along with surface water samples, the Aude river, and rain water. The geochemical approach is based on the investigation of time tracers including ³H/³He, ¹⁴C and ³⁶Cl/Cl along with the investigation of major and minor elements, stable isotope of water and strontium isotopes.

Groundwater conductivity is relatively high ranging from 800 to 2000 µS/cm for the deepest bore hole in the cretaceous formation. Br/Cl and strontium isotope ratio show that groundwater from the permeable layers plot between groundwater from the limestone aquifer and a more mineralised end-member. Groundwater from the permeable layers are slightly depleted in stable isotopes of water with respect to modern precipitation, suggesting recharge under different conditions from the current one. ³H, below detection limit, shows the lack of modern recharge. ¹⁴C, in the order of a few pmc, would suggest a residence time of a few ten thousand years. The ³⁶Cl/Cl ratio, in the order of 2x10^-14 at/at, is nearly one order of magnitude lower than modern input. Groundwaters from the Oligo-Miocene permeable layers tend to plot on or below a mixing line between recent recharge and a chloride enriched and ³⁶Cl depleted end-member, coherent with long residence time.

All tracers are coherent with relatively long residence time and low circulation potential in the porous and permeable layers.