Dissolved helium distribution in the Oxfordian and Dogger deep aquifers of the Meuse/Haute-Marne area
Résumé
The 140-m-thick, clay-rich Callovo-Oxfordian (COX) layer of the eastern Paris Basin, France, is being considered by the French Nuclear Waste Agency (Andra) as a long-term underground nuclear waste repository. Andra has selected a 250 km2 area (transposition zone) to be further characterised, especially in view of the confinement properties exhibited by the COX. This study reports the helium concentrations and isotopic ratios of water samples from the aquifers above and below the COX, which are the Oxfordian and Dogger aquifers, respectively. The samples were collected from five drilling sites (2007-2008). Both the Oxfordian and Dogger groundwaters are of meteoric origin and have accumulated radiogenic He with a 3He/4He ratio of approximately 0.02 Ra. The He concentrations in the groundwaters are two orders of magnitude higher than in the air-saturated water in the Oxfordian and approximately 10 times higher in the Dogger than in the Oxfordian. One borehole was drilled down into the Triassic sediments, allowing a sample to be collected from the Bundsandstein aquifer. Here, the He concentration is of the same order of magnitude as those measured in the Dogger, but the isotopic ratio is slightly higher at 0.04 Ra. However, this ratio is approximately 10 times lower than those measured by Marty et al. (2003) in waters collected closer to the Trias recharge and tagged with mantle-derived He. This lower ratio is most likely due to a significant and rapid slowdown of the circulation in this aquifer, allowing substantial radiogenic He accumulation. A key conclusion of this study concerns the lateral non-homogeneity of the studied area: He concentrations are higher in the northern part of the transposition zone. This observation can be explained by a longer residence time of the waters and/or a higher input of He from the basement. A simplified 2D model of He transport shows that the second hypothesis alone does not allow fitting of the data, thus implying more stagnant water in this area. The indicative residence times derived from this coarse model are 0.3-0.5 Myr (Oxfordian) and 0.5-0.6 Myr (Dogger) for the borehole located in the centre of the investigated zone; for the northernmost borehole, they reach 0.8-1.4 Myr for the Oxfordian and 1.5 Myr for the Dogger.