Establishing constraints on groundwater ages with 36Cl, 14C, 3H, and noble gases : a case study in the Eastern Paris Basin, France

Abstract Groundwaters from the Tithonian/Kimmeridgian, Oxfordian and Upper Dogger aquifers, within the eastern part of the Paris basin (France), were characterised using 3 H, 14 C and 36 Cl, and noble gases tracers, to evaluate their residence times and determine their recharge period. This information is an important prerequisite to evaluating the confinement properties of the Callovo-Oxfordian clay formation sandwiched between the Oxfordian aquifer and the Dogger aquifer, currently being investigated by the French nuclear waste management agency (Andra) for radioactive waste disposal. Data presented in this paper are used to test 4 hypotheses. (1) The Oxfordian limestone is isolated from the overlying Tithonian/Kimmeridgian surface aquifer. This first hypothesis is supported by the presence of measurable 3 H activities in groundwaters from the surface aquifer (6–11 TU), and by its absence in groundwaters derived from deeper aquifers ( (2) The Callovo-Oxfordian clay sequence in the vicinity of the URL serves as an effective barrier to advective flow and transport between the underlying Upper Dogger aquifer and the overlying Oxfordian limestone aquifer. This second hypothesis is supported by relatively low concentrations of Cl (3.5–78 mg L −1 ), Na (15–145 mg L −1 ), Br (0.08–0.4 mg L −1 ) and 4 He rad (0.2–2.3 ccSTP g −1 ) and slightly higher 36 Cl/Cl ratios (2.7–59 × 10 −15  at at −1 ) in groundwaters from the Oxfordian aquifer, compared to those from the Upper Dogger aquifer (345–4027 mgCl L −1 ; 402–2390 mgNa L −1 , 1.8–22.2 mgBr L −1 , 1.6–37.2 ccSTP He g −1 , 1–5 × 10 −15  at 36 Cl at −1 Cl). (3) Groundwaters from the Oxfordian aquifer, in the vicinity of URL are pre-Holocene, with residence times exceeding 10 ka. Geochemical and isotopic lines of evidence for this hypothesis include: (a) δ 13 C values measured on Oxfordian aquifer waters that are close to δ 13 C of the aquifer matrix, reflecting strong isotopic exchange over several thousand years; (b) noble-gas temperatures that indicate recharge temperatures between 3 and 8.6 °C, lower by 2–7 °C than the modern average temperature in this area; (c) radiogenic 4 He concentrations between 0.16 × 10 −5 and 2.3 × 10 −5  ccSTP g −1 of water, about 2–3 orders of magnitude greater than the 4 He concentration of air-equilibrated water. (4) Groundwater residence time for the Upper Dogger aquifer in the vicinity of the URL probably exceed several hundred thousand years. Such long residence times are indicated by the accumulation of radiogenic He-up to 37 × 10 −5  ccSTP g −1 assuming a closed system. Primary contributors to uncertainties in interpretation of the data presented in this paper are the lack of: – well-defined 36 Cl/Cl input ratio; – knowledge about the 4 He rad sources for Oxfordian and Dogger aquifers; – knowledge about transport processes occurring through the Dogger aquifer, and the extensive dilution of initial 14 C activities by dissolution/exchange with aquifer matrix. Overall, however, although the data presented in this paper remain semi-quantitative due to inevitable intrinsic limitations of the methods, none contradict the hypotheses of long residence times and aquifer isolation in the vicinity of the URL.