Fracture groundwaters collected from the indurated Toarcian/Domerian clayrock in the IRSN Underground Research Laboratory (URL) at Tournemire (south of France) are rare and limited in volume. The present study aims at resolving an apparent contradiction observed for these groundwaters: how can these groundwaters in transmissive fractures contain indicators of both geologically young and old residence times? Young residence times on the order of a few thousand years are indicated by the presence of 14C, and old residence times appear to be indicated by the extraordinarily positive δ37Cl values, between 5 and 8‰ vs SMOC. These highly enriched δ37Cl values might be attributed to a long diffusive transport (on the order of tens of million years) between the semi-permeable clayrock sequence and its surrounding aquifers, as estimated by modeling in a companion paper [1].Knowledge of the geometry of the fractures and the diffusion parameters in the intact clayrock allowed us to propose a 2D modeling for the transport of chloride and its stable isotopes using the Hytec code. Simulations applied to borehole M3 data show that the measured Cl values in the fracture groundwater are consistent with a diffusive exchange between the unaltered argillite porewater and meteoric waters that infiltrated in the fracture about 26 ka ago and have become isolated since then. This result is in good agreement with previously estimated maximum 14C residence times for these fracture groundwaters. The modeled δ37Cl value at 26 ka would be enriched by only 0.02‰ with respect to the initial imposed value for the argillite, which remains in the analytical uncertainty of ±0.2‰. These simulations provide evidence that the aqueous chloride and high δ37Cl values may reflect values of the unaltered argillite porewater.