Detailed vertical profiles of dissolved Cl⁻ and its isotopes (³⁶Cl and δ³⁷Cl) provided new information on the origin and systematics of this conservative tracer in pore waters of a thick aquitard system. The aquitard system consists of surficial Quaternary clay‐rich till (80 m thick) deposited 30–20 kyr B.P., overlying Cretaceous marine clay (76 m thick) deposited ∼71 Ma. The distribution of Cl⁻, δ³⁷Cl, and Br⁻ showed the presence of five distinct end‐members for Cl⁻: the top of the unoxidized till, a regional aquifer underlying the Cretaceous clay, two localized geological heterogeneities (sand streaks) in the till, and glacial meltwater emplaced with the till and still present at depths of between 36 and 60 m. Numerical simulations of the transport of Cl⁻ from the sand streaks indicated that this geochemical profile has been developing throughout most of the Holocene. The ³⁶Cl measurements showed that the age of the dissolved Cl⁻ in the upper Cretaceous clay is likely between 0.75 and 1.9 Myr. The ³⁶Cl measurements further suggested that the dissolved Cl⁻ in the till was not directly derived from the underlying Cretaceous clay. Finally, it was not possible to quantify the effects of isotopic fractionation of ³⁷Cl relative to ³⁵Cl because of diffusion in this aquitard system.