Chloride and chlorine isotopes (Cl-36 and delta Cl-37) as tracers of solute migration in a thick, clay-rich aquitard system

Detailed vertical profiles of dissolved Cl- and its isotopes (Cl-36 and del ta(37)Cl) provided new information on the origin and systematics of this co nservative tracer in pore waters of a thick aquitard system. The aquitard s ystem consists of surficial Quaternary clay-rich till (80 m thick) deposite d 30-20 kyr B.P., overlying Cretaceous marine clay (76 m thick) deposited s imilar to 71 Ma. The distribution of Cl-, delta(37)Cl, and Br- showed the p resence of five distinct end-members for Cl-: the top of the unoxidized til l, a regional aquifer underlying the Cretaceous clay, two localized geologi cal heterogeneities (sand streaks) in the till, and glacial meltwater empla ced with the till and still present at depths of between 36 and 60 m. Numer ical simulations of the transport of Cl- from the sand streaks indicated th at this geochemical profile has been developing throughout most of the Holo cene. The Cl-36 measurements showed that the age of the dissolved Cl- in th e upper Cretaceous clay is likely between 0.75 and 1.9 Myr. The Cl-36 measu rements further suggested that the dissolved Cl- in the till was not direct ly derived from the underlying Cretaceous clay. Finally, it was not possibl e to quantify the effects of isotopic fractionation of Cl-37 relative to Cl -35 because of diffusion in this aquitard system.