A detailed vertical profile of the stable isotope ΔD in pore water was measured through a thick aquitard system consisting of surficial Quaternary clay-rich till (80 m thick) and an underlying Cretaceous marine clay (76 m thick). Numerical modeling was used to simulate one-dimensional (vertical) groundwater flow and transport of ΔD. Best fit simulations to the data provided an independent estimate of long-term groundwater velocity through the aquitard and estimates of the timing of late Pleistocene and Holocene events. Best fit simulations to the measured isotope profile across the till-clay interface yielded a groundwater velocity of 0.75--1.0 m per 10 ka for a transport time of between 20 ka and 30 ka. The estimate of velocity agreed well with that calculated from hydraulic data and suggested that hydraulic conductivities of these aquitards are independent of volume tested. The 20-30 ka time frame required for the ΔD profile to develop across the till-clay interface reflects the timing of till deposition and shows that the till is the Battleford Formation, a younger till than previously believed. Numerical transport modeling of ΔD in the upper 30 m of the profile yielded a nonunique fit. Assuming a similar groundwater velocity to that determined across the till-clay interface, a best fit was obtained for a transport times of 7.5--10 ka. This range compared favorably with that reported for the start of the Holocene (about 10 ka B.P.). This study shows that the application of ΔD, and by analogy Δ18O, to the study of thick aquitard systems not only can provide independent, long-term estimates of very low groundwater velocities but can also provide insight into the timing of major geologic events such as glaciations. ¿ 1999 American Geophysical Union