Implications of the distribution of delta D in pore waters for groundwaterflow and the timing of geologic events in a thick aquitard system

A detailed vertical profile of the stable isotope delta D in pore water was measured through a thick aquitard system consisting of surficial Quaternar y 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 delta D. Best fit simulations to the da ta provided an independent estimate of long-term groundwater velocity throu gh the aquitard and estimates of the timing of late Pleistocene and Holocen e events. Best fit simulations to the measured Isotope profile across the t ill-clay interface yielded a groundwater velocity of 0.75-1.0 m per 10 ka f or a: transport time of between 20 ka and 30 ka. The estimate of velocity a greed well with that calculated from hydraulic data and suggested that hydr aulic conductivities of these aquitards are independent of volume tested. T he 20-30 ka time frame required for the delta D profile to develop across t he till-clay interface reflects the timing of till deposition and shows tha t the till is the Battleford Formation, a younger till than previously beli eved. Numerical transport modeling of delta D in the upper 30 m of the prof ile yielded a nonunique fit. Assuming a similar groundwater velocity to tha t 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 repor ted for the start of the Holocene (about 10 ka B.P.). This study shows that the application of delta D; and by analogy delta(18)O, to the study of thi ck aquitard systems not only can provide independent, long-term estimates o f very low groundwater velocities but can also provide insight into the tim ing of major geologic events such as glaciations.