Implications of the distribution of σD in pore waters for groundwater flow and the timing of geologic events in a thick aquitard system

Abstract

A detailed vertical profile of the stable isotope σD 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 σD. 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 σD 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 σD 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 σD, and by analogy σ18O, 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.