Millennial-scale diffusive migration of solutes in thick clay-rich aquitards: evidence from multiple environmental tracers

Abstract

High-resolution one-dimensional profiles of naturally occurring environmental tracers (3H, δD, δ18O, 14C-DIC, 14C-DOC, 36Cl, 4He, and major ions) and hydraulic data were used to study residence times, transport mechanisms, and sources of pore water and solutes in an aquitard system in Saskatchewan, Canada. The aquitard system consisted of 80 m of plastic clay-rich Battleford till discomformably overlying 77 m of Late Cretaceous plastic marine clay. Individual tracers independently revealed molecular diffusion as the dominant transport mechanism in the unoxidized nonfractured till and clay. Transport modeling indicated that late Pleistocene-age pore water at 35–55 m below ground was emplaced 15–30 thousand years Before Present (ka BP) during till deposition and ice retreat. The late Holocene climatic transition was estimated to have occurred 7–12 ka BP. Differences in the timing of events determined with different tracers were attributed to inaccuracies in transport parameters and boundary condition assumptions. This study showed that solute transport in clay-rich aquitards can be accurately predicted for time scales beyond 20 ka, and proved such aquitards are suitable for long-term isolation of most wastes.