Evidence for recent groundwater flow through Late Wisconsinan till near Toronto, Ontario

Abstract

The Northern till is a thick (~50 m) Late Winsconsinan diamict unit that occurs throughout south-central Ontario. The till has generally been regarded as massive and uniform, with a very low vertical hydraulic conductivity. It is similar to many other till units of mid-continental North American glaciated terrain in that it is believed to inhibit recharge to underlying aquifers and afford a high degree of protection to these aquifers from surface and near-surface sources of contamination. Standard methods of estimating hydraulic conductivity (K) for the Northern till, such as laboratory testing of core samples (other studies) and rising–falling head field piezometer tests (this study and other studies), characteristically yield values on the order of 10–11 to 10–9 m/s. Typically, these values indicate advective traveltimes through the till on the order of hundreds to thousands of years. In contrast, isotopic evidence (2H, 18O, and 3H) from till pore waters indicates the presence of modern (post-1952) waters at depths of up to 50 m, suggesting either that certain facies of the till are considerably more permeable or that minor sand lenses or hydrogeologically active secondary permeability structures are locally important. In some areas, vertical flow velocities may approach 1 m/year. By comparing pore-water isotopic data from cores acquired using mud (sodium bentonite) and dry rotary methods, this study further demonstrates that representative pore-water samples can be obtained using a drilling fluid providing care is taken in preparing core samples for analysis. Key words: till, aquitard, permeability, recharge, contaminant transport, isotopes.