EVIDENCE FOR RECENT GROUNDWATER-FLOW THROUGH LATE WISCONSINAN TILL NEAR TORONTO, ONTARIO

The Northern till is a thick (similar to 50 m) Late Wisconsinan diamic t unit that occurs throughout southcentral Ontario. The till has gener ally been regarded as massive and uniform, with a very low vertical hy draulic conductivity. It is similar to many other till units of mid-co ntinental North American glaciated terrain in that it is believed to i nhibit recharge to underlying aquifers and afford a high degree of pro tection to these aquifers from surface and near-surface sources of con tamination. Standard methods of estimating hydraulic conductivity (K) for the Northern till, such as laboratory testing of core samples (oth er studies) and rising-falling head field piezometer tests (this study and other studies), characteristically yield values on the order of 1 0(-11) to 10(-9) m/s. Typically, these values indicate advective trave ltimes through the till on the order of hundreds to thousands of years . In contrast, isotopic evidence (H-2, O-18, and H-3) from till pore w aters indicates the presence of modern (post-1952) waters at depths of up to 50 m, suggesting either that certain facies of the till are con siderably more permeable or that minor sand lenses or hydrogeologicall y active secondary permeability structures are locally important. In s ome areas, vertical flow velocities may approach 1 m/year. By comparin g pore-water isotopic data from cores acquired using mud (sodium bento nite) and dry rotary methods, this study further demonstrates that rep resentative pore-water samples can be obtained using a drilling fluid providing care is taken in preparing core samples for analysis.