SEISMIC-REFLECTION, BOREHOLE AND OUTCROP GEOMETRY OF LATE WISCONSIN TILLS AT A PROPOSED LANDFILL NEAR TORONTO, ONTARIO

The search for new landfill sites in the Greater Toronto area of south ern Ontario, Canada, is producing a wealth of data regarding the subsu rface stratigraphy and geometry of Late Wisconsin (< 25 ka) till depos its. Till strata are favoured as landfill substrates because of their wide surface extent, thickness (maximum similar to 60 m), high degree of overconsolidation, apparently massive character, and low permeabili ty. However, problems are emerging where surface contaminants have mig rated through till deposits into underlying aquifers along poorly unde rstood transport paths. This paper reports the results of a detailed s hallow seismic reflection investigation of a proposed 275 ha landfill site 40 lan northeast of Toronto near Whitevale, where previous hydroc hemical analysis and hydrogeological monitoring identified rapid verti cal recharge of contaminated surface waters through Late Wisconsin til ls up to 60 m thick. Seismic reflection data are ground truthed by dri lling (36 holes; total drilled 3157 m), coring (1600 m), downhole geop hysical logging, and outcrop data. The site stratigraphy at Whitevale consists of an uppermost Late Wisconsin till (Halton Till) separated f rom a lower till (informally named Northern till) by a silt, sand, and gravel complex. Seismic reflection profiles identify the presence of well-defined reflectors within the Northern till, which are correlated in outcrop with laterally extensive erosion surfaces overlain by shee t-like sands and gravels, up to 1 m thick, and boulder concentrations. Erosion surfaces and associated sediments record episodic scouring by subglacial meltwaters and provide potential ''hydraulic windows'' for the movement of surface contaminants through the till into underlying aquifers.