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The Newmarket Till forms a regionally extensive aquitard separating two major aquifer systems in the Greater Toronto area,
Canada. The till is incised, and sometimes eroded entirely, by a network of sand- and gravel-filled channels forming productive
aquifers and, locally, high-conductivity windows between aquifer systems. Leakage through the till may also be substantial
in places. This study investigates the spatial variability of aquitard leakance in order to assess the relative importance
of recharge processes to the lower aquifers. With a large database derived from water-well records and containing both hard
and soft information, the Sequential Indicator Simulation method is used to generate maps of aquitard thickness and window
probability. These can be used for targeting channel aquifers and for identifying potential areas of recharge to the lower
aquifers. Conductivities are modeled from sparse data assuming that their correlation range is much smaller than the grid
spacing. Block-scale leakances are obtained by upscaling nodal values based on simulated conductivity and thickness fields.
Under the "aquifer-flow' assumption, upscaling is performed by arithmetic spatial averaging. Histograms and maps of upscaled
leakances show that heterogeneities associated with aquitard windows have the largest effect on regional groundwater flow
patterns.
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