Geostatistical mapping of leakance in a regional aquitard, Oak Ridges Moraine area, Ontario, Canada

The Newmarket Till forms a regionally extensive aquitard separating two maj or aquifer systems in the Greater Toronto area, Canada. The till is incised , and sometimes eroded entirely, by a network of sand- and gravel-filled ch annels forming productive aquifers and, locally, high-conductivity windows between aquifer systems. Leakage through the till may also be substantial i n places. This study investigates the spatial variability of aquitard leaka nce 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 Simulat ion method is used to generate maps of aquitard thickness and window probab ility. These can be used for targeting channel aquifers and for identifying potential areas of recharge to the lower aquifers. Conductivities are mode led from sparse data assuming that their correlation range is much smaller than the grid spacing. Block-scale leakances are obtained by upscaling noda l values based on simulated conductivity and thickness fields. Under the "a quifer-flow" assumption, upscaling is performed by arithmetic spatial avera ging. Histograms and maps of upscaled leakances show that heterogeneities a ssociated with aquitard windows have the largest effect on regional groundw ater flow patterns.