Evaluation of heterogeneity and field-scale groundwater flow regime in a leaky till aquitard |
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| Authors: | Richard E Gerber Joseph I Boyce Ken W Howard |
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| Institution: | (1) Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada,;(2) Applied Geophysics Group, School of Geography and Geology, McMaster University, Hamilton, Ontario L8S 4K1, Canada,;(3) Groundwater Research Group, University of Toronto at Scarborough, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada, |
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| Abstract: | Recent work in southern Ontario, Canada, demonstrates anomalously high vertical groundwater flow velocities (>1 m/year) through
a thick (as much as 60 m), sandy silt till aquitard (Northern till), previously assumed to be of very low permeability (hydraulic
conductivity <10–10 m/s). Rapid recharge is attributed to the presence of fractures and sedimentary heterogeneities within the till, but the
field-scale flow regime is poorly understood. This study identifies the nature of physical groundwater pathways through the
till and provides estimates of the associated groundwater fluxes. The aquitard groundwater flow system is characterized by
integrating details of the outcrop and subsurface sedimentary characteristics of the till with field-based hydrogeologic investigation
and numerical modeling. Outcrop and subsurface data identify a composite internal aquitard stratigraphy consisting of tabular
till beds (till elements) separated by laterally continuous sheet-like sands and gravels (interbeds) and boulder pavements. Individual till elements
contain sedimentary heterogeneities, including discontinuous sand and gravel lenses, vertical sand dikes, and zones of horizontal
and vertical fractures.
Hydrogeologic field investigations indicate a three-layer aquitard flow system, consisting of upper and lower zones of more
hydraulically active and heterogeneous till separated by a middle unit of relatively lower hydraulic conductivity. Groundwater
pathways and fluxes in the till were evaluated using a two-dimensional aquitard/aquifer flow model which indicates a step-wise
flow mechanism whereby groundwater moves alternately downward along vertical pathways (fractures, sedimentary dikes) and laterally
along horizontal sand interbeds within the till. This model is consistent with observed hydraulic-head and isotope profiles,
and the presence of tritiated pore waters at various depths throughout the till. Simulations suggest that a bulk aquitard
vertical hydraulic conductivity on the order of 1×10–9 m/s is required to reproduce observed hydraulic-head and tritium profiles.
Electronic Publication |
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| Keywords: | heterogeneity groundwater flow hydraulic properties confining units Canada |
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