Under non-glacial conditions, groundwater is typically recharged in topographically high areas and discharged from topographic lows. Our simulations indicate, however, that the Lake Michigan Lobe altered or reversed topographically driven pressure gradients, resulting in groundwater flow patterns and velocities that differed substantially from those of current conditions. The non-glacial (current conditions) simulation shows a flow pattern consisting of recharge in the high-elevation areas north and south of Lake Michigan and upward discharge through aquifers underlying the lake. South of Lake Michigan, groundwater velocity is substantially reduced, and flow directions are variable.
Under glacial conditions, simulated groundwater velocities are high, and flow is consistently directed toward the southern end of the flow line. Along the northern two-thirds of the flow line, groundwater has a relatively strong downward component in the drift aquifer. Along the southern one-third of the flow line, flow in the drift aquifer is negligible, owing to flow through the Mahomet bedrock valley and discharge at the ground surface. South of the ice margin, groundwater in deep bedrock aquifers is directed upward and discharges to regions that currently serve as recharge zones.
Figure 11.1 Simulated flowline (dashed line). The ice limit is shown as the dark grey line and the ice cover is represented by the grey pattern. A and A' show the boundaries of the finite-difference grid. The approximate location of the Mahomet Bedrock Valley is shown where it is crossed by the simulated flowline.
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