NATURAL DISCHARGE OF GROUND WATER IN SOUTHEASTERN WISCONSIN

The surface-water bodies in southeastern Wisconsin consists of rivers, streams, headwater creeks, lakes, ponds, wetlands, seeps, springs, and agricultural tile as well as Lake Michigan. Together they produce a relatively dense network:

Map of U.S. Geological Survey stream gage network superimposed on surface-water bodies in southeastern Wisconsin (source: U.S. Geological Survey Fact Sheet 116-03)

The model incorporates this network using three types of input, one for Lake Michigan, one for major rivers, streams, and lakes, and one for secondary features such as wetlands and agricultural tile.

Map of representation of surface-water bodies in ground-water flow model (source: Wisconsin Geological and Natural History Survey Open-File Report 2004-01)

All these features are available in the model as potential locations for ground-water discharge. Under the influence of pumping, Lake Michigan and the major water bodies are also available as sources of ground-water recharge to wells.

When ground water leaks downward from an unconfined aquifer to an underlying confined aquifer, or leaks upward from a deep, hydrogeologic unit to a regional sink, then the exchange can be considered a kind of internal discharge within the flow system. For southeastern Wisconsin, we are particularly interested in the vertical exchange that occurs across the top of the sandstone units that represent the main regional aquifer for the area. The uppermost sandstone unit is the St. Peter Formation; it corresponds to the most shallow sandstone unit stippled in the block diagram shown below:

Under natural conditions, both downward flow to the St. Peter and upward flow from the St. Peter occurs. The complexity of the pattern is shown in the following plot. White areas represent locations where there is upward flow or no vertical exchange at all. Colored areas represent locations of downward leakage, where blue/green zones indicate low rates of leakage and red/yellow zones represent high rates of leakage.

Model output: Map of downward leakage from shallow to deep parts of flow system in southeastern Wisconsin before onset of pumping (source: Wisconsin Geological and Natural History Survey Open-File Report 2004-01)

1. In the absence of pumping, upward flow occurred in the sandstone toward the shallow part of the flow system all along the Lake Michigan coastline as well as under the Lake. That is, the Lake acted as a strong regional sink.
2. There was continuous zone of downward leakage along a belt from north to south in the middle of southeastern Wisconsin, with higher rates occurring where the Maquoketa shale is absent.
3. Zones of upward and downward flow intermingled in the western part of the study area. This pattern indicates that ground water in the St. Peter sandstone exchanged freely with shallower ground water and discharged locally to surface water bodies. The relatively high rates of downward leakage suggest that a large part of the recharge to the water table circulated to the sandstone before discharging back to the surface.

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