SURFACE-WATER AND NATURAL GROUND-WATER DIVIDES IN SOUTHEASTERN WISCONSIN

Local topography and arrangement of surface-water bodies control shallow ground-water flow in southeastern Wisconsin. The variations in lateral direction of flow and the alteration between upward and downward flow formed many relatively small-scale flow systems that discharged to local water bodies (streams, wetlands, lakes, seeps). These flow lines are relatively short (generally less than 2 miles); even along the subcontinental divide flow directions are determined by the relative location of many small surface-water bodies.

Model output: Map showing direction of ground-water flow at water table (source: Wisconsin Geological and Natural History Survey Open-File Report 2004-01)

The ground-water flow pattern is much different in the deep sandstone aquifer than it is at the water table. Flow directions shown for the St. Peter Formation in the figure below are representative of flow in most of the deeper sandstone units. The St. Peter is the top sandstone layer.

Model output: Map showing direction of ground water flow in deep sandstone aquifer, pre-pumping (source: Wisconsin Geological and Natural History Survey Open-File Report 2004-01)

Prior to development under natural conditions, small-scale flow systems characterized the flow in the sandstone aquifer toward the west (indicated by the variety of arrow directions) while regional flow was dominant to the east (indicated by uniform arrow directions). A ground-water divide separated the two areas. This deep ground-water divide is closely related to the western edge of the Maquoketa shale and marks the transition from a confined to unconfined flow system.

Note that the ground-water divide was distant from the subcontinental surface-water divide marking the edge of the Lake Michigan basin. For example in central Waukesha County, the deep ground-water divide was 18 miles west of the subcontinental divide. West of the divide, deep ground water interacted freely with the shallow part of the flow system, while deep ground water east of the divide flowed over long distances toward Lake Michigan.

Thus, under natural, pre-development conditions in southeastern Wisconsin, the Great Lakes basin with respect to tributary ground water was much larger than the basin with respect to tributary surface water. Under pumping conditions the discrepancy became even larger.

The pattern of upward and downward flow in the St. Peter sandstone was distinctive. West of the regional ground-water divide, the vertical direction changed over short distances as leakage from the shallow part of the flow system was carried downward into the sandstone aquifer, circulated laterally, and then moved upward to local discharge locations. East of the divide, the pattern is much more uniform. Ground water moved eastward and downward deeper into the sandstone aquifer, traveled long distances laterally within the aquifer, and eventually tilted upward toward the single ground-water sink for the region - Lake Michigan. Under pumping conditions, this natural pattern is disrupted.

To next topic --> Natural discharge of deep ground water to Lake Michigan