EFFECT OF PUMPING ON GROUND-WATER DIVIDES IN THE DEEP SANDSTONE IN SOUTHEASTERN WISCONSIN

Pumping has moved the regional ground-water divide in the deep part of the system westward almost entirely outside the seven-county southeastern Wisconsin region.

The regional ground-water divide marks the boundary east of which deep ground water is tributary to Lake Michigan. Even before pumping this divide was far to the west of the subcontinental divide that marks the Great Lakes watershed boundary.

Model output: Map of ground-water flow directions in deep sandstone aquifer for natural conditions (source: Wisconsin Geological and Natural History Survey Open-File Report 2004-01)

The figures below show the positions of the regional ground-water divide in the deep part of the flow flow system for 1950 and 2000 relative to its predevelopment position under natural conditions. In 1950, the regional flow in the deep sandstone aquifer converged on the pumping center under Milwaukee . By 2000, flow paths converge under eastern Waukesha County . Over that time the regional ground-water divide has moved from Waukesha County westward about 10 miles into Jefferson County , a location 27 miles from the subcontinental divide at the western edge of the Lake Michigan watershed basin. This displacement of the divide in response to pumping is directly related to the increase in leakage to the deep part of the flow system in areas where the Maquoketa shale is absent.

Model output: Map of 1950 ground-water flow directions in deep sandstone aquifer	Model output: Map of 2000 ground-water flow directions in deep sandstone aquifer
(source: Wisconsin Geological and Natural History Survey Open-File Report 2004-01)

NOTE that the regional ground-water divide has moved west over time, but that deep ground water that was formally tributary to Lake Michigan now flows to pumping centers in Wisconsin and Illinois.

The flow-direction plots for the St. Peter Formation, representative of the deep flow system, also show the changing location of the ground-water divide between a regional ground-water system centered in southeastern Wisconsin and another centered in northern Illinois. The model shows that in 1950 the divide was located along the Kenosha/Racine County boundary. Increases in northern Illinois pumping after 1950 moved the divide north into eastern Racine County, but the development of local cones of depression around Union Grove moved the divide south in western Racine and in Walworth Counties. While northern Illinois pumping decreased overall in the 1990s, the 2000 divide is still north of the 1950 boundary in some places.

1. the shale that confines the deep sandstone,
2. Lake Michigan that served as a regional sink under natural conditions, and,
3. the pumping centers that capture flow under current conditions:

Model output: Cross sections containing representative ground-water flow lines for natural conditions and for 2000 conditions (source: D.T. Feinstein, U.S. Geological Survey)

The dramatic changes evident in the deep flow pattern between pre-development and 2000 are not reflected in the shallow part of the flow system. Most of the shallow ground water continues to discharge to local water bodies over short flow paths. However, less ground water than before discharges to streams and lakes; a greater fraction now leaks downward in response to the demand from wells.

Summing up:

	Schematic cross sections of ground-water flow system before well development (source: J.T.Krohelski and D.T. Feinstein, U.S. Geological Survey)
	Schematic cross sections of ground-water flow system after well development (source: J.T.Krohelski and D.T. Feinstein, U.S. Geological Survey)

To next topic --> Effect of pumping on direct & indirect ground-water flow to Lake Michigan & on ground-water flow between the Mississippi River and Lake Michigan basins

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