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Ground water in the Great Lakes Basin: the case of southeastern Wisconsin

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Graphic Link - Concept, Schematic flow systemOVERVIEW OF GROUND-WATER FLOW SYSTEM IN SOUTHEASTERN WISCONSIN

The setting for the case study is a seven-county region that constitute southeastern Wisconsin, also known as the SEWRPC (Southeastern Wisconsin Regional Planning Commission) area. Four of the counties border Lake Michigan:

Map of area encompassed by case study (55 kb) Map of area encompassed by case study
(source: K.R. Bradbury, Wisconsin Geological and Natural History Survey)

The subcontinental divide that crosses the study area is the watershed boundary for the Great Lakes Basin. West of this divide, rivers and the waters that replenish them do not discharge to the Great Lakes but, instead, are tributary to the Mississippi River. The subcontinental divide is very close to Lake Michigan in southeastern Wisconsin. The map shows that more than half the case study area lies west of the divide and, therefore, falls in the Mississippi River Basin.

Studies indicate that the aquifers below southeastern Wisconsin are the source of potable water for 700,000 persons, or about 37% of the resident population in the seven counties. The remaining 63% of the population is provided with public water supply drawn from Lake Michigan. A good source for information on the role of ground water in the water supply for southeastern Wisconsin is a publication issued by the Southeastern Wisconsin Regional Planning Commission and by the Wisconsin Geologic and Natural History Survey called "Groundwater Resources of Southeastern Wisconsin", SEWRPC Technical Report No. 37, June 2002.

The area of greatest ground-water use is around the growing city of Waukesha. Because Waukesha is west of the subcontinental divide, there are legal constraints on its ability to draw on Lake Michigan water that have, so far, obliged the city and its surrounding area to rely almost entirely on ground water. Increasing drawdown at all deep wells and radium concentrations above Federal standards at some deep wells have prompted the city to investigate alternative sources of drinking water, including shallow wells, distant deep wells, and lake water transferred by pipeline. Such efforts have intensified interest in expanding our knowledge of the regional ground-water system and its connection to surface water.

A large part of any improved understanding of the regional ground-water system depends on developing a quantitative tool to study how ground water interacts with the huge body of water that sits off the Wisconsin coast - Lake Michigan. Between 2000 and 2003, a team of hydrogeologists from several state and federal agencies in Wisconsin cooperated to develop such a tool in the form of three-dimensional ground-water flow model capable of not only describing the system at a given time, but also accounting for historical changes.

The basis of the model is the representation of the physical geology in the subsurface. The geology and its influence on ground-water flow (that is, the "hydrogeology") are not changed by human activity. In contrast, the network of wells that grew over the course of the 20th century has had a profound effect on the ground-water flow system.


We will look briefly at these key elements of the model.

Hydrologic Framework in Southeast Wisconsin
  • There are three principal groups of rocks, each containing aquifers:
    1. Shallow unlithified material containing sand and gravel aquifers (Shallow part of flow system)
    2. Shallow bedrock containing the fractured dolomite aquifer (Shallow part of flow system)
    3. Deep part of the flow system containing the sandstone aquifer
  • Along Lake Michigan, the shallow and deep parts of the flow system are separated by the Maquoketa shale, an aquitard that keeps the deep sandstone under pressure.
  • Most municipal pumping in population centers near Lake Michigan is from the deep bedrock.
3D block diagram of aquifers and aquitards under southeastern Wisconsin (68 kb) 3D block diagram of aquifers and aquitards under southeastern Wisconsin
(source: K.R. Bradbury, Wisconsin Geological and Natural History Survey)

Representative cross sections of rock units under southeastern Wisconsin A) West to east section, B) south to north section (64 kb) Representative cross sections of rock units under southeastern Wisconsin
A) West to east section, B) south to north section

(source: Wisconsin Geological and Natural History Survey Open-File Report 2004-01)

The hydrogeologic units shown in the west-to-east cross section dip to the east and thicken under Lake Michigan.

It is worth stressing that close to Lake Michigan the shallow part of the flow system consists of two formations - the unlithified glacial tills and outwash with alluvial deposits overlying fractured Silurian dolomite. The deep part of the flow system is capped by the Sinnipee dolomite and St. Peter sandstone. The two parts are hydraulically separated by the resistive Maquoketa shale. Farther west where the shale is absent, the shallow part of the flow system consists only of unlithified deposits, while the dolomite and sandstone at the top of the deep part of the system are closer to the land surface and in better connection with shallow, local flow systems.

A stratigraphic column provides a detailed look at the units considered by the model, from the oldest rocks (the Mt. Simon Formation at the bottom of the deep sandstone aquifer) to the youngest (Quaternary deposits, i.e., glacial and alluvial deposits). The column also provides the range of hydraulic conductivity values used in the model to describe the ease of flow through each unit.

Stratigraphic column for southeastern Wisconsin (56 kb) Stratigraphic column for southeastern Wisconsin
(source: Wisconsin Geological and Natural History Survey Open-File Report 2004-01)

The hydrostratigraphy for Southeastern Wisconsin is also represented along different slices in a VIDEO CLIP (1189 kb). The viewer is looking northeast from the Illinois/Wisconsin boundary. The slices are aligned with the southeast dip of the bedrock units. Yellow rocks are sandstone units, light green are silty sandstone, light blue and dark blue are dolomite, red is Maquoketa shale, and dark green is unlithified material (glacial tills and outwash, river deposits).

The model consists of 18 layers which constitute all or part of an individual rock unit. The shallow system above the Maquoketa shale is represented by 6 layers. The shale itself and an underlying carbonate unit are represented by 4 layers. The units that constitute the deep sandstone aquifer are divided into 8 model layers.

Each model layer is assigned parameter zones that account for the properties of the rock that affect ground-water flow, One important property is hydraulic conductivity. The following figure gives an example of the horizontal and vertical hydraulic conductivity distribution in a clean, permeable sandstone, called the Wonewoc Formation. It lies in the middle of the deep sandstone aquifer:

Model input: Maps of the horizontal and vertical hydraulic conductivity distribution for one unit within deep sandstone aquifer (54 kb) Model input: Maps of the horizontal and vertical hydraulic conductivity distribution for one unit within deep sandstone aquifer
(source: Wisconsin Geological and Natural History Survey Open-File Report 2004-01)

Shallow and deep wells penetrate the various rock units to various depths:

Schematic cross section of relation of well depths to rock units in southeastern Wisconsin  (72 kb) Schematic cross section of relation of well depths to rock units in southeastern Wisconsin
(source: K.R. Bradbury, Wisconsin Geological and Natural History Survey)

To provide a picture of where the big "high-capacity" wells are located that in 2000 supplied most of the ground water to municipalities and industries, the next set of figures show three-dimensional views of the shallow and deep network. Shallow wells pump from unlithified deposits and the Silurian dolomite (where present above the shale), deep wells pump from Sinnipee dolomite and (mostly) from deep sandstone:

3D views of high-capacity wells in southeastern Wisconsin A) SHALLOW wells (27 kb)3D views of high-capacity wells in southeastern Wisconsin
A) SHALLOW wells

3D views of high-capacity wells in southeastern Wisconsin B) DEEP wells (31 kb)3D views of high-capacity wells in southeastern Wisconsin
B) DEEP wells
(source: D.T. Feinstein, U.S. Geological Survey)

 

 

 

 

 

The network of high-capacity wells for southeastern Wisconsin grew up gradually over time between the late 19th century and the present. The growth of the network is reflected in the total ground-water demand for the SEWRPC counties and three bordering western counties:

Graph of pumping rates over time in counties within and adjoining southeastern Wisconsin (74 kb) Graph of pumping rates over time in counties within and adjoining southeastern Wisconsin
(source: D.J. Hart, Wisconsin Geological and Natural History Survey)

A closer look at the well-use data show the separate trends for shallow and deep wells:

Graph of shallow and deep pumping by county (113 kb) Thumbnail graph of shallow and deep pumping by county (LARGE FILE)
(source: Wisconsin Geological and Natural History Survey Open-File Report 2004-01)

The pumping history varies by county. Municipal and industrial pumping has decreased in some counties that have shifted from ground water to Lake Michigan water (for example, Milwaukee County), but has increased in other counties farther from the Lake that have growing populations (for example, Waukesha County). Some counties rely heavily on shallow wells in sandy deposits (for example, Rock County) or fractured shallow dolomite (for example, Ozaukee County) while withdrawals in others are focused in the deep sandstone units (for example, Jefferson County).

The spread of the well network and the increase in withdrawal rates has caused a regional drawdown cone to develop in southeastern Wisconsin. The rate of drawdown is greatest in the confined portions of the deep sandstone aquifer overlain by the Maquoketa shale. Individual deep wells currently average 7 feet a year of additional drawdown in much of the area:

Plot with graphs of observed and simulated water levels in deep observation wells showing significant drawdown since 1940 (118 kb) Plot with graphs of observed and simulated water levels in deep observation wells showing significant drawdown since 1940 (LARGE FILE)
(source: Wisconsin Geological and Natural History Survey Open-File Report 2004-01)

The graphs show not only the observed drawdown at wells, but also the match achieved by the ground-water model to the observed drawdown. The model simulates the historical response of the ground-water system to pumping that is reflected at the observation wells. The generally good agreement between measured and simulated trends is evidence that the model properly captures the historical behavior of the regional flow system.

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