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

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SCHEMATIC FLOW SYSTEM

Water cycles continuously between the earth's atmosphere, surface and subsurface.

Hydrologic Cycle
Schematic block diagram of hydrologic cycle (90 kb)

Schematic block diagram of hydrologic cycle

(source: U.S. Geological Survey Circular 1223)

Graphic link to Case Study, Flow System
The amount of water in each part of the cycle can be estimated:
Schematic section of amount of water in parts of hydrologic cycle (21 kb) Schematic section of amount of water in parts of hydrologic cycle
(source: U.S. Geological Survey Circular 1139)
 

 

 

 

 

 

 





A significant amount of water is stored in and flows through the subsurface as ground water. Strictly speaking, ground water refers only to water below the water table where the pores in loose sediment or lithified rock are fully saturated with water. When the pores are saturated, the water is more free to move toward streams or wells.

We call the circulation of ground water, from the water table where it is recharged to its destination where it is discharged, the ground-water-flow system. Most of the flow is lateral (that is, in a horizontal direction). However, there is also a vertical component to the flow that can be especially important whereground water enters the flow system, when it moves between parts of the flow system, and where it exits the flow system. One way to understand typical ground-water behavior is to consider flow that moves mostly in a single direction (say from west to east) and draw flow lines along a vertical cross section aligned with the direction of flow. Each of the following section plots is meant to emphasize different aspects of ground-water flow system. In viewing them, keep in mind that the vertical dimension is stretched (we say "exaggerated") to bring into relief certain features.


Unconfined vs. Confined Parts of Ground-Water Flow System:

Schematic section of unconfined and confined parts of gw flow system (36 kb)
Schematic section of unconfined & confined parts of ground-water-flow system
(source: U.S. Geological Survey Circular 1186)

Most ground water flows short distances (no more than one or two miles) from where it enters the water table as recharge to where it exits the subsurface as discharge. Recharge occurs over most of the land surface as water infiltrates down through the unsaturated zone to the water table. The most common discharge locations are streams, lakes, and wetlands; they occupy a relatively small proportion of the landscape.

The bulk of ground-water flow in settings such as the Great Lakes Basin occurs through permeable material lying at shallow depths below the water table. However, some of the flow can move to deeper permeable material separated from the shallow part of the flow system by less permeable rock. The shallow material (often consisting of loose deposits laid down by rivers or glaciers) is called an "unconfined aquifer". The resistive layer (often consisting of lithified material like shale) is called an "aquitard" or "confining bed". The deeper layer (often consisting of lithified material like sandstone) is called a "confined aquifer".

In the schematic section shown above, the ground water in all three "hydrostratigraphic" units forms a single flow system with a single discharge point at a local stream. Although united in a single flow system, the unconfined aquifer (whose top surface is the water table) and the confined aquifer (whose top surface is the bottom of an aquitard) will react differently to stresses such as pumping.


Local vs. Regional Ground-water Flow:

Schematic section of local and regional ground-water circulation (63 kb) Schematic section of local and regional ground-water circulation
(source: U.S. Geological Survey Circular 1186)

When ground-water systems are mapped at a large scale, we find that while most of the water moves short distances to nearby streams and lakes, some moves under local water bodies to more distant discharge features. Such "regional sinks" include both natural features (e.g., a large water body) and manmade features (e.g., a deep, high-capacity well). In other words, although the "recharge area" or "capture area" for most surface-water features is a small basin defined by local topography, large bodies of water (and some wells), can receive some of their water from distant basins. Consider any of the Great Lakes. The source of most of the ground water that discharges to the Lake is precipitation and recharge along its coastline, but there can be a component of deep tributary flow that originates tens of miles inland from the coast.


Exchange between Shallow and Deep Parts of a Ground-Water Flow System:

Schematic section of natural downward leakage from shallow to deep part of ground-water flow system (50 kb) Schematic section of natural downward leakage from shallow to deep part of ground-water flow system
(source: D.T. Feinstein and J .T. Krohelski, U.S. Geological Survey)

It is often important to quantify how much of the ground water circulates locally through shallow parts of the flow system as opposed to how much circulates regionally through deeper parts of the system. It is also useful to estimate the exchange between the shallow and deep parts of the system. The schematic shows downward "leakage" from an unconfined to confined aquifer. Most of the shallow flow is directed horizontally toward a stream; the downward leakage represents a small fraction of the total flow. The downward leakage is spatially focuses and has a strong vertical component; most occurs where the resistive aquitard is thin or absent. Once in the deep part of the flow system, the ground water moves largely in a horizontal direction toward a regional sink.


Rapid vs. Slow Ground-Water Circulation:

Schematic section of typical ground-water travel times as function of depth and distance of flow (44 kb) Schematic section of typical ground-water travel times as function of depth and distance of flow
(source: U.S. Geological Survey Circular 1139)

Shallow flow to local sinks is relatively rapid, which means that the time between when the water is recharged at the water table and the time when it exits is typically on the order of months or years. Flow is relatively rapid because distances tend to be short and hydraulic gradients tend to be high. Deep flow to regional sinks can circulate for much longer times, on the order of hundreds or thousands of years. In many areas around the Great Lakes, multiple aquifers are stacked on top of one another, and the travel times vary accordingly. Flow from a hillside to a shallow well open to glacial outwash in the adjacent valley is much more rapid than flow across a series of resistive units to a well open to a deep sandstone.

Each Great Lake is also a system with flows that enter and exit. Consider Lake Michigan:

Thumbnail of Diagram showing inflow and outflow rates for Lake Michigan (53 kb) Diagram showing inflow and outflow rates for Lake Michigan
(source: U.S. Geological Survey Water-Resources Investigations Report 00-4008)

The biggest source of water to Lake Michigan is the rain and snow that falls on it. Ground water is a relatively small component of its "budget" if you consider only the part that directly discharges across the lakebed near the coastline. However, ground water is much more important if you consider the part in the Lake budget played by tributary flow from rivers and streams. All these surface-water bodies that empty into the Lake are fed by ground water. They are the discharge locations for large and small ground-water flow systems everywhere between the Lake Michigan coastline and the topographic divide that defines its inland watershed. It is estimated that up to 80% of the water in streams tributary to Lake Michigan originated as ground water. Streams in southeastern Wisconsin receive somewhat less, about 40-50% of their flow from ground-water discharge.

For more information on the role of ground water in the Great Lakes basin, see the USGS publication "The Importance of Ground Water in the Great Lakes Region", Grannemann and others, 2000, USGS Water-Resources Investigation Report 00-4008.

The components of the hydrologic cycle are connected. In particular, ground water and surface water form a single resource:

Block diagram illustrating water flow in a natural landscape (39 kb) Block diagram illustrating water flow in a natural landscape
(source: U.S. Geological Survey Circular 1139)

This connection has an important consequence: when cities develop, water that once discharged to streams and wetlands and lakes is diverted to wells installed for water supply. This change from a natural ground-water system to a developed system has occurred extensively within the Great Lakes Basin. Shallow and deep wells have changed ground-water flow directions, have moved the boundaries of ground-water systems, and have decreased the rates of ground-water discharge to the surface.

To next Concept -- > Natural recharge to ground water

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