The Importance of Ground Water in the Great Lakes Region
Water Resources Investigations Report 00 - 4008

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Why do we need to know more about ground-water conditions in the Great Lakes Region?

The Great Lakes constitute the largest concentration of unfrozen fresh surface water in the western hemisphere–about 5,440 mi³. Because the quantity of water in the lakes is so large, ground water in the Great Lakes Basin is often overlooked when evaluating the hydrology of the region. Ground water, however, is more important to the hydrology of the Great Lakes and to the health of ecosystems in the watershed than is generally recognized.

Figure 1. Surficial geology of the Great Lakes Basin.

Although more than 1,000 mi³ of ground water are stored in the basin–a volume of water that is approximately equal to that of Lake Michigan–development of the ground-water resource must be carefully planned. Development of the ground-water resource removes water from storage and alters the paths of ground-water flow. Ground water that normally discharges to streams, lakes, and wetlands can be captured by pumping (the most common form of development), which may deplete or reduce inflows to the Great Lakes.

Ground water is important to ecosystems in the Great Lakes Region because it is, in effect, a large, subsurface reservoir from which water is released slowly to provide a reliable minimum level of water flow to streams, lakes, and wetlands. Ground-water discharge to streams generally provides good quality water that, in turn, promotes habitat for aquatic animals and sustains aquatic plants during periods of low precipitation. Because of the slow movement of ground water, the effects of surface activities on ground-water flow and quality can take years to manifest themselves. As a result, issues relative to ground water are often seemingly less dire than issues related to surface water alone.

Ground water is a major natural resource in the Great Lakes Region that helps link the Great Lakes and their watershed. This linkage needs to be more fully understood and quantified before society can address some of the important water-resources issues in the region.

What are the major ground-water issues on the Great lakes Region?

A major attraction of the Great Lakes Region is the abundant water supply on which manufacturing, power generation, transportation, agricultural, and recreational sectors have historically relied. Most large public water supplies are obtained from the lakes themselves, but ground water is the source of drinking water for about 8.2 million people within the watershed. Although most residents of Chicago use water from Lake Michigan, many people in the Chicago suburbs who live outside of the watershed, but are close to it, use ground water as a source of supply. As the suburban areas near the watershed boundary expand, more and more people depend on ground water to supply household water needs. Small manufacturing companies in suburban locations also are increasing their ground-water use. As communities encroach upon agricultural areas, conflicts between agricultural and other ground-water users will increase (Alley and others, 1999). Therefore, ground-water resources need to be characterized according to their occurrence, availability, quality, and use to develop a sustainable supply for all uses.

Pumping ground water can capture water from or intercept flow to streams and alter the area that contributes ground water to the Great Lakes. Thus, ground-water withdrawals can divert ground water that would normally discharge to the Great Lakes system.

Figure 2. (A) Bedrock aquifers of the Great Lakes Basin (modified from great Lakes Commission, 1975; (B) Approximate extent of the freshwater bearing carbonate aquifer on Ohio, Indiana and parts of Michigan and Wisconsin ( modified from Casey, 1996, figure 14); (C) Approximate extent of the sandstone aquifer west of Lake Michigan (modified from Young, 1992, figure 16).

In addition to water quantity issues in the Great Lakes Region, water quality also can be of concern. As development increases, activities that could threaten the quality of ground water also increase. Human health needs to be safeguarded, as does the health of many other organisms that rely on clean water. Thus, the major ground-water resource issues in the Great Lakes Region revolve around 1) the quantity of ground water, 2) the interaction of ground water and surface water, 3) changes in ground-water quality as development expands, and 4) ecosystem health in relation to quantity and quality of water. In summary, ground water is an essential part of the Great Lakes Region water-supply system. It is a critical resource for maintaining human health and healthy ecosystems.

Geology establishes the framework for aquifers

Ground water is present throughout the Great Lakes Basin, but the quantity that can be withdrawn varies depending on the characteristics of the water-bearing rocks and sediments (aquifers). Unconsolidated material that was deposited at or near the land surface as a result of large-scale glacial ice advances and retreats during the last 2 million years make up the most productive aquifers. These deposits are as much as 1,200 feet thick in parts of Michigan and are several hundred feet thick in buried bedrock valleys in Illinois, Wisconsin, and New York. The deposits are thin or nonexistent in areas where bedrock that was not easily eroded by glacial ice is exposed at land surface. Most glacial deposits are composed of mixtures of sand and gravel, and silt and clay (fig. 1). Sand and gravel deposits (outwash and ice-contact deposits) are the most productive aquifers because they have greater permeability and effective porosity than do the finer grained deposits. Some areas with silt and clay at the surface (till or glacial lake deposits) contain more permeable deposits at depth and are able to yield moderate to large amounts of water to wells. In general, however, the silt and clay deposits are not aquifers.

Bedrock aquifers are generally widespread throughout the region and are more continuous than the aquifers in glacial deposits. Some bedrock aquifers in the region extend far beyond the watershed boundaries. The relations between ground water in these aquifers and water in the Great Lakes is complicated because ground-water divides and watershed boundaries may not coincide. Carbonate rocks (limestone and dolomite) are the most common bedrock aquifers in the region (fig. 2A). Natural processes may increase permeability by dissolving carbonate minerals in these aquifers, but this increased permeability makes the aquifers more vulnerable to contamination. The most extensive carbonate aquifer in the region consists of a series of limestones and dolomites that underlie a large part of the upper Midwest (fig. 2B). Sandstone aquifers are the next most common bedrock aquifer. An extensive sandstone aquifer underlies much of the northern Midwest and even extends under Lake Michigan (fig. 2C). In general, shale, and igneous and metamorphic bedrock have limited water-yielding capacity, and they are not considered regional aquifers.

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