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

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How does ground water move in the Great Lakes Region?

Aquifers and confining units (relatively impermeable rocks and sediments) make up the ground-water system in the Great Lakes watershed. This system stores water and acts as a conduit for water to move from recharge areas to discharge areas (fig. 3). Recharge takes place between streams in areas that occupy most of the land surface. Ground water moves in both local and regional flow systems.

Figure 3. Generalized local and regional ground-water flow systems in the Great Lakes Region.

Most ground water moves in local flow systems

Ground water in local flow systems commonly travels relatively short distances underground before discharging to a stream, lake, or wetland. The Great Lakes Region has an abundance of small streams, and most ground-water flow takes place in these shallow systems. The amount of ground water moving through these systems is not well quantified, however, because most water-supply studies have focused on deeper regional flow systems. The most productive shallow aquifers are composed of sand and gravel (fig. 1). The extent of these deposits near the land surface is commonly known and illustrated on maps, but the thickness and capability to transmit water often is not well known. To improve our understanding of the importance of ground-water flow in unconsolidated aquifers in the Great Lakes watershed, new geologic maps that show the extent, thickness, and boundaries of these aquifers are needed (Central Great Lakes Geologic Mapping Coalition, 1999).

Most ground water for municipal supply comes from regional ground-water flow systems

Regional ground-water flow systems are usually deeper below land surface and have longer flow paths than local flow systems (fig. 3). Confining units that restrict flow of water between the systems commonly separate local from regional flow, but thick, unconfined aquifers may have regional scale ground-water flow. In the Great Lakes Region, regional ground-water flow occurs in both glacial deposits and bedrock aquifers, depending on the hydraulic properties of the aquifers and confining units, and the topographic relief.

Figure 4. Estimated ground-water withdrawal rates for some major U.S. metropolitan areas (data not available for Canadian areas).

Glacial deposits usually consist of a complex assemblage of sediments (fig. 3). In some parts of the region, glacial deposits are as much as 1,200 feet in thickness. As thickness increases, the complexity of the sediment assemblage usually increases. These sediments need to be mapped using established three-dimensional mapping techniques to understand their geological framework (Bhagwat and Berg, 1991). Hydraulic characteristics of the sediments also need to be determined for the aquifers that are increasingly being tapped for water supply. Armed with this hydrogeologic characterization, water managers will be able to make better determinations of sustainable withdrawal rates from the region's aquifers.

The extent, thickness, hydraulic properties, and general directions of flow in the most used bedrock aquifers have been described by regional aquifer studies conducted by the USGS (Sun and others, 1997) and by State and local agencies (Bleuer and others, 1991; Batten and Bradbury, 1996; and Passero and others, 1981). Although these studies provide a baseline of hydrologic and geologic information, more work needs to be done to define and quantify the interactions between regional ground-water flow and ground-water discharge to the Great Lakes. Divides that are transient barriers to ground-water movement are established by a combination of natural and human-induced stresses on the aquifers. In some areas, bedrock aquifers may discharge large quantities of water to the lakes, but the data needed to quantify the amount of flow have not been collected. In addition, the effects on the Great Lakes of pumping from regional aquifers are unknown. Many ground-water issues take time to be recognized, but, because of the large volumes and resulting long travel times for water in regional flow systems, the time lags expected are usually much longer than for local flow systems. Thus, adverse effects of withdrawals may take years to manifest themselves.

How is ground water replenished?

Recharge is the term that is commonly used to describe the process of adding water to the ground-water system. Although it is difficult to directly measure the amount of recharge, it is important to estimate recharge rates to understand the effects of ground water on other hydrologic processes in the basin and to assess how activities at the land surface may change the recharge rates. The amount of recharge can vary considerably throughout the basin depending on soil type, precipitation (rates, types, timing, and amounts), and other factors, including the extent of impervious surfaces (roofed and paved areas) and storm sewers. For example, the amount of water that infiltrates into a sandy soil is usually greater than that into clayey soil. Recharge rates in Michigan's Lower Peninsula range from nearly 0 to about 23 inches per year (Holtschlag, 1997). Ground-water recharge rates estimated in previous studies represent the approximate range of recharge to the water table in the entire Great Lakes Region. A comprehensive study for the entire watershed is needed to more completely determine the importance of ground water in the hydrologic budget of the Great Lakes.

Figure 5. Generalized ground-water flow (A) under natural conditions and (B) affected by pumping (Note that surface - and ground-water divides are coincident in A but not B).

Urban development may reduce recharge amounts because impervious surfaces (such as roads, buildings, and paved areas) often drain to storm sewers, a situation that increases surface runoff and reduces infiltration. These processes may significantly alter ground-water conditions in many urban settings by "short-circuiting" to streams and lakes water that would have infiltrated to the water table. They also may increase flood potential. Currently, only 7 percent of the Great Lakes watershed is classified as urban; therefore, the effects of urbanization on ground-water recharge are likely to be localized and the effects on the watershed as a whole may be minimal. Because urban areas are rapidly expanding, however, it is important to continue to monitor the effects of urbanization on ground-water recharge rates. Other activities associated with urban expansion, such as increased ground-water pumping, along with reduced recharge rates may increase the drawdown of water levels caused by pumping.

Figure 6. Decline in water levels in the sandstone confined aquifer, Chicago and Milwaukee areas, 1864-1980. (Modified from Avery, 1995.)

Recharge to bedrock aquifers is less well understood than that to unconsolidated aquifers because infiltrating water may need to move through several layers of geologic material before reaching the bedrock aquifer. Direct measurement of recharge rates to bedrock aquifers is difficult. Estimates of these rates have been made in the USGS Regional Aquifer-System Analysis studies (Sun and others, 1997) mostly by simulating regional ground-water flow with digital models. These rates vary considerably from place to place, but generally are much lower than the estimates of recharge to the water table, especially for non-pumping conditions.

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