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

| Previous Page | Back to Contents | Next Page |

How much ground water is pumped in the Great Lakes Region?

Total ground-water withdrawal in the Great Lakes Region is estimated to be about 1,510 Mgal/d or 2,336 ft³/s (Solley and others, 1998). An additional 200 Mgal/d or 309 ft³/s is withdrawn from outside the basin but near Lake Michigan in the Chicago area to supply commercial, industrial, domestic, and public-supply customers. For comparison, the average discharge of the St. Clair River at Port Huron is about 120,850 Mgal/d or 187,000 ft³/s. On a basinwide scale, ground-water withdrawal is a small part of the overall hydrologic budget and only about 5 percent of this water is consumed. The remainder is returned mostly as surface water effluent. Nevertheless, ground water is the source of drinking water for more than 8 million people on the U.S. side of the border in the basin (about one third of the total number of residents) and continues to be a concern in both the U.S. and Canada. The areas where large quantities of ground water are pumped on the U.S. side of the Great Lakes watershed are shown in figure 4. The largest withdrawal takes place in an eight-county area near Chicago, where an unknown amount of the return flow is discharged outside the Great Lakes watershed. At the same time, it should be noted that much of the regional ground-water flow in this area also originates outside of the watershed. An analysis of the amount of ground water pumped from wells in areas just outside the Great Lakes watershed would help identify the magnitude of this diversion.

Some areas where the effects of ground-water pumping have been evaluated

Pumping water from aquifers results in lower ground-water levels (fig. 5) and creates a cone of depression around a well. Because water must converge on the well from all directions and because the cross-sectional area through which the flow occurs decreases toward the well, the hydraulic gradient must get steeper toward the well (Heath, 1983). Cones of depression caused by large withdrawals from extensive confined aquifers can affect very large areas (Heath, 1983). Any water withdrawn from the ground-water system will divert part of the water that eventually would have discharged to a stream, lake, or wetland, or been transpired by vegetation. Even ground water withdrawn at some distance from the Great Lakes will reduce flow to the lakes depending on how much of that ground water is returned to streams as wastewater effluent. If the amount of water-level decline is sufficient, ground water that would normally discharge to the Great Lakes may cease and the ground-water divide may be altered (fig. 5). In some cases, water may be drawn from streams or the Great Lakes into the ground-water system. Measurable effects of ground-water withdrawal have been documented at a few locations near the Great Lakes. In order to understand the effect of pumping on the water budget, detailed analyses of ground-water systems near the Great Lakes will be required.

Figure 7. Aquifers, confining unit, and direction of ground-water flow near Green Bay, Wisconsin.

Chicago-Milwaukee Area

The effects of ground-water pumping in the Chicago-Milwaukee metropolitan area where, in 1980, about 300 Mgal/d was withdrawn from a very productive sandstone aquifer system (fig. 2C), are documented in Young (1992). Prior to large-scale withdrawal of ground water, recharge and discharge for the aquifer were in balance at about 350 Mgal/d. When wells were first drilled into the sandstone aquifer along Lake Michigan, the initial ground-water level at Milwaukee was reported to be 186 feet above the surface of Lake Michigan; in Chicago, it was reported to be 130 feet above the lake surface. By 1980, large-scale pumping had caused the water levels in wells to decline as much as 375 feet in Milwaukee and 900 feet in Chicago. At some locations, the quality of ground water was altered when water levels were drawn below the layer that confines the aquifer. Ground-water levels below the confining layer will allow parts of the sandstone aquifer to be exposed to oxygen in the air, which can trigger some chemical reactions that do not take place in the absence of oxygen. By 1994, ground-water withdrawals in Chicago for public supply decreased to about 67 Mgal/d and total ground-water withdrawals decreased to about 200 Mgal/d. These withdrawals were concentrated west and southwest of the earlier pumping centers. As a result, ground-water levels in some parts of the Chicago area have risen by as much as 250 feet, although levels continue to decline in the southwestern Chicago (Visocky, 1997) and the Milwaukee metropolitan areas.

Figure 8. Simulated potentiometric surface in the sandstone aquifer, northeastern Wisconsin, (A) in 1957; and (B) in 1990 ( from Conlon, 1998).

Computer simulations of the sandstone aquifer system indicate that, for 1980 pumping conditions, depressed water levels in the system have caused additional recharge and have resulted in a reduction of natural discharge (Young, 1992). To keep withdrawals in balance with recharge and discharge, for 1980 pumping rates, water was withdrawn from storage in the aquifer system thus accounting for lower ground-water levels. As a result, in 1980, the ground-water divide in the aquifer system was displaced, in some places, about 50 miles west of its original (pre-pumping) position (fig. 6). The rates of recharge, discharge and removal from storage will vary depending on pumping rates, hydraulic properties of the aquifer and confining units, and sources of water. The hydrologic system is further complicated by the fact that most effluent from ground-water withdrawals in the Chicago area is discharged to the Mississippi River Basin via the Chicago Ship and Sanitary Canal–one of the few places where water is diverted from the Great Lakes Basin. However, the amount of ground-water diverted from the Great Lakes Basin by pumping is unknown because some of the water captured by pumping is recharged to the aquifer or removed from storage in the aquifer west of the surface-water divide. These sources of water need to be more accurately quantified in order to assess whether, on a net basis, water is being diverted from the Great Lakes by ground-water withdrawals.

Green Bay-Fox River Area

The sandstone aquifer also is used as a water-supply source in the Green Bay-Fox River area of Wisconsin. A depiction of the potentiometric surface for the aquifer indicates that water-level declines of as much as 300 feet have occurred (fig. 7). The depressed water levels were deep enough in 1957 (fig. 8A) that the city of Green Bay began pumping water from Lake Michigan to supplement ground-water sources. Withdrawals of ground water were reduced, so that by 1990, ground-water levels had risen by about 100 feet in Green Bay (fig. 8B); levels decreased to the south, however, because of increases in ground-water withdrawals by outlying communities.

Toledo, Ohio Area

The Toledo, Ohio metropolitan area obtains ground water from wells open to the carbonate aquifer and from quarry dewatering near Lake Erie. Pumping has lowered water levels in wells as much as 35 feet below the average level of Lake Erie (fig. 9). In addition, pumping has induced water from Lake Erie into the ground-water system and intercepted water that would have discharged from the ground-water system to Lake Erie (Breen, 1989; Eberts and George, in press; and Eberts, 1999). Although water-level data indicate that these interactions are taking place, the amounts of water being induced from the lake and intercepted by the pumping have not been quantified. Figure 9. Potentiometric surface for the carbonate aquifer near Toledo, Ohio, July 1986.

Figure 9. Potentiometric surface for the carbonate aquifer near Toledo, Ohio, July 1986.

Figure 10. Approximate average water budget for Lake Michigan.

Irrigation throughout the Great Lakes watershed

Irrigation is the largest consumptive use of water in the Great Lakes watershed, and ground-water sources contribute about half of the water used for irrigation. In areas where surface-water sources are not readily available, it is likely that ground water will be the water source if new irrigation systems are installed.

Figure 11. Average ground-water and surface-runoff components of selected watersheds in the U.S. portion of the Great Lakes Basin (from Holtschlag and Nicholas, 1998).

Ground-water and surface-water interactions

Streams interact with ground water in three basic ways: they gain water from inflow of ground water through the streambed, they lose water to ground water by outflow through the streambed, and they do both, gaining in some reaches and losing in others (Winter and others, 1998). For ground water to discharge into a stream channel, the altitude of the water table in the vicinity of the stream must be higher than the altitude of the stream-water surface. Conversely, for water in a stream or lake to flow into the ground, the altitude of the water table in the vicinity of the stream must be lower than the altitude of the stream-water surface. The complexity of these interactions may vary from stream to stream as well as over time.

Ground-water flow into the Great Lakes

An approximate water budget for Lake Michigan helps place the role of ground water in perspective. This water budget quantifies the flow of water into and out of Lake Michigan (fig. 10). Inflow of water to Lake Michigan consists of precipitation on the lake (about 53,000 ft³/s); direct surface runoff into the lake (about 8,800 ft³/s); indirect ground-water discharge to the lake (about 32,000 ft³/s); direct ground-water discharge to the lake (about 2,700 ft³/s); diversions into the lake (about 50 ft³/s); and return flows into the lake from water users (about 6,000 ft³/s). Outflow of water from Lake Michigan consists of evaporation from the lake surface (about 41,000 ft³/s); outflow from Lake Michigan to Lake Huron (about 52,000 ft³/s); surface-water withdrawals from the lake (about 7,500 ft³/s); and ground-water withdrawals in the watershed (about 2,100 ft³/s) (Croley and Hunter, 1994; written commun., Great Lakes Commission; Holtschlag and Nicholas, 1998; and Grannemann and Weaver, 1999). Although small in comparison to the amount of water in storage in the Great Lakes, ground water directly and indirectly contributes about 80 percent of the water flowing from the watershed into Lake Michigan. On the basis of these data, it is evident that ground water is an important component of the hydrologic budget for the Great Lakes Region.

A relatively small amount of ground water flows directly to the Great Lakes

The Great Lakes are in topographically low settings that, under natural flow conditions, causes them to function as discharge areas or "sinks" for the ground-water-flow system. Most ground water that discharges directly into the lakes is believed to take place near the shore (Grannemann and Weaver, 1999). Of all the Great Lakes, Lake Michigan has the largest amount of direct ground-water discharge (2,700 ft³/s) because it has more sand and gravel aquifers near the shore than any of the other Great Lakes (Grannemann and Weaver, 1999). Although this is a relatively low inflow compared to the total streamflow into the lake from land areas (41,200 ft³/s) (Croley and Hunter, 1994), it is nearly equal to the amount of water diverted from Lake Michigan through the Chicago Ship and Sanitary Canal (Oberg and Schmidt, 1996).

Ground water keeps streams flowing during periods of low surface runoff

In most instances, the flow of a stream includes both a surface-water runoff component and a ground-water inflow component. The fraction of total streamflow that originated from ground water must be known to analyze and understand the interaction between surface water and ground water in the stream. Holtschlag and Nicholas (1998) used a method called "hydrograph separation" to estimate the amount of ground water in the total streamflow that discharges to the Great Lakes. They call this quantity of water "indirect ground-water discharge" to the lakes. Prior to this study, indirect ground-water discharge was not explicitly considered in estimates of Great Lakes Basin water supply. Instead, it was incorporated into the streamflow component of the supply. Surface runoff is a short-term component of flow that results from precipitation moving overland to a stream without percolating into an aquifer. Ground-water discharge is a long-term, persistent component that results from that part of precipitation that infiltrates into the soil, percolates into an aquifer, and then flows to a stream.

Although Holtschlag and Nicholas (1998) used data from 195 streamgaging stations in the watershed for their analysis, the combined drainage areas to these stations covered only 13.6 percent of the total drainage area of the Great Lakes Basin. These results were extended to the entire basin by assuming that the average ground-water component of streamflow estimated for the ungaged streams is about the same as that estimated for gaged streams in the basin. Using this approach Holtschlag and Nicholas estimated that the average ground-water component of streamflow ranges from 48 percent for Lake Erie to 79 percent for Lake Michigan (fig. 11).

Ground water, wetlands, and stream ecology

Ground water and wetlands

Wetlands, once perceived as worthless land, are now recognized as a necessary component of a vital landscape (Hunt, 1996). They are often considered the "kidneys of the landscape" because of their role in mitigating and filtering the effects of human activity on water resources in the watershed. Wetland functions have been shown to include storm and floodwater retention, shoreline protection, and water-quality improvement. Wetlands also provide wildlife habitat. More than one-third of endangered species in the United States are associated with wetlands even though wetlands comprise less than five percent of the landscape. Vast areas of wetland acreage –more than 50 percent in the United States, and more than 95 percent in some states that border the Great Lakes–have been destroyed, modified, or converted to other uses since presettlement time. Although the effects of these losses are beginning to be understood, more study is needed to improve our knowledge about the role of these important wetland systems.

Wetland hydrology is widely recognized as the primary effect on wetland ecology, development, and persistence. An understanding of the hydrology is essential to identify and quantify wetland functions and processes. For example, ground-water flow has been shown to be important for the physical and chemical environment of other aquatic systems because the amount of dissolved solids carried by ground water is typically much higher than that carried by surface water. Thus, ground water can have a profound effect on the acid susceptibility and nutrient status of the wetland (Hunt and others, 1997). It is widely recognized that linkages between water-budget components and wetlands are not well known, due, in large part, to poor understanding of how ground water flows into and out of wetlands.

While problems associated with ignoring ground water in water-budget analyses are well known (Winter, 1981), traditional hydrologic analyses have had limited success in showing the linkage of ground water to physical and chemical hydrology of wetlands. Previous work on ground water in wetlands has often relied on methods used in aquifer-scale studies, such as widely spaced sample intervals and aquifer tests. Recently, non-traditional investigations of wetlands have shown substantial complexity within wetland hydrologic systems (Harvey and Nuttle, 1995; Hunt and others, 1996). Moreover, this research is showing that ground water has profound effects on the physical and chemical environment of a wetland (Hunt and others, 1999).

Ground water provides refuge for aquatic organisms

Ground-water discharge to streams may help provide important habitat for aquatic organisms, including fish. In addition, because ground-water temperatures are nearly constant throughout the year, stream reaches with relatively large amounts of ground-water discharge can provide refuge to organisms from heat in summer and from cold in winter. For example, some stream reaches in the region remain unfrozen even though air temperatures are well below 32° Fahrenheit. Other possible benefits to the survival of aquatic organisms related to ground-water discharge to streams include increasing concentrations of dissolved oxygen; adding small amounts of nutrients that are essential to the health of organisms; providing cold pockets of water in summer; and maintaining streamflow during dry periods.

| Previous Page | Back to Contents | Next Page |