The Importance of Groundwater in the North American Great Lakes Region
The North American Great Lakes constitute about 20 percent of the Earth’s unfrozen fresh surface water. Because the amount of surface water in the Great Lakes is so large, groundwater in the Great Lakes Basin is often overlooked when evaluating the hydrology of the region. It is, however, more significant than is generally recognized (Grannemann and others, 2000). This lesson is designed to illustrate how groundwater and surface water interact in this water-rich region and can be used as a guide to understanding the complexity of this interaction in other similar regions.
Discussion Questions:
After reading the following sections and studying the accompanying figures, answer the following critical thinking discussion questions:
- Briefly describe why there is so much water in the Great Lakes Region (What happened to create the large lake basins and what role does groundwater play?)
- In regions where surface water is abundant like the Great Lakes, why is groundwater use still significant? (What factors push humans to use groundwater instead of surface water?)
- Given that groundwater discharge to streams in the Great Lakes Basin is so important, provide some examples of why some streamflow gages cannot be used to analyze groundwater discharge.
- Provide the rationale for using base flow as a model calibration tool and to estimate recharge.Is there a bias to using base-flow estimates for estimating recharge?
- In areas of high base flow from groundwater discharge, why might it be harder to reduce nutrient input to the Great Lakes from agricultural or residential sources?
How much water is in the Great Lakes?
The North American Great Lakes consist of Lakes Superior, Michigan, Huron, Erie, and Ontario in descending order by elevation. Superior and Huron are connected by the St. Mary’s River, Michigan and Huron are connected by the Straits of Mackinac, Huron and Erie are connected by the St. Clair and Detroit Rivers with Lake St. Clair in between, Erie and Ontario are connected by the Niagara River which includes Niagara Falls, and the outlet of Ontario is the St. Lawrence River which discharges to the Atlantic Ocean. It is a complex water system whose lake surface covers 94,250 square miles and contains 5,440 cubic miles of water (about 6 quadrillion gallons) (USEPA, 1995). By surface area it is, by far, the largest freshwater lake system in the world; however, Lake Baikal, which is only about the size of Lake Erie and is located in Siberia near the border with Mongolia, contains more water than the five Great Lakes because of its depth. Lake Baikal’s maximum depth is 5,387 feet compared to Lake Superior’s 1,332 feet. Lake Victoria, in the African Rift Valley at the headwaters of the Nile River, is the world’s second largest lake by area but its maximum depth is only 266 feet.
Thirty two percent of the Great Lakes watershed is the water surface of the lakes which is a relatively high ratio compared to other large lake systems. For example, the water surface area of Lake Baikal is only 5.7 percent of the Lake’s watershed. The rivers and streams that discharge into the Great Lakes travel relatively short distances from headwaters to lake. As a result, groundwater that feeds these rivers and streams, are mostly from local, relatively short flow systems unlike the preponderance of regional flow systems in other large watersheds around the world.
How were the North American Great Lakes formed?
The Great Lakes were formed at or near the edge of the Canadian Shield. But they are not the only large lakes that formed in this geologic setting (Figure 1). One of the Shield’s most obvious features are the many small lakes that resulted from glacial erosion during the ice age. In addition to the many small lakes on the Shield, the eastern shore of Great Bear and Great Slave Lakes, Lake Athabasca, and Lake Winnipeg touch the Shield but extend into the sedimentary rocks that overlap the Shield’s metamorphic rocks. Great Bear is the eighth largest lake (by area) in the world, Great Slave is the tenth largest, and Lake Winnipeg is the twelfth largest. Lakes Superior and Huron are also situated at the edge of the Canadian Shield. Figure 2 shows sandstones of the Jacobsville Formation directly overlapping much older metamorphic crystalline rocks.
Figure 1 – Map of Canada showing the location of lakes that surround Hudson Bay, with 8 of the 15 largest lakes in the world (Hydrological Atlas of Canada).
Figure 2 – Sandstones of the Jacobsville Formation (foreground) overlapping much older metamorphic crystalline rocks (right background) on the Lake Superior shoreline. The sandstones are much more easily erodible. (Photo from Norman Grannemann)
Geologic History of the North American Great Lakes
As with any story in history, it is hard to know where to begin. Most of what we know about formation of the Great Lakes relates to glacial erosion and deposition. But if we want to know the origin of the softer rocks in the Basin that were eroded and the harder rocks that resisted erosion by glacial ice, we need to reach back over a billion years when the Midcontinent Rift formed.
Over geologic time, continents fuse together and rift apart. Rifts are features along which continents stretch and split apart to form new ocean basins that separate parts of continents. Not all rifts proceed to continental separation; some fail to fully separate. The Midcontinent Rift, which was in an eruptive phase from about 1.1 billion to about 980 million years ago, is such a failed separation that, in spite of its failure, had a dramatic impact on the area that later became the Great Lakes.
The Rift has a horseshoe shape that extends from Kansas north to Lake Superior and southeast to approximately Lake St. Clair in Michigan. Some geophysical evidence indicates that it may extend further south and could even interact with the St. Lawrence Rift to the east. The Midcontinent Rift explains the large copper deposits in basaltic rocks and shale in the western part of Lake Superior and the origins of the Lake Superior and Michigan sedimentary basins that, in turn, created the conditions for continental glacial erosion and deposition. The Great Lakes were scooped out by glaciers with ice over a mile thick and the debris was deposited ahead, within, and under the ice. The sedimentary basins were created when basalts rose from an underlying mantle plume to spread thick basalt flows in the rift for about a million years (see lesson on igneous aquifers, basalt flows in another lesson in this series). After rifting failed and the underlying magma chamber collapsed, so did the surface of the rift and the collapse formed the Lake Superior and Michigan sedimentary basins. The ensuing rift valley then filled with sediment that became lithified. This is similar to the present day African Rift valley that runs from Lebanon, through the Sea of Galilee, Dead Sea, Red Sea, and to the headwaters of the Nile River. Most of the sedimentary rocks filling the basins were easily eroded by glacial ice but some were resistant, thus directing the flow of glacial ice that centered on Canada’s Hudson Bay. In the center of the Lake Superior Basin, sedimentary rocks were as much as 4 miles thick, plenty of material to be scooped up, added to rocks from the Shield, and redeposited to the south during four major periods of continental glaciation. In the center of the Michigan Basin, sedimentary rocks are as much as 3 miles thick.
Figure 3 – Location of the Mid Continent Rift. Intrusive volcanic rocks in red, extrusive volcanic rocks in purple, sedimentary rocks in green. Native copper and silver was extensively mined in the western Lake Superior area from 1841 to 1997 (https://www.usgs.gov/media/images/mid-continent-rift-story-map).
Figure 4 – Gravity anomaly map of the Midwestern and part of eastern United States showing the Midcontinent Rift (MCR on the map) (Stein and others, 2018). The eastern end of the Rift on this map ends about at Lake St. Clair which coincides with the Chatham Sag structural low. The western end of Lake Erie is shallower because of the Findlay Arch which is also associated with the Rift.
Glaciers finalize the current shape of the Great Lakes
As indicated earlier in this lesson, glaciation put the final touches on the orientation, depth, and size of the Great Lakes, as dictated by ice movement and thickness as well as erodibility of the underlying rocks. Periods of continental glaciation have occurred several times in geologic history but the ice advances and retreats that most shaped the Great Lakes was the Quaternary ice age that began about 2.6 million years ago and lasted until about 10,000 years before present. During that time, the ice sheets advanced to about the present location of the Missouri and Ohio Rivers. However, these advances took place in four different intervals with the ice melting back to Hudson Bay after each advance. In North America, each interval is named for a state of the United States that approximately represents its southernmost advance. From oldest to youngest, these are the Nebraskan, Kansan, Illinoian, and Wisconsinan advances (Figure 5). Most of the glacially related surficial features in the Great Lakes are from the Wisconsinan ice interval but some relic deposits of the Illinoian advance may remain.
Figure 5 – Extent of recent ice sheets in the northern United States (Encyclopedia Britannica, 2014).
The unconsolidated glacial deposits from the final retreat of glacial ice are the most extensive aquifers in the Great Lakes. Because the ice did not melt back in one continuous motion, the deposits associated with melting ice are intertwined and complex. A map of the morainic systems of Michigan illustrates how ice would retreat in pulses (Figure 6). Each dark splotch on the map represents a position where the ice mass did not move and deposited unsorted rocks and soils (a moraine) at the edge of the ice. Water flowing off of the ice carried sediment as outwash sand and gravel that became productive aquifers of the glacial deposits (also referred to as the surficial aquifer).
Note that the ice mass consisted of lobes that were associated with the most erodible rocks where the ice was thickest and would later become the most prominent water features of the Great Lakes. For example, Figure 6 illustrates moraines associated with what is now Green Bay, Lake Michigan, Grand Traverse Bay, Saginaw Bay, Lake Huron, and Lake Erie. Where these lobes came together, the most extensive sand a gravel deposits can be found. For example, in the middle of the northern Lower Peninsula of Michigan the interlobate sand and gravel is over 1,000 feet thick and constitutes one of the most prolific aquifers in the Great Lakes.
Figure 6 – Morainic systems of Michigan (Michigan Geological Survey, 1975).
Another effect of the thick glacial ice was that its weight caused the earth’s crust to be depressed under the ice. It also caused the crust in front of the ice to rise. This is referred to as isostatic adjustment. When the glacier melted, the initial crustal response was a fairly rapid rebound where the ice was thickest. The land surface in front of the glacial ice, where a bulge occurred when ice was present, began to sink after ice melt. These changes were dramatic enough that the initial outlets of the lakes were cut off by the rising land and, ultimately, the outlet of the upper lakes shifted from the Illinois and Ottawa Rivers to the St. Lawrence River. The adjustments are still taking place but at slower rates. Chicago and Duluth are still sinking by about 2 mm per year and the north shore of Lake Superior is rising by 2 to 4 mm per year. As interesting as this adjustment may be, it has little influence on groundwater flow.
How much groundwater is stored in the Great Lakes Basin?
Most large cities in the Region obtain their water from the Great Lakes. Indeed, the easy access to large quantities of water and shipping access has been a driving factor in urban development. In rural areas, however, as water demands increase, the role of groundwater to meet those needs also increases. Estimation of the amount of water stored in aquifers is an essential first step to assessing the water resources. Coon and Sheets (2006) conducted a study of the amount of groundwater in the bedrock and surficial aquifers in the Basin. Results of their study are presented in Tables 1 and 2 below. The volume of groundwater was computed as a function of storativity (also called storage coefficient), the volume of water released from (or taken into) storage per unit surface area of aquifer per unit decline (or rise) in hydraulic head (Freeze and Cherry, 1979). The calculations are different for unconfined and confined aquifers because of gravity drainage and higher storativity values (see Coon and Sheets, 2006, page 11 for details). The study concluded that there is about 1,300 cubic miles of groundwater in the Basin but only about 984 cubic miles is freshwater. This is close to the amount of water in Lake Huron which is 850 cubic miles. Note that this estimate does not include groundwater storage in Canadian aquifers which could nearly equal that in the U.S. Thus, groundwater in the Basin can be thought of as a sixth Great Lake.
Table 1 – Aquifer properties for bedrock and surficial aquifers in the Great Lakes Basin (Coon and Sheets, 2006).
Table 2 – Estimates of groundwater in storage in bedrock and surficial aquifers in the Great Lakes Basin (Coon and Sheets, 2006).
What are the most important groundwater issues in the Great Lakes Basin?
Even with 984 mi3 of fresh groundwater in storage and human use of that water primarily by rural residents and small to medium-sized communities, there are still supply issues related to groundwater. First, this volume of water is not fully available to meet water-supply needs because complete dewatering of any aquifer is not desirable either environmentally or economically. Secondly, significant lowering of either the water table or the potentiometric surface can create both hydraulic and water quality issues. For example, as urban communities begin to encroach on agricultural areas, conflicts between agricultural and other groundwater users increase.
Base Flow
The largest use of groundwater in the Great Lakes Basin is groundwater discharge to surface water, which constitutes most of the base flow of streams. Earlier, we have illustrated the large amount of groundwater in storage in the Basin. The amount of groundwater is dynamic even though it moves through the groundwater system slowly compared to the flow of water in a stream. As we learned in the lesson on the water cycle, precipitation regularly infiltrates to the water table and groundwater leaves the system by base flow to streams, lakes, and wetlands. It also is pumped for human uses but the amount pumped is tiny compare to the amount that naturally discharges to streams. Therefore, to understand the full importance of groundwater, it is necessary to evaluate the rates of groundwater flow to streams. This is accomplished by using streamflow gaging station records to separate the amount of streamflow that is from surface runoff versus groundwater discharge. An example of such a base flow separation is shown in Figure 6. The example in Figure 6 has most of the water flowing in the Nith River at New Hamburg, Ontario originating as groundwater.
Figure 7 – Base-flow separation for the streamflow record of the Nith River at New Hamburg, Ontario (Neff and others, 2005).
The amount of groundwater discharge in a given stream varies primarily by the amount of infiltration which is greater in sandy soils and less in clayey soils. A study by USGS and Environment Canada conducted base-flow separations for streamflow data from 959 gaging stations in the Great Lakes Basin and reported the results as Base-Flow Index values where 0.00 is no groundwater discharge and 1.00 is 100 percent groundwater discharge (Figure 8). The results vary from Index values less than 0.20 to more than 0.90 and have a clear relationship to the amount of sand or clay in the surficial deposits. Higher Base-Flow Index values are in the Lake Michigan, Lake Superior, Lake Ontario, and northern Lake Huron Basins whereas lower values are more common near Lake Erie. Basinwide, the average Base-Flow Index is about 0.70 which means that nearly 70 percent of the surface water discharging to the Great Lakes originated as groundwater. The entire Great Lakes system relies on groundwater more than would be expected.
Figure 8 – Results of base-flow separation for streamflow gaging station records in the Great Lakes (Neff and others, 2005).
A water budget for Lake Michigan is shown in Figure 9 to illustrate estimated rates of groundwater discharge to streams that later discharge to the Lake. This figure also illustrates the rate of groundwater withdrawal and other components of the budget for comparison.
Figure 9 – Approximate average water budget for Lake Michigan (Grannemann and others, 2000).
Groundwater Withdrawals
Large-scale groundwater withdrawals, such as for irrigation, can capture water from or intercept flow to streams which may cause conflicts between people or disrupt groundwater dependent ecosystems. This is especially true for wells drilled in headwaters areas where the impacts of pumping are more quickly apparent. Certain fish species can be harmed and other biological effects are of particular concern.
Groundwater is the source of supply for most inland municipalities such as Lansing, Michigan and Elkhart, Indiana. Although water directly from the Lakes supply municipal water for most large cities in the Region (Chicago, Milwaukee, Detroit, Cleveland, Toronto) there is still considerable groundwater withdrawn in the suburban ring of most major cities (Figure 10). Note that the amount of groundwater withdrawn is small compared to base flow.
Figure 10 – Estimated groundwater withdrawal rates for some major U.S. metropolitan areas (Granneman and others, 2000).
Groundwater Quality
Many shallow aquifers in the Great Lakes have high quality freshwater but the groundwater may be saline at depth. When withdrawals are increased, it is not uncommon for saline water to be induced into shallow aquifers. In addition, groundwater contamination has long been and remains a concern because it degrades the ecosystem and adversely impacts human use of groundwater. Contaminants include nutrients, salts, petroleum and chlorinated hydrocarbons, pharmaceuticals, pathogens, and, most recently, fire retardants. Other lessons in this series will address these issues and how to use groundwater models to more fully understand them.
References:
Coon, W.F., and Sheets, R.A., 2006, Estimate of Ground Water in Storage in the Great Lakes Basin, United States, 2006; U.S. Geological Survey Scientific Investigations Report 2006-5180, 19 p.
Freeze, R.A., and Cherry, J.A., 1979, Groundwater, Prentice-Hall, Inc., 604 p.
Grannemann, N.G., Hunt, R.J., Nicholas, J.R., Reilly, T.E., and Winter, T.C., 2000, The Importance of Ground Water in the Great Lakes Region; U.S. Geological Survey Water-Resources Investigations Report 00-4008, 13 p.
Michigan Geological Survey, 1975, Morainic Systems of Michigan, Map
Neff, B.P., Day, S.M., Piggott, A.R., Fuller, L.M., 2005, Base Flow in the Great Lakes Basin: U.S. Geological Survey Scientific Investigations Report 2005-5217, 23 p.
Stein, S., Stein, C., Kley, J., Keller, R., Merino, M., Wolin, E., Wiens, D., Wysession, M.E., Al-Equabi, G., Shen, W., Frederiksen, A., Darbyshire, F., Jurdy, D., Waite, G., Rose, W., Vye, E., Rooney, T., Moucha, R., Brown, E.; 2016, New Insights into North America's Midcontinent Rift, EOS, Vol. 97, No. 18, pp. 10-16.
United States Environmental Protection Agency, 1995, Environmental Atlas of the Great Lakes, https://www.epa.gov/greatlakes [accessed 4/30/2020]
United States Geological Survey, Mid-Continent Rift Story Map https://www.usgs.gov/media/images/mid-continent-rift-story-map [accessed 4/30/2020]