A look at Groundwater from Bob Otwell
Bob Otwell, FLOW Board of Directors
Most of us in northern Michigan drink groundwater and use it to bathe. Outside of metro Detroit, the majority of Michigan’s public water supplies along with water in rural homes comes from groundwater. Groundwater also is used to water golf courses and supply the growing thirst of irrigated farm land. We would not have trout in our northern streams if they were not nourished during the heat of the summer by cold groundwater. This is our invisible resource.
This blog is the first in a two-part series examining groundwater; this article will provide the reader a better understanding of the physics, and the second one will examine current groundwater regulations.
Groundwater is simply rainfall and snowmelt that has percolated into the ground. In northern Michigan, about one third of our annual 33 inches of precipitation ends up as groundwater. The remainder runs off on the surface to lakes and streams, or is taken up by plants and is lost through evapotranspiration. In the Great Lakes Basin, abundant groundwater is stored in the layers of sand and gravel left behind by the glaciers, and in sandstone and limestone bedrock. The temperature of groundwater is generally the average annual air temperature above the ground. In northern Michigan, this means 50 degrees Fahrenheit all year round. This temperature cools trout streams and provides a nice cool drink in the summer, and it also helps keep small streams from freezing in the winter.
Groundwater flows naturally by gravity through permeable sands and other porous materials, and continues moving downhill until it seeps into wetlands, springs, streams, rivers or lakes. We’ve all seen groundwater percolating into a spring, or felt the cool currents on our feet while swimming in one of our clear lakes. But groundwater discharge to surface water bodies is in fact continuous throughout the bottom of the stream, lake, etc., even though we can’t see it. They are connected, and if you care about a certain babbling brook, you in fact care deeply about the groundwater that makes it what it is. Rivers and streams flow at a velocity measured in feet per second, whereas groundwater flows at a rate of feet per day. This sure and steady seepage provides the base flow that makes a perennial stream flow all year round.
Groundwater also flows unnaturally where the “downhill” direction is altered through the installation of wells and pumps. This pumping creates a “drawdown cone” around the well. If a small well is installed, there is a small blip in the groundwater table. By contrast, if a large well is placed with a large capacity pump, the groundwater table can be altered dramatically. Where there are many large wells, serious regional impacts can take place. The High Plains (Ogallala) aquifer that extends from South Dakota to Texas has been over-pumped for decades, resulting in a lowering of the groundwater table in some areas of over 150 feet. This significant drawdown forces other groundwater users to deepen their wells, increasing their costs and energy requirements. This “mining” of water has created a net loss of groundwater in the High Plains of 340 km³. What would be the effect if this volume of water was taken from Lake Michigan? If spread out over the surface area, this would reduce the lake level by 20 feet.
Large wells can also dry up springs and streams. The most vulnerable springs and streams are those near the headwaters, where flowing tributary groundwater is limited. Ironically, due to FDA requirements, this area is where bottled water companies must install their wells if they want to label the bottle “Spring Water.” Pump a gallon out of the ground in these areas and you lose a gallon in the stream.
Groundwater, springs, wetlands, rivers and lakes are all interconnected. To care about one, is to care about all. Are we taking care of our groundwater in Michigan?
To be continued next time.
Note: I have simplified the discussion above to aid in understanding. Hydrogeology is complicated by a combination of confined, unconfined and perched aquifers, separated by discontinuous layers of less permeable soils (silt, clay and glacial till). In addition, we only know for sure what we find in a soil boring at a specific location, and we must then interpolate between the borings. Our knowledge is dependent on the funds available to install multiple borings.