Have you seen any good movies lately? How about the one where a little camera slowly descends into a well several hundred feet deep in Afton State Park? You can see water and rocks and, well…you know, it sure captivated a room full of science geeks.
With funding from the Metropolitan Council, the Clean Water, Land, and Legacy Amendment, and the Legislative-Citizen Commission on Minnesota Resources (LCCMR), the Minnesota Geological Survey (MGS) is wrapping up a pair of research projects that have vastly improved understanding of how water moves underground in the Twin Cities metro area. The research supports previous findings that high capacity wells are causing groundwater to move more quickly and go deeper than it would otherwise. New data collected also show that water is moving both horizontally and vertically below ground.
To better explain this research, let me first offer a basic primer on how groundwater works. Groundwater is the water beneath the land’s surface that fills the spaces between rocks and sediment. If you think of the earth as a layer cake, there are porous layers where water is stored, known as aquifers, and confining layers of impenetrable rock that water cannot move through. Thus, there can be several layers of aquifers, one on top of another, with the deepest aquifers containing the oldest water. Statewide, groundwater provides about 75% of our drinking water, and in Washington County it provides 100% of the water we use for drinking, irrigation, and commercial uses.
Groundwater interacts with surface water in many different ways. There are recharge areas where rainwater collects and soaks easily into the ground, as well as spring-fed lakes and streams where groundwater flows out to the land’s surface. We humans are able to access groundwater by drilling wells that go hundreds of feet deep into the ground to reach both shallow and deep aquifers.
As part of the County Geologic Atlas program, MGS has worked with many counties in Minnesota to map the sediments, rocks and aquifers below ground, as well as public and private wells. These geologic atlases show where aquifers connect with surface water resources and also include information about geology, mineral resources, and natural history in each county. Additional maps, created by the Minnesota Department of Natural Resources, show water levels in aquifers, direction of groundwater flow, water chemistry, and sensitivity to pollution.
According to Robert Tipping, a Senior Scientist involved in recent MGS research, there is a correlation in the metro area between locations where there is deeper groundwater pumping, and those with higher levels of chloride in the groundwater. This information, paired with other water quality data collected, tells the MGS scientists that human activities (such as putting down road salt) are not only increasing chloride levels in groundwater, but also causing the chloride to move down into deeper aquifers.
By installing measurement equipment and dropping video cameras down into a couple of test wells, MGS was also able to document that water is moving both horizontally and vertically beneath ground. In a test well in Edina, Tipping estimates that water was flowing downward at a rate of 30-40 gallons per minute, which was much faster than they had previously imagined. Video footage from a temporary well built in Afton State Park showed water shooting out of a horizontal fault in one of the bedrock layers and then rushing downward toward a deeper level aquifer. Watching the video, it is easy to see how a poorly placed or improperly constructed well could act as a wormhole, allowing contaminants from a shallow aquifer to travel through confining layers into a deeper aquifer. To prevent this from happening in Afton, the test well was carefully controlled during the investigation and sealed at the end of the project.
In future research, MGS hopes to gain a better understanding of how groundwater travels through or across vertical fractures in bedrock, as well as how it moves through bedrock valleys. As for the thrilling footage from the bottom of a well, maybe you’ll find it on YouTube some day!
Learn more about the Minnesota Geological Survey at www.mngs.umn.edu. Find information from the Minnesota DNR’s County Atlas project at www.dnr.state.mn.us/waters/groundwater_section/mapping.