February 24, 2015
By Aaron R. Conklin
Given the tens of millions of gallons of water that are pumped from municipal wells in Wisconsin’s Dane County each and every day, it would be awfully naïve to believe that there’s no been no long-term effects on the deep aquifers that provide the groundwater.
And sure enough, there have been long-term effects—significant effects. In fact, Wisconsin hydrogeologists have verified that all that pumping has actually reversed the directional flow of groundwater: Where water once flowed from aquifers into rivers and lakes, now it moves the other way.
“The lakes are now losing water (albeit relatively small volumes relative to the size of the lakes) to the groundwater system,” said Jean Bahr, a professor of hydrogeology with the University of Wisconsin-Madison.
But flow isn’t the only thing that may have been altered by long-term municipal well pumping—the groundwater chemistry itself may have been affected, including the possibility of elevated levels of metal contaminants like chromium, iron and manganese into the groundwater. With the support of funding from the UW Water Resources Institute, Bahr, UW-Madison graduate student Joshua Olson and Madeline Gotkowitz, a hydrogeologist with the Wisconsin Geological and Natural History Survey will use a new groundwater flow model to determine the extent of the changes.
The Dane County Regional Flow Model, developed by the Wisconsin Geological and Natural History Survey and the U.S. Geological Survey, is a sophisticated and geologically detailed model of groundwater flow in the area that includes large municipalities such as Madison, Verona, Middleton and Fitchburg. Olson, a graduate student in the UW’s Nelson Institute for Environmental Studies who’s pursuing master’s degrees in hydrogeology and water resources management, will use the model to run simulations looking at physical water flow and current conditions in the Dane County watershed.
Olson will focus on a technique called particle tracking—creating an imaginary water molecule and tracing its path through the model, focusing on advective flow, i.e., what moves along with the groundwater itself. The larger long-term goal of the project will be using the tool to identify hydrogeological units that may be contributing to elevated concentrations of chromium, iron and manganese.
Gotkowitz explained that in shallow groundwater, the water is younger and more suffused with oxygen; in deeper aquifers, older groundwater contains little to no oxygen, which can lead to increased levels of metals. When the flow is reversed, as it has been in Dane County, the two types of water mix.
“What we could find is that changes in the flow paths could be changing redox conditions [the chemical reaction in which an atom’s oxygen levels are reduced] that control movement of metals,” said Bahr.
That’s the big question.
“How the flow has changed is obviously of interest,” said Gotkowitz. “But when we use that groundwater for drinking, iron and manganese levels are important. It’s in part an aesthetic question—these metals tend to impact the color of the drinking water—but also, there’s a bigger question of safe levels.”
Another aspect of the project that intrigues Gotkowitz is gauging the impact of what’s called short-circuiting—the points during a day when water utility managers turn well pumps on and off to account for fluctuations in water use/demand. In large, three-foot-diameter wells, these short-circuit points can provide another conduit for water to move from shallow to deep, providing a potential contributor to trace metal contamination.
The project’s only just beginning, and won’t likely report final results until sometime in 2017.
“The impacts may not be huge,” Bahr said. “But this could give us ways to think about both how we pump from our municipal wells and when. This is information that a water utility could use to improve the way they operate.”