Volume 3 2011

Research Assistant Ashley Winker and UW-Oshkosh Professor Bob Stelzer investigate the role that deep stream sediments play in removing nitrogen from the environment. Photo: John Karl.
Water Resources Research

Stream Sediments Serve Important Role in Ecological Balance

By Carolyn Rumery Betz

The Green Revolution—not the kind that celebrates Earth Day, solar panels and recycling but the one related to food production—has used various technologies to meet the world’s nutritional needs by growing more food per acre. The use of nitrogen fertilizer is one tool that has allowed for increased food production. Crops need nitrogen to make their own food, but since they can’t take it directly from the air, nitrogen fertilizer, often in the form of anhydrous ammonia, is added to enrich the soil and maximize yields. Adding more fertilizer than crops need to grow results in surface and groundwater contamination.

Worldwide, there is more available nitrogen in our environment than ever before because of fertilizers and burning of fossil fuels, according to Bob Stelzer, associate professor of biology at UW–Oshkosh. Stelzer is interested in the overall balance of the many forms of nitrogen in the environment.

Too much nitrogen in our lakes and streams can lead to excessive plant growth, degraded recreational experiences and fish kills. The cumulative effects of excess nitrates moving downstream from the Corn Belt have created a “dead zone” in the Gulf of Mexico.

“Can the rate of removal of nitrates keep up with the supply of nitrogen fertilizers?” asks Stelzer. “Right now, supply is winning, and the excess is ending up in our drinking water and in our coastal ecosystems.”

Excess nitrates in our drinking water can pose serious human health threats by compromising the ability of blood to carry oxygen, potentially leading to serious health complications. This is particularly dangerous for infants and young children. Excessive nitrate consumption has been linked to increased risks for certain cancers. In Wisconsin, an estimated 9 percent of the state’s private wells exceed the safe drinking water standard for nitrates. This problem affects every county in the state and costs millions of dollars to correct. More than 70 percent of the state relies on groundwater for all uses, including drinking water, agricultural, manufacturing and industrial uses.

In a research project funded by the Water Resources Institute, Stelzer is examining the ability of bacteria in sediments below the stream surface to convert nitrates, a form available to plants, into atmospheric nitrogen, which plants cannot use to grow. It is a location where denitrification can occur and has not been previously well studied.

Nutrients can be carried from groundwater to surface water since the groundwater flows from under the stream bed into the surface water. Stelzer has installed a series of wells and specialized groundwater samplers in eight streams in the Waupaca River watershed, one of the hotspots of Wisconsin’s excess nitrates. This allows him to create a vertical nitrate profile for each sample location, yielding better understanding about how nitrate behaves in the groundwater beneath the stream.

Preliminary results show that while denitrification rates are higher in the shallower sediments, the deeper sediments are responsible for a large portion of denitrification overall. The denitrification and nitrate profile results suggest that nitrate retention removal is widespread in deep stream sediments, particularly in watersheds with high groundwater nitrate concentrations. Wetlands, rivers, sediments and wet soils are all places where nitrates are converted back to atmospheric nitrogen that can help balance the overall ecosystem. If a river ecosystem is degraded, such as being channelized and lined with concrete, groundwater and surface water are less likely to interact, and nitrate removal will be reduced.

“It’s very important that ecosystems remain as natural and healthy as possible so that removal of excess nitrates and nutrients can take place,” Stelzer said.

The Aquatic Sciences Center is the administrative home of the
University of Wisconsin Sea Grant Institute & University of Wisconsin Water Resources Institute.

©2011 University of Wisconsin Board of Regents