University of Minnesota researchers, along with colleagues at institutions from the University of Kansas (KU), University of California-Irvine (UCI) and others, found that wetland restoration and construction along waterways are the most cost-effective way to reduce nitrate and sediment loads in large streams and rivers.
The research, published in the Proceedings of the National Academy of Sciences, relied on computer modeling to examine the Le Sueur River Basin in southern Minnesota, a watershed subject to runoff from intense agricultural production of corn and soybeans - crops characteristic of the entire Upper Midwest region.
"Excessive nitrate or sediment affect local fish populations, the amount of money we have to spend to treat drinking water, and there's a downstream effect also," said study author Amy Hansen, who led the research as a postdoctoral researcher at the College of Science and Engineering and in her current position as an assistant professor at KU. "Our rivers integrate what's happening across the landscape, so that location that you love to go and fish or swim - whether that continues to be a great place to fish or swim has a lot to do with the choices that people are making further upstream."
As excess pollution travels downstream into a reservoir, lake or ocean, it causes algal blooms or hypoxic areas called dead zones. The dead zone near the Mississippi River Basin in the Gulf of Mexico is directly linked to excess nitrate from the Mississippi River.
Currently, most methods to improve water quality rely on voluntary participation by individual farms and are implemented by a patchwork of different agencies. The researchers found these methods are less effective at improving water quality than a coordinated approach. Through their work, the team compared potential watershed-level approaches to improving water quality, such as cutting runoff from farms, cover crops and adding wetlands. They then gauged the economic costs of each approach, including combinations of strategies dispersed across the watershed. The team found restored wetlands were most effective because they enhance removal of excess nutrients by vegetation and microbes and lowers peak stream flows.
"A key takeaway from our work is that management to increase water storage on the landscape is crucial," said study co-author Jacques Finlay, a professor in the College of Biological Sciences and fellow at the Institute on the Environment at the University of Minnesota. "Our work shows that holding back water in wetlands should be emphasized more because it addresses influences of climate change on agricultural watersheds, and helps solve two problems: preventing the excess nitrate from being transported from fertilized fields and slowing erosion of banks and bluffs during high water flows."
While the research team found that small, shallow wetlands and stabilizing ravines were more cost-effective than field management, wetland restoration strategies required optimal placement. Additionally, large-scale wetland restoration can often be too expensive for a single farm or government agency to put in place. As a result, researchers conclude that a comprehensive strategy must address an entire watershed as a system, combining funds from different sources and pinpointing locations for wetlands that will lead to the greatest reduction in nitrates and sediments reaching waterways.
"This work shows that it will be very difficult to make greater progress toward our goals for improving water quality in agricultural areas with more of a business-as-usual approach," said Finlay. "Instead, conservation actions, and the investments that support them, can be more effective if they consider the interactions that underlie the source of water pollution and how different management options influence them."
The research, funded by the National Science Foundation, brought together experts from a variety of fields including hydrology, ecology and environmental economics.
"The sustained NSF support allowed us to take a fresh view of the problem and take the time needed to collect extensive field data, build new models and engage with stakeholders," said study co-author Efi Foufoula-Georgiou, a professor at UCI. "We hope that our results will affect policy and management as the clock ticks to meet the water quality targets of the state."
The researchers used the Le Sueur River Basin as a proof-of-concept watershed. However, they say their findings could be applied to agricultural regions throughout the Midwest.
Researchers from the University of Minnesota on this project include Se Jong Cho and Jonathon Czuba with the College of Science and Engineering; Brent Dalzell with the College of Food, Agricultural and Natural Resource Sciences; Christine Dolph with the College of Biological Sciences; Peter Hawthorne from the Institute on the Environment; and Karen Gran from the Swenson College of Science and Engineering at the University of Minnesota Duluth. Additionally, the team included researchers from Iowa State University, U.S. Geological Survey, Cornell University, University of Washington, and Utah State University.
