Constructed wetlands are best protection for agricultural runoff into waterways

Scene taken from the Le Sueur River Basin constructed wetland. Credit: Amy Hansen A new paper by a University of Kansas lead author shows that wetlands built along waterways are cost-effective ways to reduce sediment and nitrate loads in large rivers and streams. The research suggests that conservation efforts using wetlands should not be limited to individual farms. The paper was published in the Proceedings of the National Academy of Sciences. It used computer modeling to study the Le Sueur River Basin, in southern Minnesota. This watershed is subject to runoff from intensive agricultural production of soybeanscrops and corn, which is typical of the entire Upper Midwest region. Amy Hansen, an assistant professor of civil and environmental engineering at KU, said that excessive nitrates or sediment can affect fish populations in the area, as well as increase costs for treating drinking water. There is also a downstream effect. "Our rivers incorporate what's going on in the landscape. Whether that location continues to be a great spot to fish or swim depends on the choices made further upstream. Pollution can cause algal blooms, hypoxic zones or other problems in water bodies downstream. The Mississippi River Basin nitrate is the direct cause of the dead zone in northern Gulf of Mexico. The researchers compared different watershed strategies to improve water quality. They compared the potential methods, including reducing runoff from farms or adding wetlands. Finally, they analyzed the economic cost of each. The researchers discovered that most of the methods are less effective because they require voluntary participation from individual farms and are implemented through a network of agencies. "Currently there is individual management or conservation practices. These include cover crop, high precision fertilizer application and reduced tillage. Hansen stated that these are just a few of the many practices we considered. However, management of non-point source is voluntary in the U.S. via incentive programs. The scale at which these conservation practices are applied is often the farmer. A coordinated approach is far more effective. It's a sort of recycling program, where everyone recycles one thing better than all the others. While some recycling is better that none, a coordinated approach to recycling will be more cost-effective and efficient. Hansen and her coauthors discovered that constructed wetlands were the most efficient of all these practices. This is especially true if they are evaluated at the scale a watershed, an entire area that drains into one common waterway. According to Hansen, wetlands have two main benefits: They slow down water's flow toward rivers and streams, and they contain plants and microbes that can use nutrients for fertilizer. Maple Wetland is a fluvial habitat on a tributary of the Maple River in Le Sueur River Basin. It was built 15 years ago. Amy Hansen Hansen stated that nitrate is actually removed by plants and microscopic organisms in wetlands. "With sediment, however, what fluvial wetlands do is hold water back during high flows. This is reducing near-channel sediment that's being carried downstream. Hansen is a water quality researcher, but her co-authors, who hail from the University of Minnesota and the University of California, Irvine, brought multidisciplinary perspectives to the problem of improving agricultural water quality. A National Science Foundation award supported the collaboration. Efi Foufoula from the University of California, Irvine, who is the principal investigator, said that this work would not be possible without the wide range of expertise and perspectives of the team made up of ecologists, hydrologists, geomorphologists and biogeochemists. "The NSF sustained support allowed us to see the problem from a new perspective and to spend the time necessary to collect extensive field data and build new models, as well as engage with stakeholders. As the clock ticks towards meeting the state's water quality targets, we hope our results will have an impact on policy and management. The economics of stabilizing ravines and small fluvial wetlands is a key part of the new study. The investigators found that such measures were more cost-effective than field management. However, the researchers discovered that wetlands performance required optimal placement. Often, wetlands are too costly for one farm or agency to install. According to the PNAS paper, a comprehensive strategy must be developed for an entire watershed. This includes combining funds from various agencies and identifying locations where fluvial wetlands can be found that will reduce nitrates or sediments in waterways. "This study shows that we cannot make real progress towards our goals for improving water quality and agricultural areas using more of a business as usual approach," said Jacques Finlay, a co-author of the study. He is a professor at the College of Biological Sciences of the University of Minnesota. Conservation actions and investments can be more successful if they consider the interplays that lead to water pollution and how different management options impact them. Researchers used Le Sueur River Basin to demonstrate their concept, but they believe that the findings can be applied to other agricultural areas in the Midwest. Continue reading Study finds that wetlands reduce nitrogen pollution at landscape scale Further information: Amy T. Hansen and colleagues, "Integrated Assessment Modeling reveals near-channel Management as cost-effective for improving water quality in agricultural waterssheds," PNAS (2021). Information from the Journal: Proceedings of the National Academy of Sciences Amy T. Hansen and colleagues, "Integrated Assessment Modeling reveals near-channel Management as cost-effective for improving water quality in agricultural waterssheds," (2021). www.pnas.org/cgi/doi/10.1073/pnas.2024912118

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