The most cost-effective approach to reducing nitrate and sediment loads in major streams and rivers, according to new research by a lead author from the University of Kansas, is to build wetlands along waterways. Rather than focusing on individual farms, the study proposes that wetlands conservation initiatives be undertaken at the watershed scale.
The paper, which was recently published in the Proceedings of the National Academy of Sciences, used computer modeling to investigate the Le Sueur River Basin in southern Minnesota, a watershed subject to runoff from intensive corn and soybean crop production that is typical of the 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 Amy Hansen, lead author and assistant professor of civil, environmental, and architectural engineering at KU. “Our rivers integrate what’s going on across the landscape, so whether or not the spot where you want to go fishing or swimming remains a fantastic spot to fish or swim has a lot to do with the decisions that people make farther upstream. Excess pollution produces algal blooms or hypoxic or ‘dead zones’ in a water body downstream, such as a reservoir or the ocean. The nitrate from the Mississippi River Basin is directly linked to the dead zone in the northern Gulf of Mexico.”
The researchers examined several watershed techniques to enhancing water quality, such as reducing agricultural runoff and adding wetlands and then calculated the financial implications of each. The researchers discovered that most strategies are ineffective because they rely on individual farmers’ voluntary involvement and are executed by a patchwork of different authorities.
“Currently, there’s individual management or conservation practices, and those include cover crop, high-precision fertilizer application, reduced tillage, constructed wetlands, and ravine tip management. Those are some of the different practices we considered,” Hansen said. “However, in the United States, non-point source management is voluntary through incentive programs, and the scale at which these conservation measures are frequently regarded is the individual farmer, whereas a coordinated approach is considerably more successful. In a way, it’s like a recycling program where you’re saying, ‘Anyone recycling one thing is better than no one recycling.’ True, any recycling is preferable to none, but a coordinated effort will save money and improve efficiency.”
Constructed wetlands are the most successful of these measures, according to Hansen and her co-authors, especially when the size and placement are examined at the scale of a watershed, or a whole region that drains into a shared river. Wetlands, according to the KU researcher, do two things well: they slow down water as it flows toward streams and rivers, and they contain plants and microorganisms that can digest fertilizer nutrients.
“Microbes and plants within wetlands are actually removing the nitrate from the water,” Hansen said. “When it comes to sediment, the fluvial wetlands hold water back during high flows, resulting in lower peak stream flows and a reduction in the quantity of near-channel material carried downstream.”
While Hansen’s study focuses on water quality, her co-authors from the University of Minnesota, the University of California-Irvine, and other schools throughout the country contributed a variety of views to the problem of enhancing agricultural water quality. A grant from the National Science Foundation aided the partnership.
According to Efi Foufoula, the project’s lead principal investigator from the University of California-Irvine, “This work would not have been possible without the diverse expertise and perspective of the team composed of hydrologists, ecologists, geomorphologists, biogeochemists, social scientists, and environmental economists.”
“We were able to take a fresh look at the problem because to the NSF’s ongoing assistance, which allowed us to collect substantial field data, develop new models, and interact with stakeholders. We hope that our results will affect policy and management as the clock ticks to meet the water quality targets of the state.”
Indeed, the economics of installing tiny, shallow fluvial wetlands and stabilizing ravines is a major emphasis of the current study. According to the investigators, such measures “were clearly more cost-effective than field management.” However, the researchers discovered that excellent wetland performance necessitated ideal location and that even cost-effective wetlands might be prohibitively expensive for a single farm or agency to implement.
A complete plan, according to the PNAS study, must treat an entire watershed as a system, integrating money from several programs and organizations and selecting sites for fluvial wetlands that would result in the largest decrease in nitrates and sediments reaching rivers.
According to research co-author Jacques Finlay, a professor in the University of Minnesota’s College of Biological Sciences, “This work shows that we can’t make real progress toward our goals for improving water quality in agricultural areas with more of a business-as-usual approach.” “Instead, conservation measures, as well as the investments that support them, can be more effective if they take into account the interactions that underpin the source of water pollution, as well as how alternative management approaches affect them.”
The Le Sueur River Basin was utilized as a proof-of-concept watershed, but the researchers think their findings may be extended to agricultural districts across the Midwest.