Constructed Wetlands are Best Protection for Agricultural Runoff into Waterways

A new paper from a lead author based at the University of Kansas finds wetlands constructed along waterways are the most cost-effective way to reduce nitrate and sediment loads in large streams and rivers. Rather than focusing on individual farms, the research suggests conservation efforts using wetlands should be implemented at the watershed scale.

The paper, just 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 lead author Amy Hansen, assistant professor of civil, environmental & architectural engineering 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. Excess pollution goes to a water body downstream like a reservoir or the ocean and causes algal blooms or hypoxic or 'dead zones.' The dead zone in the northern Gulf of Mexico is directly correlated with nitrate that comes from the Mississippi River Basin."

The research team compared potential watershed approaches to improving water quality, such as cutting runoff from farms and adding wetlands, then gauged the economic costs of each. Because most methods rely on voluntary participation by individual farms and are implemented by a patchwork of different agencies, the researchers found they're less effective.

"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. "But management of non-point sources is voluntary in the U.S. through incentive programs, and the scale these conservation practices are often considered at is the individual farmer when a coordinated approach is much more effective. In a way, it's like a recycling program where you're saying, 'Anyone recycling one thing is better than no one recycling.' This is true, some recycling is better than no recycling, but a coordinated approach will save money and be more effective."