Riverside wetlands stop nitrates and fertilisers seeping into waterways

Excessive nitrate or sediment levels can affect local fish populations and increase the cost to treat drinking water. The pollutants also find their way into water bodies downstream like a reservoir or the ocean and create algal blooms or hypoxic or ‘dead zones.’ According to the researchers, the dead zone in the northern Gulf of Mexico is directly correlated with nitrate that comes from the Mississippi River Basin.

They compared different 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 that they tend to be less effective.

“Currently, there’s individual management or conservation practices, and those include cover crop, high-precision fertiliser application, reduced tillage, constructed wetlands and ravine tip management. Those are some of the different practices we considered,” said lead author Dr Amy Hansen. “But management of non-point sources is voluntary in the US 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.”

The team found that constructed wetlands are the most effective of these practices, especially if the size and location are evaluated at the scale of a watershed—an entire region that drains into a common waterway. This is because wetlands both slow down water as it heads toward streams and rivers and contains vegetation and microbes that can process nutrients used as fertiliser on crops.

“Microbes and plants within wetlands are actually removing the nitrate from the water,” Hansen said. “With sediment, on the other hand, what the fluvial wetlands are doing is holding water back during these high flows—and by holding that water back you’re getting lower peak stream flows, which is reducing the amount of near channel sediment that’s being transported downstream.”

The researchers used the Le Sueur River Basin in Southern Minnesota as a proof-of-concept watershed, but say their findings could be applied to agricultural regions throughout the Midwest US.

E&T has previously looked at the increasing popularity of wetland restoration projects as a way to mitigate climate change and create a home for different types of wildlife.