Small Wetlands, Big Benefit: How to Harness Nature to Filter Agriculture Runoff

A 12-Year Study on Constructed Wetlands

The results of this long-term study are in: Constructed wetlands are a cost-efficient, long-term, highly effective tool to reduce nutrients exiting fields that could otherwise affect local waterways. A relatively small wetland, around 6% of the tile-drained agricultural area, can reduce nitrogen by nearly 50%. If applied at scale, this approach could help address excess nutrients from farms throughout the Midwest. These types of wetlands are one of many "edge of field" and in-field practices that farms can deploy to support conservation. 

"The wetlands do work really well,” says Maria Lemke, TNC’s director of conservation science for Illinois and the lead scientist on the study. “They’re effective over the long term and relatively low maintenance.” Her team found that installing wetlands on the borders of fields reduced both nitrate nitrogen and dissolved phosphorus runoff, significantly improving water quality flowing into downstream stream and river systems.

How Can Wetlands Reduce Runoff?

In nature, wetlands are havens of biodiversity for wildlife. They also store carbon, and they help to keep an area’s hydrology in balance by wetlands filtering out pollutants, soaking up floodwater and releasing it gradually during dry periods, helping to keep the area’s hydrology in balance. Putting wetlands to work on farms is a practical extension of their function in nature. On the Franklin family farm, Lemke’s team built three sets of wetlands, each with a series of three wetland cells, and measured how much the nutrient load was reduced as the water flowed from one to the next.

At times, she said, “we saw the water coming in was carrying 20 milligrams per liter of nitrate, and going out it was 10 milligrams or less." Wetland plants absorbed some of the nutrients, and microbes converted some into nitrogen gas, an inert, non-greenhouse gas. Wetland plants can also sequester carbon from the air, helping to mitigate climate change.

An important focus of Lemke’s work was figuring out the optimal size of wetland for nutrient reduction. “The last thing we want to do is put in something bigger than you need, because this land is so productive, but we also don’t want to put in something too small to have effective conservation benefit,” she explains. The team compared wetlands that were scaled to 3%, 6%, and 9% of the cultivated area they drained.

“Even the smallest one showed  really good reduction of nitrate, about a 15% to 38% reduction, which is really great for these small wetlands,” Lemke shares. The wetlands also removed 53–81% of dissolved phosphorus. Larger wetlands removed more nutrients, but ultimately, Lemke concluded, the size of the wetland should depend on landowner preferences and goals for nutrient reduction.

The team worked with Dr. David Kovacic, an environmental science professor at the University of Illinois; the Natural Resources Conservation Service (NRCS); and the local Soil and Water Conservation District. A local contractor constructed the wetlands, and wetland plants colonized the wetlands on their own, either from seeds that had been in the soil or from wind dispersal. The wetlands were enrolled in the U.S. Department of Agriculture’s Conservation Reserve Program (CRP) and Environmental Quality Incentives Program (EQIP), and funding from the state Department of Natural Resources, NRCS, a federal Conservation Innovation Grant and in-kind assistance from AGREM, LLC, made the work possible.

Though the benefits of wetlands for water quality are clear, installing a wetland is still a difficult task that requires the services of an expert contractor, and the costs of design and construction can run into the tens of thousands depending on wetland size. Farmers can receive compensation for the expense through CRP, EQIP and other programs, but they often must pay out of pocket and wait for reimbursement. In addition, although putting a wetland on their property provides an enormous environmental service, landowners see few personal benefits from them.

“We need to make it easier to do this,” Lemke says. “Landowners need better incentives, such as 100% reimbursement of construction costs and 120% annual rental rates, because they’re providing a benefit to downstream users.” She notes that they could also benefit from technical assistance to apply for cost-share programs, manage the construction and keep up with minor maintenance of the wetlands over time.

Twelve Midwest states, including Illinois, participate in a compact to reduce nutrient loss to waterways. Adding more constructed wetlands to farms could help Illinois meet its goals for nutrient reduction. But the practice needs to be widely adopted to be effective. That’s where assistance to farmers and promotion of the practice by the state could make a difference.

“We have a long way to go,” Franklin says, “but farmers have an incentive to do this, to become part of the solution in reducing nutrient runoff. Everyone lives downstream from someone."