Wetlands efficiently remove nitrogen pollution from surface water, leading to cost savings for municipalities

(Press-News.org) URBANA, Ill. – Wetlands are an important part of the ecological system, providing a myriad of benefits for people, wildlife, and the environment. They also serve as “nature’s kidneys,” filtering out pollutants from surface water. A new study from the University of Illinois Urbana-Champaign finds that wetlands along the Mississippi River Basin effectively clean up nitrogen runoff from agricultural fields. The researchers also show this can lead to significant savings for local drinking water treatment facilities.

Non-point source pollution from agricultural production is a major problem for water quality in the U.S. Nutrient runoff can lead to algal blooms, hypoxia zones, and contaminated drinking water.

“Many of our solutions to reduce excess nutrients focus on the stage before the nutrients reach the surface water system. However, wetlands can intervene after the nutrients have already entered the system and help with cleanup and removal,” said study co-author Marin Skidmore, assistant professor in the Department of Agricultural and Consumer Economics, part of the College of Agricultural, Consumer and Environmental Sciences at U. of I.

“Wetlands facilitate the nitrogen cycle and can help nitrogen undergo denitrification, releasing it as N2, a harmless gas that is abundant in Earth’s atmosphere,” she explained. 

The study focuses on the Agricultural Conservation Easement Program (formerly the Wetland Reserve Program), which is among the USDA’s smaller conservation initiatives. In these programs, landowners enter a long-term contract to retire a portion of their land from farming. The farmer receives a per-acre payment, and USDA Natural Resources Conservation Service staff converts the area to a wetland.

“Wetlands have a great impact on water quality, but most studies have focused on single wetlands or watersheds. Our study complements existing work by using large-scale data to estimate the average effect over time,” said co-author Nicole Karwowski, agricultural economist at Montana State University.

Skidmore and Karwowski studied the effect of wetland easements along the Mississippi River Basin from program inception in 1990 until 2018. At the end of this period, there were about 9,000 permanent easements, accounting for approximately 0.22% of land in the region. They included water quality data at the sub-watershed level (smaller units within a watershed), evaluating changes in water quality trends over time. They also looked at data about weather patterns and seasonality to more accurately capture the aggregated effects of the easement programs.

Their analysis focused on three main nutrients: phosphorus, ammonia, and total Kjeldahl nitrogen (TKN), which measures the combination of ammonia and organic nitrogen. 

They found that initial wetland restoration reduced ammonia concentration by 62% (0.08 milligrams per liter), while TKN decreased by 37% (0.20 milligrams per liter), and effects increased with additional wetland areas. It took, on average, 3 years after restoration for these water quality improvements to materialize, but the benefits are likely to last for longer than a decade.

“These are very meaningful results considering how difficult it is to meet the goals Illinois and neighboring states have set for nitrogen levels in the Mississippi River Basin in the Nutrient Loss Reduction Strategy and similar plans,” Skidmore said.  

The study did not find significant long-term impacts on local phosphorus levels, although there were some downstream reductions.

The researchers also considered the presence of cropland within the watershed, which affects the amount of nutrient runoff.

“We might be concerned that these wetlands can be overrun by too many nutrients, so they can’t filter them effectively. But we’re finding a large impact in areas with lots of nutrients. Thus, wetlands are not only able to filter average waters in the Mississippi River Basin; they're very good at doing it in places that have the biggest problems with nutrients,” Karwowski said.

Finally, they calculated the potential savings for local water treatment facilities.

The Safe Drinking Water Act requires that drinking water provided through public water systems meets certain requirements, such as a maximum level of nitrates to protect against detrimental effects on human health. Local water systems pay treatment costs and may need to invest in costly technology to meet the standards, especially as the nitrogen levels in the surface water increase.

The researchers estimated that restoring 100 acres of wetland in a sub watershed can potentially decrease treatment costs for a large public water system by up to $17,000 per year. The initial wetland investments may pay off within 20-30 years, and these wetlands can potentially generate annual benefits of $200 million.

“The wetland easement programs are essentially a cost transfer from the federal government, who is paying for the restoration, to local communities. And the savings at the local level match the spending at the federal level in communities with nitrogen concerns,” Skidmore said.

The findings are particularly relevant in light of a recent Supreme Court decision stating that wetlands must be adjacent to navigable waters, such as a stream or river, to be protected under the Clean Water Act. This ruling affects 72% of wetlands in Illinois, making it more likely they will be used for development, and communities will lose the many benefits they provide.

In future studies, Skidmore and Karwowski plan to look at whether proximity to a stream or river affects how effectively wetlands remove pollutants. Early results indicate that even when the wetlands are not directly adjacent to water bodies, they still provide large benefits for water quality.

“Hopefully our results can help policymakers make decisions about wetlands and whether they should be protected. Wetlands can work in tandem with other nutrient reduction strategies, such as cover crops, buffer strips, and more efficient nutrient use. Different conservation practices complement each other and having them work together is a good path forward for agricultural regions,” Skidmore concluded.

The paper, “Nature’s Kidneys: the Role of Wetland Reserve Easements in Restoring Water Quality” is published in the Journal of the Association of Environmental and Resource Economists [DOI: 10.1086/739287]. Research in the College of ACES is made possible in part by Hatch funding from USDA’s National Institute of Food and Agriculture. Additional funding was provided by the National Great Rivers Research and Education Center.

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