Scientists at the University of Illinois Urbana-Champaign have developed a new way to distinguish between human-driven and hydrological sources of riverine nitrogen pollution across the Upper Mississippi River Basin. The advancement, published in Environmental Science and Technology, could lead to more targeted policy and management approaches for nitrate and nitrite—nutrients that degrade drinking water quality and contribute to the oxygen-depleted “dead zones” that form each year in the Gulf of Mexico.
“This is the first study that distinguishes changes in nitrogen loss attributable to human activities, such as fertilizer application and farm conservation practices, from hydrological variability, such as extreme rainfall and changes in streamflow. It also shows us where the hotspots of nutrient loss are and which driver — human activity or hydrological variability — is more influential in each sub-watershed location,” said Bin Peng, assistant professor in the Department of Crop Sciences, part of the College of Agricultural, Consumer and Environmental Sciences at Illinois. “Our goal in segregating these drivers is to further tailor policy and management to reduce nitrogen loss.”
Peng and his co-authors analyzed two decades of water quality data collected from monitoring sites operated by the U.S. Geological Survey throughout the Upper Mississippi River Basin. Using those records, the researchers calculated annual nitrate and nitrite loads at each site. They then calibrated a modified version of the USGS SPARROW model—an analytical framework that links stream nutrient loads with watershed characteristics such as nutrient sources, land-to-water delivery factors, and in-stream processes. To separate the impacts of human behavior from natural hydrology, the team applied a factorial scenario analysis that attributed observed changes to each driver.
The study compared two five-year periods, 2001–2005 and 2016–2020, and found that nitrogen loss increased by nearly 10 kilograms per hectare per year on average over the 20-year span. Roughly half of that increase was linked to human activity, with the remainder tied to hydrological change. While those averages applied across the full basin, the researchers found substantial variation when they examined individual sub-watersheds.
“The northwestern part of the Upper Mississippi River Basin shows high contributions from both anthropogenic activities and hydrological changes, whereas the southeastern part of the basin has a higher contribution from hydrological change,” said Qianyu Zhao, first author of the study and a doctoral student in the Department of Natural Resources and Environmental Sciences (NRES) in ACES.
Those differences suggest that nutrient reduction strategies need to be customized based on dominant local drivers. According to Peng, the northwestern region would benefit from a dual focus on reducing fertilizer and manure inputs while also managing runoff associated with increased precipitation. In contrast, areas in the southeastern portion of the basin may see greater gains by prioritizing approaches that address hydrological variability.
The research team is now expanding its analysis to the entire Mississippi River Basin, with the aim of reducing nutrient losses in ways that protect farm profitability while minimizing downstream environmental impacts.
“Expanding this analysis to the whole Mississippi Basin will also help us build our new science-based and data-driven conservation prioritization framework, which will be of many practical uses to all stakeholders, including farmers, watershed managers, state and federal policy makers,” Peng said.
“This is a good example of how deeper scientific research at the Agroecosystem Sustainability Center (ASC) at Illinois can inform practices on the ground and environmental policies,” said Kaiyu Guan, Levenick Endowed Professor for Sustainability in NRES and founding director of the ASC. “Improving nutrient management and conservation planning for cleaner water has been one of our focuses since the very beginning.”
The study, “How do hydrological variability and human activities control the spatiotemporal changes of riverine nitrogen export in the Upper Mississippi River Basin?” is published in Environmental Science and Technology (DOI: 10.1021/acs.est.5c06476). Bin Peng and Kaiyu Guan are co-corresponding authors, and doctoral student Qianyu Zhao, co-supervised by Peng and Guan, is the first author.
Research in the College of ACES is supported in part by Hatch funding from USDA National Institute of Food and Agriculture. Additional support came from the National Science Foundation, the Illinois Nutrient Research and Education Council, the U.S. Department of Energy’s Center for Advanced Bioenergy and Bioproducts Innovation, competitive funding from USDA NIFA, and the Dudley Smith Initiative in the College of ACES at Illinois.
Peng and Guan are also affiliated with the Agroecosystem Sustainability Center, the Institute for Sustainability, Energy, and Environment, the Center for Advanced Bioenergy and Bioproducts Innovation, and the National Center for Supercomputing Applications at the University of Illinois.
The news writer of the press release is Lauren Quinn, ldquinn@illinois.edu .
