Nitrate, a chemical mixture of oxygen and nitrogen often used in farm fertilizers, can rise to toxic levels in surrounding water under very dry conditions. This relationship has traditionally been attributed to agricultural runoff or human waste in the environment, but new research led by scientists at Penn State shows that a hot and dry climate elevates concentrations of nitrate in rivers and streams, even in areas without urban or agricultural development.
Using artificial intelligence (AI) algorithms to overlay data patterns recorded from over 7,000 river sites around the world, the team built a machine learning model that can reliably map nitrate concentrations across regions with scarce data. Their research, which the team said is the first of its kind at a global scale, suggests that nitrate pollution is not driven by human activity alone - climate conditions also play a critical role. They published their findings today (Nov. 12) in Science.
"Right now, roughly a half billion people are living in proximity to rivers where nitrate concentrations exceed levels that can potentially lead to negative health effects," said Li Li, the Barry and Shirley Isett Professor of Environmental Engineering and corresponding author on the paper. "This finding implies that nitrate pollution is beyond a local management issue: it's a broader problem directly related to climate, which is becoming increasingly hotter and drier."
Nitrate is one of the most prevalent forms of inorganic pollution found in rivers, streams, lakes and groundwater worldwide, with about 18% of the U.S. population exposed to drinking water with elevated nitrate levels between 2021 and 2023. In addition to harming ecosystems, high levels of nitrate in drinking water can lead to a host of health conditions, most notably methemoglobinemia - commonly known as blue baby syndrome - a potentially fatal condition where an infant's blood cannot effectively carry oxygen.
"Generally, we think nitrate hotspots derive from human input like fertilizer used in agricultural or wastewater from cities," said Li, who holds an additional affiliation with Penn State's Institute of Energy and the Environment. "There's not a lot of very-large-scale water quality work, so people don't traditionally think about climate in terms of affecting nitrate levels. However, our new model is telling us that although human input plays a major role, long-term climate such as average annual temperature and average annual rainfall of an area contributes to environmental nitrate levels, as well."
