New research from the U.S. Geological Survey shows nutrient pollution increased in many streams and rivers across the country between 2000 and 2020. The dominant source was the release of old nitrogen and phosphorus stored in soil and groundwater.
“One of the key findings is that a lot of the nutrients that are in rivers today comes from past activities,” said Olivia Miller, a USGS research hydrologist and lead author of the study. “Even when improvements are made in terms of reducing nutrient sources, water quality can take time to recover.”
Nitrogen and phosphorus cycle through the environment and are essential for life.
But excess amounts in waterways feed algal blooms, which deplete oxygen when they decompose, and contribute to the massive “dead zone” in the Gulf of Mexico. This low-oxygen area expands along the Louisiana coast each summer, suffocating bottom-dwelling creatures and pushing fish and shrimp further out.
Disruptions to fishing and spoiled beach vacations affect the seafood and tourism industries. Elevated nitrate concentrations and algal toxins also force drinking water utilities and private well-owners to invest in new technologies or look for alternative sources of water.
The USGS study aimed to understand when and how nutrients move through landscapes and river systems. Researchers looked at two sources of pollution: what scientists call "point" and "non-point" sources.
“A point source is something where you have a clear discharge point, like a pipe,” said David Cwiertny, director of the Center for Health Effects of Environmental Contamination at the University of Iowa. He was not involved in the USGS study.
Point-source pollution includes waste from sewage treatment facilities and factories.
“We've done a really good job of controlling those point sources through essentially a regulatory and permitting process,” Cwiertny said. “The diffuse sources, what we call non-point source, where things run off land, that is a much more challenging source to manage.”
In Iowa, roughly 93% of the total nitrogen in the state’s waterways come from non-point sources.
Some stems from the natural breakdown of carbon-rich portions of the soil, nitrogen-fixing plants and atmospheric deposition. But researchers say much of the excess in Iowa’s waterways comes from fertilizer and manure applied to corn and soybean fields, paired with a lack of year-round groundcover on a landscape once dominated by tallgrass prairie.
Subsurface tile drainage, which moves water from farm fields into ditches or streams, further complicates nutrient management across the Corn Belt.
Filling in the gaps
USGS has an extensive monitoring network across the U.S., which tracks water quality and quantity, including streamflow, groundwater levels, temperature and contaminants.
The network uses real-time monitoring with sensors in some places, Miller said. USGS staff also go out to collect samples. But the cost and resources required make it impossible to monitor everywhere, she added.
“Models help us fill in those gaps,” Miller said.
The USGS researchers developed models to estimate seasonal nutrient loads to streams across the contiguous U.S. between 2000 and 2020.
The models incorporated data from the agency’s monitoring network with land use, hydrology, soil types, weather and other factors.
Based on their results, the USGS researchers estimate between one-third and half of the nutrients in streams came from “lagged, non-point sources.” This means it took longer than one season for nitrogen or phosphorus to move from soil or groundwater into a waterway.
Lagged nonpoint sources of nitrogen and phosphorus contributed 48% and 35% to the loads in streams, respectively.
The second largest source in streams was fertilizer applied in the same season. Treated wastewater played a smaller role but was the dominant source of total phosphorus in streams in the eastern U.S.
Changing stream flow also contributed to higher nutrient loads, Miller said.
“In the Midwest, for example, it got a lot wetter over our study period, and that wetting, moves more nutrients into streams,” Miller said.
The researchers found the largest influxes of nitrogen and phosphorus in streams in the Midwest and other regions of the U.S. happened in spring.
Soil and groundwater can hold nutrients for months or years before they flush out into streams and rivers, Miller said.
“Some groundwater can take thousands of years to move from the place … where it enters the subsurface to where it goes into a river,” Miller said. “If you have nutrients moving in groundwater, that could be really, really slow.”
Different paces of nitrogen molecules moving through the landscape are similar to workers leaving the office at the end of the day, she explained.
“Everyone will get home at a different time,” Miller said. “Some people will go to the grocery store. Some people might walk or ride their bike, and some people drive.”
The USGS researchers said their models help “pinpoint when, where, and why nutrients are entering rivers” and inform management decisions to shape future water quality.
Nutrient Reduction Strategies
Thirty states have published nutrient reduction strategies, according to the U.S. Environmental Protection Agency. They identify significant contributors of nitrogen and phosphorus and provide a roadmap for minimizing those impacts to surface waters.
Some of the states’ strategies are connected to the Hypoxia Task Force’s goal of shrinking the “dead zone” – or hypoxic zone – in the Gulf of Mexico by curtailing pollution in the Mississippi and Atchafalaya river basins.
Earlier this summer, the task force announced progress meeting voluntary, interim targets. Data from 2025 showed total nitrogen loads to the Gulf decreased by 28% compared to the baseline. But total phosphorus loads increased by 13%.
The U.S. Environmental Protection Agency pointed to newer research that explains how phosphorus can accumulate over months or years before showing up downstream.
“As a result, the benefits of today's reductions may take years to register in monitoring data,” the EPA press release stated.
Cassandra Glaspie, a professor at Louisiana State University, runs the annual cruise that maps the seasonal hypoxic zone in the Gulf.
During a recent webinar with environmental organization Izaak Walton League of America, Glaspie emphasized the mapping is a “snapshot.” The hypoxic zone changes in size throughout the summer, and storms can mix up the low-oxygen bottom layer.
The measurement this year was roughly 1,332 square miles, making the hypoxic zone the second smallest on record in 40 years, according to a NOAA announcement Aug. 5. The press release said it was smaller than anticipated due to Tropical Storm Bertha, which passed through the Gulf on the eve of the cruise.
Still, mapping the zone helps shrimpers plan and prepare, Glaspie said. It also provides “a finger on the pulse of the nitrate problem.”
“Even though we’re far away from the Midwest, this is your metric, too,” she said.
This story was produced in partnership with Harvest Public Media, a collaboration of public media newsrooms in the Midwest and Great Plains. It reports on food systems, agriculture and rural issues.
