US Geological Survey study shows rising nutrient loads in rivers

A study from USGS shows nutrient pollution in U.S. rivers can spend a season or longer on the landscape before it reaches a stream. Pictured is the Mississippi River along Dubuque. (Photo by Perry Beeman/Iowa Capital Dispatch)

New modeling from the U.S. Geological Survey shows nutrient loads in rivers across the nation increased between 2000 and 2020, largely due to nutrients stored in the landscape from past seasons and current season agriculture nutrient applications. 

The research, according to USGS, shows why “water‑quality improvements often lag behind management actions” and emphasizes the importance of addressing legacy nutrients and non-point source pollutants. 

According to the research, “lagged” non-point sources – those that take longer than one season to appear in a stream — were the largest contributors of total nitrogen and total phosphorus to streams. Non-point sources include agricultural, atmospheric deposition, urban, natural and septic sources. 

Non-point sources with lagged delivery contributed 48% of total nitrogen to streams and 35% of total phosphorus, according to the study. Fertilizer that was applied and ended up in a stream in the same season was the second-largest contributor, at 18% of total nitrogen and 22% total phosphorus. Treated wastewater was the next highest nutrient source, contributing 10% of total nitrogen and 20% of total phosphorus. 

Olivia Miller, a hydrologist with USGS and the lead author on the study, said nutrients increased in stream sources during the study period, in part because it “got a lot wetter” over the 20-year period. 

“There’s a lot more rain, and so that kind of washed more nutrients into rivers and then higher stream flow, so you just get more movement of nutrients in those streams,” Miller said.

The study said the Midwest “dominated” the total nutrients delivered to streams. A graphic from the study shows lagged sources and fertilizer as the dominant nutrient contributors in the region. The East also had high amounts of total nitrogen and phosphorus, though treated wastewater and lagged sources made up the majority of the contributions in that region. 

A series of graphs show the contributions from major sources of total nitrogen and total phosphorus over time in U.S. regions. (Graph courtesy of USGS)

Miller said the determination of a lagged versus current season source is “baked” into the SPAtially Referenced Regression On Watershed, or SPARROW, model. The model accounts for fertilizer application rates and nutrient loads in the rivers to determine how much nitrogen or phosphorus came from the current application season, versus nutrient that was already on the land. 

While lagged non-point sources dominated the nutrient contributions, the study said “large” increases in fertilizer application over the 2000-2020 study period in the Midwest “more than offset” the nutrient declines from things like atmospheric deposition and treated wastewater, “resulting in overall increased total loading to streams, particularly in the Midwest.” 

Miller said that with the increased precipitation during the period, some areas in the Midwest saw overall concentrations of nutrients in the rivers decrease, even while the nutrient loads increased. But other areas had an increase both in load and concentration. 

An analysis of nutrient sources during different seasons showed that fertilizer was the dominant source of nutrient pollution, especially in the Midwest, during spring. More nutrients in the water can cause algal blooms, threaten water biodiversity and challenge water distribution systems.

The study also showed that rivers in the Midwest and East were in poor condition, due to total nitrogen and total phosphorus concentrations that exceed U.S. Environmental Protection Agency criteria, for the majority of the 20-year study period.

A USGS study shows widespread areas with “poor” water quality conditions due to excessive total nitrogen and total phosphorus. The closer to dark pink the line appears, the greater percentage of the time from 2000-2020 the region exceeded the U.S. Environmental Protection Agency criteria for poor condition. (Graphic courtesy of USGS)

Iowa, like most states in the Mississippi River Basin, is part of a task force dedicated to shrinking the size of the hypoxia, or dead zone, in the Gulf of Mexico by reducing the nutrient runoff that ends up in the Mississippi River. 

Per its Nutrient Reduction Strategy, Iowa is supposed to reduce its total nitrogen from non-point sources by 41% and total phosphorus from non-point sources by 29%, based on nutrient levels from 1980-1996. The strategy promotes the voluntary adoption of practices like cover cropping, reduced tillage and more strategic fertilizer application.

Miller said the USGS study shows that past activities “continue to impact” water quality. While USGS doesn’t make policy recommendations, she said she hopes the study can “help target solutions a little bit more effectively” so that water resource managers can focus their efforts where it will have the biggest impact.

“Clean water – it’s really shaped by what happened in the past, and current actions,” Miller said. “What we do today can impact future conditions for water quality and nutrients.” 

Miller said the data will soon be available on a USGS interactive data server to more readily show collected information on specific rivers and regions.