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Research

Nitrate Levels Continue to Increase in the Mississippi River; Signs of Progress in the Illinois River



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Nitrate levels in the Illinois River decreased by 21 percent between 2000 and 2010, marking the first time substantial, multiyear decreases in nitrate have been observed in the Mississippi River Basin since 1980, according to a U.S. Geological Survey (USGS) study published in October 2013 (“Nitrate in the Mississippi River and Its Tributaries, 1980–2010: An Update”).

Unfortunately, similar signs of progress were not widespread. “Nitrate levels continue to increase in the Missouri and Mississippi Rivers, including the Mississippi’s outlet to the Gulf of Mexico,” said Lori Sprague, USGS research hydrologist and coauthor of the October 2013 report.

USGS personnel collecting a water-quality sample at the Missouri River
Above: USGS personnel collecting a water-quality sample at the Missouri River at Hermann, Missouri, in July 2011. USGS photograph by Kelly Brady. [larger version]

Excessive nitrate and other nutrients from the Mississippi River strongly influence the extent and severity of the hypoxic zone that forms in the northern Gulf of Mexico every summer. This hypoxic zone, known as the “dead zone,” is characterized by extremely low oxygen levels in bottom or near-bottom waters, degraded water quality, and impaired marine life. Dead zones occur in many areas (for example, see “A Research Cruise to Investigate Natural Versus Human Impacts on Marine Ecosystems in Hood Canal, Washington,” Sound Waves, November/December 2013), but the Gulf hypoxic zone is the largest in the United States. In 2013 it encompassed 5,840 square miles, an area the size of Connecticut.

“The USGS results show that solving the problem of the dead zone will not be easy or quick. We will need to work together with our Federal and State partners to develop strategies to address nitrate concentrations in both groundwater and surface water,” said Lori Caramanian, Department of the Interior Deputy Assistant Secretary for Water and Science. 

“Expanded research and monitoring are absolutely essential to tracking progress in reducing nutrient pollution in the Mississippi River Basin,” said Nancy Stoner, acting Assistant Administrator for Water at the U.S. Environmental Protection Agency and co-chair of the Mississippi River/Gulf of Mexico Watershed Nutrient (Hypoxia) Task Force. “The Federal agencies and States that are part of the Hypoxia Task Force greatly appreciate this work by USGS and how it advances the science in the Mississippi River Basin.”

The reasons for increases or declines in annual nitrate levels are unknown. Reliable information on trends in contributing factors, such as fertilizer use, livestock waste, agricultural-management practices, and wastewater-treatment improvements, is needed to better understand what is causing increases or decreases in nitrate.

Nitrate trends from 1980 to 2010 were determined at eight long-term USGS monitoring sites in the Mississippi River Basin—including four major tributaries (Iowa, Illinois, Ohio, and Missouri rivers) and four locations along the Mississippi River—using methodology that adjusts for year-to-year variability in streamflow conditions (see map). 

Mississippi River drainage in the central United States, showing locations of the eight permanent nitrate-monitoring stations
Above: Mississippi River drainage in the central United States, showing locations of the eight permanent nitrate-monitoring stations (black dots). From “Nitrate in the Mississippi River and its tributaries, 1980 to 2010: Are we making progress?. [larger version]

Key Trends in Nitrate Concentration at Long-Term USGS Monitoring Sites

(From “Nitrate in the Mississippi River and its tributaries from 1980 to 2010: Are we making progress?”)

  • Nitrate concentrations steadily decreased by 21 percent in the Illinois River from 2000 to 2010. Decreases were also noted in the Iowa River during this time, but the declines were not as large (10 percent).
  • Consistent increases in nitrate concentrations occurred between 2000 and 2010 in the upper Mississippi River (29 percent) and the Missouri River (43 percent).
  • Nitrate concentrations in the Ohio River are the lowest among the eight Mississippi River Basin sites and have remained relatively stable over the past 30 years.
  • Nitrate concentrations increased at the Mississippi River outlet by 12 percent between 2000 and 2010. 
  • Nitrate increased at low streamflows throughout the basin, except for the Ohio and Illinois Rivers. Groundwater is likely the dominant source of nitrate during low flows. It may take decades for nitrate to move from the land where it was applied, migrate through groundwater, and eventually flow into these rivers. Because of this lag time, several years or decades can elapse before the full water-quality effects of either increased groundwater contamination or improved management practices are evident in the rivers. 

The USGS report and additional information on nitrate trends in concentration and flux for each of the eight sites are available online. More information about USGS long-term monitoring sites in the Mississippi River Basin is available at the USGS National Water-Quality Assessment (NAWQA) program’s website.

Related USGS Nitrate News

The USGS recently began using optical sensors in the Mississippi River Basin to track the movement and quantity of nitrate delivered from small streams all the way to the Gulf of Mexico in real time (see USGS news release “Real-time Monitoring Pays Off for Tracking Nitrate Pulse in Mississippi River Basin to the Gulf of Mexico”). Optical data collected every 15 minutes to 3 hours at the mouth of the Mississippi and on several major tributaries will complement the long-term monitoring described above, providing researchers with new insights into the storage and transport of nitrate and improving agencies’ ability to measure the effectiveness of management actions.

At the other end of the spectrum, new research by USGS coastal and marine scientists shows that chemical analyses of long-lived deep-sea corals can provide information about the movement of nitrogen and other elements from the Mississippi River Basin into the Gulf of Mexico over timescales stretching back hundreds to thousands of years (see “Deep-Corals Record Human Impact on Watershed Quality in the Mississippi River Basin,” this issue).


Related Sound Waves Stories
Deep-Corals Record Human Impact on Watershed Quality in the Mississippi River Basin
Jan. / Feb. 2014
A Research Cruise to Investigate Natural Versus Human Impacts on Marine Ecosystems in Hood Canal, Washington
Nov. / Dec. 2013
Scientists Go Deep to Track Algae-Feeding Nitrogen in Washington State's Hood Canal
July 2006

Related Websites
Nitrate in the Mississippi River and its tributaries, 1980 to 2010: Are we making progress?
USGS
National Water-Quality Assessment (NAWQA) Program
USGS
Real-time Monitoring Pays Off for Tracking Nitrate Pulse in Mississippi River Basin to the Gulf of Mexico
USGS
Mississippi River/Gulf of Mexico Watershed Nutrient (Hypoxia) Task Force
Hypoxia Task Force

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in this issue:

Spotlight on Sandy
cover story:
Decade of Fire Island Research Available

Using Scenarios to Improve Resilience to Major Storms

USGS Deploys Oceanographic Gear Offshore of Fire Island

Research New Geologic Explanation for the Florida Middle Ground

Deep-Sea Corals Record Human Impact on Mississippi River Basin

Nitrate Levels in the Mississippi River, Illinois River

USGS Scientist Examines Foraminifera Collected from Remote Clipperton Island

Outreach
3rd Annual St. Petersburg (Florida) Science Festival

Awards
Deepwater Canyon Study Given Prestigious DOI Award

Staff
Barbara Lidz Retires after Long Career with the USGS in Florida

Publications Jan. / Feb. Publications

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