The U.S. Geological Survey last week reported substantial decreases in nitrate levels in the Illinois River over the past several years, though some outlets along the Mississippi River basin did not test as well.
"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, even though nitrate levels in the Illinois River decreased by 21% between 2000 and 2010.
The new numbers are part of an update to a multi-year study of USGS long-term nitrate monitoring sites. USGS has been monitoring the sites for nitrate trends since 1980 as part of the SGS National Water-Quality Assessment program.
USGS says nitrates in the water can contribute to a "dead zone" that forms in the northern Gulf of Mexico every summer. The zone, which is characterized by low oxygen levels in the bottom or near-bottom waters, USGS says, impairs aquatic life.
The 2013 Gulf hypoxic zone encompassed 5,840 square miles, an area the size of Connecticut, USGS says.
According to the study, in addition to the Illinois River decreases, nitrate concentrations steadily decreased by about 10% from 2000 to 2010 in the Iowa River. Nitrate concentrations in the Ohio River are the lowest among the eight Mississippi River Basin sites and have remained relatively stable over the last 30 years.
However, consistent increases in nitrate concentrations (29% and 43%, respectively) occurred between 2000 and 2010 in the upper Mississippi River and the Missouri River. Nitrate concentrations increased at the Mississippi River outlet by 12% between 2000 and 2010.
Nitrate increased at low streamflows throughout the basin, except for the Ohio and Illinois Rivers.
"These 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.
USGS says groundwater is likely the dominant source of nitrate during low flows, and it may take several years before full water-quality effects of either increased groundwater contamination, or of improved management practices, are evident in the rivers.
The permanent nitrate monitoring stations include: Mississippi River at Clinton, Iowa; Iowa River at Wapello, Iowa; Illinois River at Valley City, Ill.; Mississippi River below Grafton, Ill.; Missouri River at Hermann, Mo.; Mississippi River at Thebes, Ill.; Ohio River near Grand Chain, Ill.; and Mississippi River above Old River Outflow Channel, La.
The USDA in recent years has pledged to assist farmers in mitigating nutrient problems by constructing wetlands and helping farmers employ cover crops and other conservation methods.
In August, USDA's Natural Resources Conservation Service estimated that controlling erosion and managing nutrients has reduced the edge-of-field losses of sediment by 35%, nitrogen by 21% and phosphorous by 52%
The report, part of USDA's Conservation Effects Assessment Project, or CEAP, also found that conservation practices have resulted in an estimated 17% reduction in nitrogen and 22% reduction in phosphorus entering the Gulf of Mexico annually.
USDA said a key takeaway from the study was the importance of the appropriate rate, form, timing and method of application for nitrogen and phosphorous.
To read the full USGS report, click here.