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.