Federal scientists report latest monitoring data and highlight challenges.
U.S. federal scientists say water quality has declined in the massive Mississippi River Basin in recent years due to the combined effects of agricultural and urban infrastructure, despite decades of conservation efforts. That's a concern both for those who rely on the river system and for those downstream on the Gulf of Mexico, where a huge "dead zone" hurts fishing and recreational opportunities.
Scientists with the U.S. Geological Survey (USGS) sampled the main stem and four tributaries of the Mississippi River and found that levels of nitrate increased at more than half the sites from 1980 to 2010. Overall, nitrate levels increased by 14 percent during that period, the USGS reported in a congressional briefing on Capitol Hill Friday.
The new findings are a warning about water quality health nationwide, and raise a troubling issue: even when policymakers and environmental advocates try to clean up the waterways, their efforts are not always successful.
Scientists often focus on levels of nitrate because it plays an important role in the environment. The nitrogen in nitrate is an essential nutrient for plants. But too much of it leads to overgrowths of algae, called blooms, which can use up too much oxygen in the water (a process called eutrophication), choke out fish and seagrasses, and in some cases release toxic chemicals.
Large algal blooms have created dead zones in Puget Sound, the Chesapeake Bay, Long Island Sound, and many other waterways. The Gulf of Mexico is now host to the world's second largest dead zone, a patch of impaired sea about the size of New Jersey. (Learn more aboutriver basins.)
"When oxygen gets too low, most fish can't live there," says Nancy Rabalais of the Louisiana Universities Marine Consortium, who studies the dead zone. "They'll swim away if they can, but if they are associated with the bottom they can't." She points to declines in shrimp catches and charter fishing trips in the area and an estimated loss of $82 million a year.
The Mississippi Basin is a good representative of the rest of the country because it has mixed urban and agricultural areas, and because "many lessons learned there can be applied throughout the U.S.," said Lori Sprague, a hydrologist with USGS's National Water-Quality Assessment Program, at the hearing.
Across the country, more than half of the streams and rivers are in "impaired condition," meaning swimming and fishing are not advisable, reports the Environmental Protection Agency.
Rising Tide of Nitrates
Sprague said that although data from decades ago are limited, available numbers show that since the 1940s, nitrate levels skyrocketed in the Mississippi Basin, from less than one million tons of input to about 13 million tons by the mid 2000s. The biggest sources are industrial fertilizers (41 percent) and animal manure (10 percent), followed by urban areas (7 percent), wastewater treatment (7 percent), and other sources.
The decades since the 1940s represent the maturation of the "Green Revolution," when agricultural productivity in the basin increased 400 percent, says Colin Wellenkamp, the director of the Mississippi River Cities and Towns Initiative for the Northeast-Midwest Institute in Washington, D.C.
Studies now show that about 16 percent of the nitrate in fertilizer is not taken up by the crops to which it is applied, Sprague said at the hearing. Instead, it runs off fields and makes its way into ground or surface waters.
Fertilizer use on farms began to plateau around 1980, and over recent years many farmers have been applying smaller amounts of chemicals, thanks to stronger formulations and the rise of "precision agriculture," in which more targeted applications are made. The same trend has persisted on many golf courses and other landscaping efforts.
Still, from 1980 to 2010, the biggest gains in nitrates were seen in the upper Mississippi and Missouri River Basins, especially in Minnesota and Wisconsin, which saw increases of 50 percent. Parts of Illinois and Iowa saw decreases in nitrates during that period.
Sprague said the data are unable to show exactly what effects are responsible for the changing nitrate levels. Modeling shows that conservation measures decrease nitrate levels, but in the real world, many factors are at play, including population growth, urban sprawl, changes in livestock operations, switching crops from soy to corn (which uses more fertilizers), and conversion of marginal land to cropland to support a booming corn ethanol market.
Written By: Brian Clark Howard
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