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Editor's Note: Mississippi's Natural Filtration May be at Capacity

Jamey Dunn
mattpenning.com 2014
WUIS/Illinois Issues

A new study indicates that the naturally occurring filtration systems in the Mississippi River are being overwhelmed by the amount of nitrogen going into the water. 

The river’s natural filtration system is remarkable but pushed to its limits. Research from hydrogeologists at the University of Texas at Austin found that practically every drop of water that travels through the 311,000 miles of the Mississippi network is filtered before it reaches the Gulf of Mexico. The study found that 99.6 percent of the water is filtered through sediment at the banks of creeks, streams and rivers. Microbes in the sand, gravel and mud suck up compounds such as oxygen and nitrates, in the water. 

While all that filtration seems like a good thing, researchers say their findings indicate that the river’s self-cleaning system may be at capacity. Despite the filtration, nitrates are making their way into the Gulf of Mexico where they deplete the water of oxygen, creating a dead zone that is on average roughly the size of Connecticut. “As a result of its filtration systems being overwhelmed, the river system operates less as a buffer and more as a conveyor belt, transporting nitrates to the Gulf of Mexico. The amount of nitrates flowing into the gulf from the Mississippi has already created the world’s second biggest dead zone, an oxygen-depleted area where fish and other aquatic life can’t survive,” said a news release from the University of Texas at Austin. 

The nitrates come from several sources, including manure used as fertilizer, runoff from urban areas and wastewater treatment. However, the biggest source for nitrates in the Mississippi River is industrial fertilizer used in commercial farming. The dead zone does fluctuate in size over the seasons and is largest in the summer when nitrate runoff is at its peak.

One problem that researchers discovered was that much of the water is not remaining in sediment long enough to have the nitrates filtered out. They found that only about 24 percent of the water was spending enough time cycling through the sediment to be filtered. “Clearly for all this nitrate to make it downstream tells us that this system is very overwhelmed,” Bayani Cardenas, a professor of hydrogeology at the university’s Jackson School of Geosciences, said in a prepared statement. Cardenas and doctoral candidate Brian Kiel looked at the Mississippi River network, which includes the watersheds of the Ohio River and Missouri River. 

The changes humans have made to the river system to bend it to our needs have also reduced its ability to clean itself. Aaron Packman, a professor in the Department of Civil and Environmental Engineering at Northwestern University in?Evanston, says that the drainage systems built by humans are designed to carry water away rapidly, but the water is better filtered if it takes longer to travel through twists and turns and sometimes even makes its way into wetlands. Humans have built systems to ensure quick drainage of farmland and cities, altered rivers to make them more easily navigable and filled in wetlands for development. All of this means that water makes its way off of fields, into the river and down to the gulf more quickly. Packman says that one way to tackle the nitrogen problem would be to restore natural systems, such as wetlands, that slow down the water, allowing more nitrogen to be neutralized.

Some Illinois farmers are working to cut down on the amount of nitrates that are flushed from their fields into the water systems. “People spend a lot money on fertilizers. Do you think they want them down in the gulf? No. They pay a lot of money for that,” says Paige Buck, spokeswoman for the Illinois Natural Resources Conservation Service. The organization is part of the U.S. Department of Agriculture and assists farmers with conservation efforts. Buck says many Illinois farmers are reducing runoff by planting cover crops and tilling fields less often. “Cover crops help hold on to that nitrogen — hold it in the soils, so it’s not going anywhere. Even in the winter, it just stays stuck there.” 

The service also works to educate farmers on how to keep their soil healthy and full of beneficial bacteria and microbes. Buck says healthy soil hangs on to water and nitrogen and requires less fertilizing. She says that keeping soil healthy helps out the environment as well as farmers. “They see the benefit. They see it in their yields. They see it in what happens when it rains. They are not having as much runoff.” This anecdotal evidence proves out in scientific research. A recent cover crop experiment at Iowa State University saw bigger corn yields while reducing the use of herbicide by 95 percent and cutting nitrogen use by 90 percent.

While these efforts contribute to reducing the nitrates going into the water system, the 5,800 square mile dead zone in the Gulf of Mexico attests that they are not being used widely enough at this point. A recent report from Ceres, a nonprofit organization that seeks to bring business leaders and environmentalists together to make industry more sustainable, found that $420 million worth of fertilizer washed off fields into the Mississippi and eventually into the Gulf in 2013. 

The U.S. Environmental Protection Agency’s Mississippi River/Gulf of Mexico Watershed Nutrient Task Force recently partnered with a dozen public universities, including the University of Illinois, to create state-level strategies to address water pollution. The group’s goal is to reduce the dead zone. They have their work cut out for them as a recent forecast predicts the zone will be at its largest this summer. The forecast, developed by the University of Michigan and Louisiana State University in cooperation with U.S. National Oceanic and Atmospheric Administration, estimates the dead zone could be between 7,286 and 8,561 square miles, or about the size of New Jersey. The forecast is created using runoff data collected by the U.S. Geological Survey. The largest gulf dead zone reported to date occurred in 2002, when it was 8,481 square miles.

Yet, few of us in the Midwest likely think of the dead zone as we go about our lives in communities bolstered by local farming economies and as we buy and consume the food grown here. “The thing that makes this challenging is that the impacts are so far away,” says Packman. Meanwhile, the runoff associated with food production in the Midwest is harming the trade of fisherman and shrimpers in the Gulf. Reducing the dead zone is going to require that we in the Midwest follow the adage from environmentalist and author Wendell Berry: “Do unto those downstream as you would have those upstream do unto you.”

But there are clear gains to be had here for cutting nitrate runoff. One is cleaner drinking water. A statistic that does hit a little closer to home, the national cost of removing nitrates from drinking water is $4.8 billion annually, according to the Ceres report. 


The proposed U.S. Environmental Protection Agency rules on carbon emission have gotten a lot of attention, but the agency also recently finalized rules to protect aquatic life from power plant and factory cooling systems. 

According to the EPA, 2.1 billion fish, crabs and shrimp are killed by water-cooling systems. The animals are pinned against water intakes, sucked into the systems or die from exposure to heat, chemicals or stress. “EPA is making it clear that if you have cooling water intakes you have to look at the impact on aquatic life in local waterways and take steps to minimize that impact,” Nancy Stoner, acting assistant administrator for water at EPA, said in a written statement. Under the rules, facilities that draw more than 2 million gallons a year for their cooling systems must choose one of seven plans provided by the EPA to reduce the number of animals killed. Environmentalists were critical of the rules because they say that power plants and factories should be limited in the amount of water they can use for cooling. They would like to see the EPA require that plants recycle their cooling water or switch to dry cooling systems. 

The EPA estimates that implementing the rule would cost about $275 million annually. Most of that cost would be covered by the plants and factories.

Illinois Issues, July/August 2014

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