Letting streams be streams
The giant web of rivers that connect the Midwest to the Gulf of Mexico are a watery conduit delivering millions of tons of nitrogen and phosphorus to the Gulf -- where the fertilizer causes a dead zone each summer. But streams and rivers are not just pipelines -- they can act as biological reactors where denitrifying bacteria remove a major dollop of nitrate (-NO3) from the water.
These bacteria are the heroes of any effort to control the dead zone, because they convert the mobile plant fertilizer nitrate into harmless nitrogen gas.
In a recent national study, Patrick Mulholland, an aquatic ecologist at Oak Ridge National Laboratory, and a large group of colleagues looked at 72 streams in natural settings, in farmland, and in cities (see #5 in the bibliography). By releasing a traceable isotope of nitrogen, the researchers measured nitrate removal on particular stretches of the streams.
The result was paradoxical: Although bacteria remove more nitrate from a stream that is heavily polluted with nitrate, that stream will retain a higher percentage of nitrate during its flow. "The total rate of denitrification increases as you increase the nitrate concentration," says Mulholland, "but the proportion of nitrate that is removed from the water declines."
Small streams "are very important filters," says Mulholland, "and denitrification is a free ecosystem service. But when you overload streams with sewage and fertilizer, they aren't as effective at removing all that nitrate you are putting in."
Curiously, even seriously degraded streams did some denitrification, courtesy of the bacteria attached to logs and other organic matter. "We were surprised that some of the agricultural and urban streams were effective," Mulholland says.
The study suggests two conclusions relevant to the dead zone. First, small streams should be preserved and enhanced to improve their biological potential. "We don't want to put them in pipes or in ditches, but they don't have to be totally pristine" to remove a measurable amount of nitrate, Mulholland says.
Second, don't expect miracles. "We can't overload the streams and expect them to strip out all the nitrate," Mulholland adds. "If we have a small or moderate nitrate load, they can be pretty effective at removing a portion. But if we overload them, they lose their effectiveness."
Photo: Courtesy Wetlands Initiative
Letting wetlands be wetlands
Wetlands can also host a massive amount of natural denitrification -- if they still exist. But 65 million acres of wetlands in the Mississippi River's watershed have been destroyed for farms and city development since colonial times. Could restoring millions of acres of wetlands mitigate the over-fertilization that is causing the dead zone?
That proposition will soon be put to a pilot-stage test in a large wetland restoration in Illinois, masterminded by a non-profit called the Wetlands Initiative. The group plans to build berms, install pumps to control water flow, and plant native vegetation to create an environment favoring natural processes that remove nitrogen and phosphorus from river water. Teams from a variety of research institutions are ready to study many aspects of the 10-year project, says Donald Hey, president of the Initiative.
The financial and legal arrangements may be more complex than the marsh restoration itself: the "nutrient farming" system that Hey envisions is designed to help polluting agencies pay landowners to remove fertilizer from water. Stringent new phosphorus limits on wastewater set by the U.S. Environmental Protection Agency could force the Chicago Water Reclamation District to spend billions removing phosphorus. Using traditional engineering, this would involve concrete tanks, pumps, and a boatload of electric energy to drive the pumps. "The District estimated the cost of removing nitrogen and phosphorus through conventional technology, at $2.5 billion," says Hey, "and we estimated the amount of money required to restore and operate a wetland as a treatment facility at $1.6 billion."
And so the District has agreed to fund a Wetlands Initiative project -- now awaiting final permits -- to build 1,300 acres of wetlands along the Illinois River and manage them to maximize the biological removal of nitrogen and phosphorus.
Hey says that under this scheme, the wetlands would remain farms, although with a different focus. "The farmer is still harvesting crops," in the form of fertilizer removal, "but he's selling a commodity to a power utility, wastewater treatment facility, industry, or any other emitter of nitrogen or phosphorus."
The EPA is pushing a similar scheme, called "nutrient credit trading," says Tony Prato, a professor of ecological economics at the University of Missouri, that would allow brokers to link buyers and sellers in a market for plant nutrient credits, along the lines of the market created to control acid rain caused by power plants two decades ago. But the program has not gone much beyond the pilot stage.
Photo courtesy Wetlands Initiative
Wetlands offer plenty of benefits beyond improving the water quality, as Prato and Hey wrote in 2006 (see #6 in the bibliography): " Besides reducing nitrogen loading to the Gulf, wetland restoration would stabilize streambanks; reduce sediment, nutrient, and pesticide loadings to receiving waters; lower peak flood flows and damages [something that is relevant after a flood-plagued spring in the Midwest]; create wildlife habitat; improve recreational water uses such as fishing and swimming; enhance consumptive uses such as drinking water; increase aesthetic values associated with wetlands; create wildlife habitat; and increase biological diversity."
Creating wetlands is expensive, and economics must play a key role in mitigating the dead zone problem, yet those who pollute rivers are not yet paying for the privilege because they have "externalized" the costs, says Hey. "We have taken the efficiency of crop production to the limits, and the externalized costs are harming other parts of our economy and environment. The nitrogen we pour on to get last bushel per acre of corn is now rolling downstream, untreated, to the Gulf of Mexico, causing hypoxia. This an artifact of the economic activity called farming. They are externalizing their waste, and that's putting a burden on the people in the Gulf."
Internalize some information from our dead-zone bibliography.
Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive