Latest Issue of Science News



Food for Thought

Janet Raloff
Food for Thought

Marsh Farming for Profit and the Common Good

Sponsor Message

May is American wetlands month, a designation the Environmental Protection Agency and other government bodies around the nation inaugurated in 1991 to draw attention to the disappearance of wetlands. Swamps, marshes, bogs, and other oft-unappreciated plots of soggy land not only provide refuge to fish, beavers, and waterfowl, but they also help remove pollutants from both water and air.

Last month, on Earth Day, President Bush announced a new wetlands initiative. Promising to do more than stem the loss of these environmentally valuable environments, the President pledged that over the next 5 years, the United States "will expand the wetlands of America." A large part of the effort will be directed at farms, he said.

For most of the past century, U.S. farmers drained such plots to increase their arable acreage or improve tillable but overly damp fields. However, a move is now afoot to get farmers to embrace wetlands as part of their business. The idea is that wetlands offer an all-natural and potentially low-cost way to clean up farm wastes.

But that's not all. Some environmental groups are considering support of a whole new class of farming that is essentially wetlands management. These facilities would harness Mother Nature to remove plant nutrients produced by other potential water polluters: upstream farms, municipalities, and industries that produce nitrogen. The novel farms' operational costs, which could prove substantial, would be paid for by upstream polluters.

Although wetland farms may immediately summon images of a humid, decaying, disease-festering environment, there's no reason they need pose a scenic or health risk to farmers or their neighbors, contends Donald L. Hey, senior vice president of the Chicago-based Wetlands Initiative, a public-interest research group. For instance, he notes, "I've never been around a designed open wetland, as opposed to a forested one, where mosquitoes were an issue." Indeed, marshes are generally treeless with plenty of birds and minnows that feed on mosquito larvae, he observes.

Most compelling, Hey argues, are the potential economics of these enterprises. Their profits could far surpass those of traditional farms, he maintains.

Today, he says, midwestern farmers may hope to clear $10 to $15 in profit per acre of tilled land. His group's analysis projects profits for nutrient farms, as these wetland operations might be termed, at up to $700 per acre. Make no mistake, Hey says, wetland farming isn't the creation of a swamp that operators would then ignore the rest of the year. These nutrient-cycling facilities would require plenty of engineering, monitoring, and management. Running most of them, he emphasizes, "would be a full-time job."

Nitrates—a large and growing problem

One of the major families of chemicals polluting waters throughout the United States is nitrates. All living things need nitrogen to grow, so in small quantities, these compounds are beneficial. Indeed, they are the foundation of most farm fertilizers.

The problem is that nitrates now taint the nation's waters in excessive amounts. Not only are elevated concentrations toxic to wildlife and even to people, but also even moderate concentrations can, if allowed to persist, fuel the explosive growth of algae. When those algae die, their decomposition leads to an equally explosive growth of bacteria, which suck the oxygen out of water. This process of rampant algae growth leading to oxygen-starved waters is known as eutrophication.

Eutrophication can occur long distances from the source of the nitrogen wastes that cause it. For instance, cities and farms throughout the upper Midwest provide a large share of the nitrate pollution responsible for the annual creation of a huge zone of oxygen-starved water—the so-called Gulf dead zone—1,000 miles downstream in the Gulf of Mexico.

However, Hey points out, bacteria in the sediment of wetlands can strip nitrogen from waterborne nitrates in a process called denitrification. Like little chemical reactors, the bacteria release the element as a gas into the atmosphere, leaving nothing nutritious to plants behind.

Marsh grasses and other plants also absorb some of the nitrates as they grow. However, this nitrogen isn't permanently removed from the ecosystem and can be re-released into the water when plants die months or years later. Scientists, therefore, look for strategies that favor denitrification over nitrogen removal by plant growth.

The nation's many natural wetlands used to remove much of the nitrate pollution produced by cities and farms. At one time, some 220 million acres of wetlands, an area about twice the size of California, existed in the lower 48 states. But as those wetlands have come under the plow or bulldozer, the environment's nitrates-buffering capacity has diminished—even as the quantity of nitrate pollution has been growing. By 1997, just 105.5 million acres of wetlands remained, and the total was falling by another 60,000 acres or so each year.

Nutrient farming

Hey's group has been a leading proponent of building new wetlands or restoring old ones to tackle the problem. And to prove it, the Wetlands Initiative has put its money into the idea. In 2001, the group bought 2,600 acres of former corn and soybean farmland on the Illinois River in north-central Illinois. This was bottomland that had been pumped 6 hours a day, 365 days a year since the 1920s to be dry enough for tilling.

Hey's group turned off the pumps and let the land flood. Today, 1,200 acres have developed into a marsh, and the rest have evolved into mostly wet prairie and a few into forest. The scientists killed carp that had colonized old drainage ditches and restocked the expanded body of water with fish native to the area. Within 6 months, managers of the site counted up to 20,000 waterfowl congregating there.

Four or five colonies of beavers continue to reconfigure the habitat. Nesting eagles patrol the air. River otters, a locally threatened species, have moved in, as has a threatened species of birds known as grebes. "This past migration season," Hey recalls, "we had 60,000 waterfowl [present] on just 1 day, sitting on those marshes we had created." Among birds winging their way through the site were 4,000 canvasback ducks, which haven't been seen along this part of the Illinois River for decades.

As exciting as these wildlife changes have been, Hey notes that potentially more important is a change in the environment's chemistry—a change that has turned the wetland from a net emitter of nitrates to an eliminator of these pollutants.

A site across the river from the wetlands experiment is being readied for similar study. And, together with several midwestern universities, Hey and his group are ready to launch additional projects at up to a dozen other sites.

"With the right financial base, we could have the research necessary to verify, confirm, or deny the efficacy of nutrient farming within 5 years," he told Science News Online. "We don't even need to buy the land" for those studies, he says. "In many cases we could just lease it for 5 years."

However, Hey says he's confident that wetlands can be farmed to successfully clean up polluted water. The trick will be grading the land to ensure that incoming water spends the right amount of time with the nitrate-destroying bacteria. Most sites will need pumps to control that flow and keep water in the marsh at an optimal level. The goal is to keep the water no more than about 2 feet deep, Hey says, for maximum exposure to the bacterial treatment.

The economics

To evaluate any wetland's efficacy in cleaning polluted water, extensive monitoring equipment must quantify the water flow, its incoming and outgoing nitrate content, and the amount of nitrogen stripping occurring. Neither water-flow management nor such monitoring will come cheap. Hey's group estimates that farmers who operate such facilities might have to invest some $5,000 to $6,000 per acre each year—far in excess of the typical $800 an acre that traditional row-crop agriculture costs.

The difference could be made up, Hey says, in the value of the process to polluters. Currently, municipal and industrial wastewater-treatment operations are paying hundreds of millions of dollars to upgrade their facilities to better remove nitrates from water. For instance, the Water Reclamation District of Chicago estimates that achieving its governmentally mandated goals for nitrate removal from city sewage could cost $144 million a year in purchase and maintenance of bigger and more sophisticated wastewater treatment facilities. Hey suspects that nutrient farmers could remove the extra nitrate for no more than $72 million a year—even after allowing themselves hefty profits. The city would save $72 million a year.

The question is how to manage the payments. And that's where the idea of pollution credits comes in. For several years, the Environmental Protection Agency has allowed companies that emit sulfur dioxide to the air to buy or sell credits on the open market. Essentially, if a company cleans up its act to more than meet the objectives of federal guidelines, it can sell the excess cleanup capacity it's achieved to some other polluter that hasn't achieved the goal.

The idea is that a company that's making an investment in pollution controls can often, for a slightly bigger investment, get a much bigger improvement. These companies can then sell that low-cost extra benefit on the open market for less money than a small company might pay to make an equal cleanup effort from scratch. In other words, it allows companies to take advantage of economies of scale in clean up investments and pass them along to others that might not qualify on their own.

Several years ago, EPA decided to look into the same kind of pollution-credit trading for water polluters. To date, most firms that have taken advantage of the concept and offered pollution credits have been municipal wastewater-treatment systems. However, Hey and others note, there's no reason why other nitrate-removal systems, such as constructed wetlands, shouldn't be allowed to qualify for such pollution-credits trading—as long as the wetlands operations can prove their nitrate-removal efficacy.

Making the grade

In small-scale lab tests, wetlands bacteria perform well. Hey's team and others have also shown in a few actual wetlands that these biological systems can remove up to 80 percent of the waterborne nitrates passing through them.

However, there are lots of scenarios to evaluate, and most wetlands haven't been studied to see whether their initial performance can be sustained over the long term. Indeed, Amy Poe has her doubts that it can.

For 3 years, this environmental researcher at the University of North Carolina was part of a team that studied the performance of a 12-acre constructed wetland built to treat nitrogen wastes draining off a 2,200-acre area of fields planted with corn and soybeans. The wetland is part of a 44,000-acre farm, the largest row-crop facility east of the Mississippi.

The farm's owner was rather progressive, Poe says, and invited the researchers in to investigate the extent to which adding a wetland might help the operations manage nitrate wastes. Explains Poe, the farmer was already employing what was known as "best management practices," such as minimizing excess nitrate-fertilizer application to fields. The wetland was to be an extra nitrogen-management step.

In the experiment, the farm didn't manage its wetland with pumps, Poe explains. The design let rains wash nitrate wastes off fields and into drains that fed the marsh. As such, she explains, the nitrogen wastes came in uncontrolled pulses. Moreover, when rains were heavy, the waste fertilizer washed through the system quickly.

Overall, her team found, the wetland removed about 50 percent of the nitrates that came from the fields.

With other factors accounted for, she notes, "denitrification acted to remove less than 20 percent of the nitrogen." Most of the nitrogen removal from the water instead came from the pollutant's incorporation in wetland plants.

For the first few years, the constructed wetland sopped up nitrogen. However, by year 3, the scientists' measurements indicated that the wetland had become a net source of dissolved organic nitrogen—the type that plants release—in water flowing into an adjacent estuary.

What appears to have happened, says Poe, is that as the wetland matured, the amount of nitrogen that its adult plants could take up diminished, simply because the plants were no longer in the growth spurt of youth. As plants began to die, they released their stored nitrogen. In other words, they merely held nitrogen for a few years and then released it to the water.

"Denitrification is the hardest thing to measure" in wetlands, Poe says, so most researchers don't attempt to do it. Instead, they simply "assume" that bacterial denitrification goes on at high rates. But if it doesn't, then the nitrogen-stripping effect of wetlands could be from plant growth and therefore much less reliable than denitrification, says Poe. In fact, she says, "plant uptake is not permanent nitrogen removal—just a [temporary] transformation."

What all this means, says Poe, is that evaluating nitrate removal by wetlands for any pollution-credits-trading scheme may prove a lot more complicated than some wetlands proponents have anticipated.

"I don't want to sound negative," Poe cautions, "because wetlands are, overall, a great thing." However, she cautions, they may not be an easy fix to nitrate pollution, but one that requires sophisticated engineering and monitoring. Hey agrees but notes that even the costs of such stewardship of created wetlands could prove far less than wastewater-treatment facilities would otherwise be forced to spend. Moreover, he adds, new and expanded wetlands offer the benefits of providing a home for wildlife and helping President Bush fulfill his pledge.

Citations

Donald L Hey

Wetlands Initiative

53 W. Jackson Boulevard, Suite 1015

Chicago, IL 60604

Web site: [Go to]

Amy C. Poe

Institute of Marine Sciences

University of North Carolina at Chapel Hill

3431 Arendell Street

Morehead City, NC 28557
Further Reading

Durham, S. 2003. Designing the best possible conservation buffers. Agricultural Research 51(December):4-7. Available at [Go to].

Brown, L.C., and J.W. Johnson. Nitrogen and the hydrologic cycle. Ohio State University Extension Fact Sheet (AEX-463-96). Available at [Go to].

Kellogg, R. 2004. Agricultural runoff potential: 1990-1995. April 2. Available at [Go to].

Poe, A.C., et al. 2003. Denitrification in a constructed wetland receiving agricultural runoff. Wetlands 23(December):817-826. Abstract.

Sohngen, B., and C.-Y. Yeh. 1999. Total Maximum Daily Loads (TMDLs). Ohio State University Extension Fact Sheet. (AE-7-99). Available at [Go to].

King, D.M., and P.J. Kuch. 2003. Will nutrient credit trading ever work? An assessment of supply and demand problems and institutional obstacles. Environmental Law Reporter 33:10352. Available at [Go to].

More from Science News