The wetland, which was built in an agricultural area, reduced levels of phosphorus by nearly 60 percent and nitrates by 40 percent. Phosphorus and nitrates are prime ingredients in both fertilizers and in water pollution.
High levels of these nutrients can cause algae to flourish, often to the detriment of fish and other animals that depend on waterways for survival. Algae essentially rob oxygen from water in a pond, lake and even the ocean.
"We saw a pretty significant reduction in phosphorus and nitrate concentrations – close to the kind of decrease we typically see in a natural wetland," said William Mitsch, a study co-author and director of the Olentangy River Wetland Research Park at Ohio State University. "The water was cleaner when it left the wetland than when it came in."
The results appear in the current issue of the journal Ecological Engineering. Mitsch conducted the study with Daniel Fink, a student in the environmental science graduate program at Ohio State.
Often called the "kidneys" of the environment, wetlands act as buffer zones between land and waterways. They also act as sinks – wetlands filter out chemicals in water that runs off from farm fields, roads, parking lots and other surfaces, and hold on to them for years to come.
But the jury is still out on how long a wetland can remain a sink for certain nutrients, Mitsch said. For example, phosphorus doesn't degrade, so after 30 or 40 years, the wetland in this study could possibly become a phosphorus source.
"Wetlands' capacity to store phosphorus declines with time," Mitsch said. "But a wetland that reaches that point has extremely rich soil, which could be harvested and spread over a farm field. Or the wetland itself could be turned into a field again, assuming there are other wetlands nearby to take its place."
The story isn't quite the same with nitrates – a nitrogen-based compound found in fertilizer.
"As a wetland gets older, it gets better at handling and storing nitrogen," Mitsch said. "The idea is not to overload wetlands in the first place."
The three-acre wetland in the study was roughly the size of three football fields and drained a watershed about 14 times that large. It is located in prime agricultural territory in west central Ohio – on the shores of Great Miami River, a tributary that flows into a local lake and then on to the Ohio River, which drains to the Mississippi River.
The researchers studied the wetland for two years, collecting and analyzing water samples twice each month. They tested the samples for concentrations of phosphorus, nitrates and several other chemicals.
The researchers also gathered data on water flowing into the wetland from rain and snowfall and from ground sources. They wanted to know if the design of this experimental wetland would work.
Apparently, it did.
"It was a relatively simple, inexpensive way to create a wetland, and it worked," said Mitsch, who is a professor of natural resources at Ohio State. The wetland was constructed by bulldozing a series of basins into the field. The five basins sloped downward toward the river – the second basin was at a slightly lower elevation than the first, the third basin was slightly lower than the second, and so on. Surface water would come into the first basin and slowly seep toward the other basins. All of the basins were connected by water flowing beneath the surface.
No plants were planted in the wetland; rather, the builders let nature take its course, Mitsch said. Stands of trees surrounded part of the wetland.
"This area once was a wetland," Mitsch said. "So it reverted pretty quickly to that kind of ecosystem when it had the chance."
Over the two-year study, the wetland reduced concentrations of nitrates by 40 percent and phosphorus by 59 percent. It also retained these nutrients during periods of high precipitation, such as during storms and torrential downpours, which saturated the ground.
"It's encouraging that this wetland acted as an effective sink, even when runoff from fertilized fields was especially high," Mitsch said. "Of course, that could change as time goes on, however."
Most of the water entering the wetland came as runoff from farm fields, and nitrate and phosphorus levels all peaked after fertilizer application in the spring. Still, the wetland was able to retain a significant portion of these nutrients.
Algae thrive on phosphorus and nitrogen. While phosphorus is more of a concern in freshwater areas, as freshwater algae thrive on this nutrient, nitrogen is a considerable problem in the Gulf of Mexico.
Each spring, the rush of chemicals that runs into lakes and streams in the Mississippi watershed – an area encompassing about 40 percent of the United States – ultimately turn more than 7,000 square miles of the Gulf of Mexico into a "dead zone," a condition known as hypoxia.
Hypoxia happens when excess nutrients, such as nitrates and phosphorus, accumulate in a body of water and cause excessive growth of algae and algal blooms. These blooms thrive on the nutrients and deplete the water of nearly all of its oxygen.
Wetlands experts estimate that the American Midwest has lost about 80 percent of its wetlands in the last two centuries, compared to a 50 percent loss overall in the lower 48 states.
About 577,000 acres of wetlands have already been created or restored under current conservation programs, Mitsch said. But about 10 to 25 times that many are needed in the Mississippi watershed region in order to see a significant reduction of nitrogen levels in the Gulf.
The Indian Lake Watershed Project and Natural Resources Conservation Service helped fund this study.