Wetland Buffers

Groundwater Bulletin No. 2, Winter 1995/96

Artificial Wetlands Are Working to Reduce Nitrogen Pollution

Wetlands and farmlands needn't always be at odds. If early results from research at the University of Illinois at Urbana-Champaign are any indication, wetlands and farmlands may even be able to work together as a system, reducing the amount of nitrogen entering streams and lakes.

Researchers estimate that as much as 44% of all nitrogen fertilizer applied in the Mississippi River watershed has made its way to the Gulf of Mexico. Most of this nitrogen moving off-site is in the form of nitrate. Excessive levels in drinking water can pose health risks. And high nitrate levels in bodies of water can trigger algae blooms that deplete oxygen--sometimes resulting in "dead zones" where fish and other aquatic organisms cannot survive. Such a dead zone reportedly extends for approximately 3500 square miles where the Mississippi drains into the Gulf.

But artificial wetlands offer hope as one of many tools to combat the problem of nitrogen moving from farmland to lakes and streams, says David Kovacic, UI researcher with the Departments of Landscape Architecture and Forestry.

Kovacic, along with Mark David and Lowell Gentry in UI's Department of Forestry, have constructed artificial wetlands as buffer zones between cropland and the Embarras River in central Illinois. According to preliminary data gathered between November 1994 and March 1995, the wetlands reduced the amount of nitrate reaching the stream by approximately 67% and the amount of phosphorus by about 20% to 50%.

In the study, the UI researchers constructed three 1-1/2- to 5-acre wetland buffer strips adjacent to the Embarras River bank. Berms, constructed 50 feet from the river, contain the tile drainage water coming from the adjacent farmlands, causing it to pool and forming the wetlands. Through this process of redirecting and retaining tile drainage water in the wetlands, nitrate is either taken up by the wetland vegetation or is converted to atmospheric nitrogen by microbes (denitrification). Consequently, there is less chance of nitrate reaching the river.

Kovacic says the research team was pleasantly surprised that even in winter, while the ice was just beginning to melt and vegetation was dormant, the constructed wetland was using up nitrogen.

"The new wetlands need to stabilize over time," he says, "and these preliminary results need to be repeated over several years to be certain that the wetlands are effective in removing nitrogen. Still, the results illustrate promising possibilities."

The wetlands for the UI study were established on sloping land that was fairly wet in its original state. Besides absorbing nitrogen, Kovacic says the wetlands offer an opportunity to return a small area of habitat back to its original condition.

But why construct an artificial wetland when you could use a simple grass or forest filter strip? After all, previous UI research found that 50-foot-wide forest buffer strips and 180-foot-wide grass strips both effectively removed the nitrate in water draining from farm fields.

The problem is that filter strips are not useful on land that is tile-drained, which accounts for about 50% of Illinois farmland. Drainage tiles carry subsurface water directly to streams and other outlets, bypassing the filter strips. With the wetland buffer strips, however, the tiles do not bypass the buffer system.

Currently, Kovacic says 1 acre of wetland can handle the nitrogen moving from 20 to 40 acres of field, but the team will not know the actual efficiency of wetlands for several years.

In a closely related project, David, Gentry, and Kovacic are developing a better understanding of how nitrogen is processed as it passes through the agricultural system bordering the constructed wetlands. The farmland (about 136 acres) is in a corn-soybean rotation. The team is tracking the movement of nitrogen after application; studying the effect of rainfall, soil processes, and plant absorption; and measuring the amount of nitrogen flowing from tile drains into the wetland buffers.

By thoroughly understanding the nitrogen budget for this site, the UI researchers hope to develop environmentally and economically sound prescriptions for fertilizer application. Information gained from this research will be used to fine-tune fertilizer recommendations and the timing of fertilizer application.

For more information about the UI projects, contact David Kovacic at (217) 244-5133.