Tuesday, November 14, 2006 - 1:15 PM

The Role of Midwestern Agricultural Constructed Wetlands in Water Quality.

David Kovacic, Univ of Illinois, 101 Buell Hall, Champaign, IL 61820

Present U.S. annual agricultural fertilizer use has increased from virtually zero before WWII to 10 million metric tons.  As a consequence of fertilizer use and fixation by leguminous crops, 81% of all nitrogen losses to the Mississippi River Basin can be attributed to Midwestern agriculture.  Locally, nitrogen losses (primarily as nitrate-nitrogen) cause the contamination of municipal water supplies.  Regionally these losses are the primary source of nutrients contributing to hypoxia in the Gulf of Mexico.  Tile drainage installed in nearly half of all Midwestern farmland transports drainage water and dissolved nitrate rapidly to downstream waters contributing a disproportional 80 to 90% of the total nitrate entering the Gulf.  Economically viable solutions to reduce nitrogen transport in tile drained areas are few.  Eliminating tile drainage is not an option because drainage is required for efficient crop production.  Fertilizer use can be reduced but research typically shows the reduction required to significantly reduce nitrate transport also significantly reduces crop production.  Constructed wetlands that intercept tile drainage are suggested as a partial solution to nitrate loading in the Gulf.  Tile-drainage wetlands can intercept and remove nitrogen at its source before it enters adjacent streams and still accommodate conventional agricultural practices.  Research in central Illinois indicates that constructed wetlands can effectively remove 37 to 46% of the nitrate-nitrogen from tile-drainage water.  Wetland to watershed land area requirements in this region are approximately 5%.  Wetland construction costs are estimated at $6,000 to $7,000 per ha.  Extrapolating from Illinois research, results indicate that a wetland area of 450 ha would be required in the Lake Bloomington, IL watershed to reduce N loading by 46%, at a construction cost ranging from 2.7 to 3.2 million dollars.  Results support the growing evidence that agricultural wetlands can effectively reduce NPS pollutant loading in the Mississippi River Basin.