Managing Denitrification In Constructed Wetlands.

Constructed wetlands can be designed to intercept tile drainage waters, removing nitrate through denitrification in flooded sediments. They have been installed and monitored for effectiveness in removing nitrate in several upper midwestern states, including Iowa, Illinois, and Ohio. Ratios of wetland to drained land areas range from about 0.5 to 5%, and wetland size is typically 0.5 to 3 ha, with a 1 m depth. Removal efficiency is primarily a function of hydraulic loading rates and nitrate concentrations, with temperature also important. The longer water containing nitrate is in the wetland (residence time), the greater the removal. Reported nitrate removal efficiencies (% of nitrate load removed per year) range from 25 to 90% of tile inputs, with higher flows (and therefore hydraulic loading) during colder periods of the year leading to reduced removal rates. Denitrification has been shown to be the major removal process in these wetlands, with plant uptake typically not an important process. Unknowns include long-term performance of wetlands, nitrous oxide and other greenhouse gas emissions, and improved design information to optimize the wetland to drained land ratio. In addition, human constraints of cost and social acceptance are major barriers to widespread adoption. Overall, constructed wetlands on tile outlets can greatly reduce nitrate loads, need little maintenance once constructed, and are an additional option to reduce nitrate loads in the upper Midwest while maintaining current agricultural production systems.