Effectiveness of constructed wetlands in reducing nitrogen and phosphorus export from agricultural tile drainage

Much of the nonpoint N and P entering surface waters of the Midwest is from agriculture. We determined if constructed wetlands could be used to reduce nonpoint N and P exports from agricultural tile drainage systems to surface waters. Three treatment wetlands (0.3 to 0.8 ha in surface area, 1200 to 5400 m(3) in volume) that intercepted subsurface tile drainage water were constructed in 1993 on Cole soils (fine-silty, mixed, superactive, mesic Cumulic Endoaquoll) between upland maize (Zea mays L,) and soybean [Glycine mar (L,) Merr,] cropland and the adjacent Embarras River. Water (tile now, precipitation, evapotranspiration, outlet flow, and seepage) and nutrient (N and P) budgets were determined from 1 Oct. 1994 through 30 Sept. 1997 for each wetland. Wetlands received 4639 kg total N during the 3-yr period (96% as NO3-N) and removed 1697 kg N, or 37% of inputs. Wetlands decreased NO3-N concentrations in inlet water (annual outlet volume weighted average concentrations of 4.6 to 14.5 mg N L-1) by 28% compared with the outlets. When the wetlands were coupled with the 15.3-m buffer strip between the wetlands and the river, an additional 9% of the tile NO3-N was apparently removed, increasing the N removal efficiency to 46%, Overall, total P removal was only 2% during the 3-yr period, with highly variable results in each wetland and year. Treatment wetlands ran be an effective tool in reducing agricultural N loading to surface water and for attaining drinking water standards in the Midwest.