Effectiveness of Constructed Wetlands in Reducing Nitrogen and Phosphorus Export from Agricultural Tile Drainage

nutrient in estuaries. Phosphorus enters surface waters in both dissolved and particulate form and is generally Much of the nonpoint N and P entering surface waters of the the limiting nutrient in lakes. A large portion of the N Midwest is from agriculture. We determined if constructed wedands could be used to reduce nonpoint N and P exports from agricultural and P now entering U.S. freshwater and marine ecosys­ tile drainage systems to surface waters. Three treatment wetlands (0.3 tems as nonpoint diffuse nutrient loading can be attrib­ to 0.8 ha in surface area, 1200 to S400 m3 in volume) that intercepted uted to agriculture and associated land use practices in subsurface tile drainage ,,:ater were constructed in 1994 on Colo soils the Midwest (USEPA, 1989, 1990b). These practices (fine-silty, mixed, superactive, mesic Cumulic Endoaquoll) between have contributed to reduced ground water quality, re­ upland maize (Zea mays L.) and soybean [Glycine max (L.) Merr.] duced surface water quality, and are also believed to cropland and the adjacent Embarras River. Water (tile flow, predpita­ be a contributing cause of hypoxia in the Gulf of Mexico tion, evapotranspiration, oudet Bow, and seepage) and nutrient (N and anoxia in estuarine and ocean ecosystems (Turner and P) budgets were determined from 1 Oct. 1994 through 30 Sept. and Rabalais, 1991; Vitousek et aI., 1997; Jickells, 1998; 1997 for each wetland. Wetlands received 4639 kg total N during the Burkart and James, 1999). 3-yr period (96% as NOrN) and removed 1697 kg N, or 37% of inputs. Wetlands decreased NOrN concentrations in inlet water (an­ Thirty-seven percent of the Cornbelt and Great Lakes nual outlet volume weighted average concentrations of 4.6 to 14.5 mg cropland is artificially drained by surface channels, sub­ N L-1) by 28% compared with the outlets. When tbe wetlands were terranean tiles, or a combination of the two (Fausey et coupled witb the IS.3-m buffer strip between the wetlands and the aI., 1995). Artificial drainage lowers water tables in river, an additional 9% of tbe tile NOrN was apparendy removed, many poorly drained soils of the Midwest so they can increasing the N removal efficiency to 46%. Overall, total P removal be farmed, but it also shunts N03-N laden drainage was only 20/0 during the 3-yr period, with highly variable results in waters more rapidly and directly to surface waters, fur­ each wetland and year. Treatment wetlands can be an effective tool ther exacerbating the N03-N problem (Fausey et aI., in reducing agricultural N loading to surface water and for attaining 1995; Drury et aI., 1996; Lowrance et aI., 1997). Phos­ drinking water standards in the Midwest. phorus export to surface waters has been conventionally associated with surface erosion and surface runoff; how­ ever, recent studies show that artificial drainage systems CONSTRUCTED wetlands can effectively treat a variety can also be a major source of nonpoint P (Sims et aI., ofpoint-source pollutants. Examples include nutri­ 1998). ents, pesticides, and heavy metals, from sources such as In the upper Embarras River watershed of central municipal wastewater, liquid mine waste, urban Illinois, where tiles drain 70 to 850/0 of the cropland, stormwater drainage, and feedlot runoff (Hammer, total N losses averaged 39 kg ha -1 yrl from 1990 to 1996 1989; Moshiri, 1993; Kadlec and Knight, 1996). Success­ (David et aI., 1997a). Tiles contributed an estimated 75 ful use of constructed wetlands to treat point-source to 91 % of the total N load to the Embarras River in pollution has led to broad interest in their potential 1995 and 68 to 82% of the total N load in 1996. Dissolved to ameliorate the effects of nonpoint pollution from P export in the Embarras River from 1993 to 1996 aver­ agricultural sources (USEPA, 1990a; Baker, 1992; aged 0.9 kg ha-1 yr-1• During 1995 to 1996, an estimated Campbell, 1995). 46 to 59% of the dissolved P export to the upper Embar­ Two of the most important agricultural nonpoint pol­ ras River resulted from tile drainage (Xue et aI., 1998). lutants are Nand P. Nitrogen enters ground water and These studies demonstrate that in heavily tile-drained surface water through seepage and lateral subsurface watersheds most of the Nand P added to surface waters flow primarily as N03-N and is generally the limiting is transported through the tiles. To remedy the nonpoint-pollution problem, method­ ologies must be devised to reduce export to aquatic D.A. Kovacic, Dep. of Landscape Architecture, 101 Temple Buell Hall, Univ. of Illinois, 611 E. Lorado Taft Dr., Champaign, IL 61820; ecosystems. Several forms of output control have been M.B. David, L.E. Gentry, and K.M. Starks, Dep. of Natural Resources discussed in the literature. Riparian buffer strip systems and Environmental Sciences, W-503 Turner Hall, Univ. of Illinois, have the potential to reduce diffuse nutrient inputs into 1102 Goodwin Ave., Urbana, IL 61801; and R.A. Cooke, Dep. of surface waters and are effective in many environments Agricultural Engineering, 338 Agricultural Engineering Science Building, Univ. of Illfuois, 1304 W. Pennsylvania Ave., Urbana, IL 61801. Received 16 July 1999. *Corresponding author (dkovacic@ uiuc.edu). Abbreviations: DOC, dissolved organic carbon; MCL, USEPA maxi­ mum contaminant level; WA, Wetland A; WB, Wetland B; WC, Published in J. Environ. Qual. 29:1262-1274 (2000). Wetland C; WD, Wetland D.