The effects of season and hydrologic and chemical loading on nitrate retention in constructed wetlands: a comparison of low- and high-nutrient riverine systems

We compared the nitrate removal efficiency of two constructed wetlands rece iving ambient river water to one constructed municipal wastewater treatment wetland over the same 2-year period in central Ohio, USA. The wastewater w etland represents a high-nutrient system, with an average nitrate plus nitr ite load of 12.3 kg N ha(-1)day(-1) and an average nitrate and nitrite infl ow concentration of 12.5 mg N l(-1). The riverine wetland loadings and conc entrations were approximately 60% lower (4.6-4.7 kg N ha(-1)day(-1) and 4.6 mg N l(-1)). Percent nitrate removal by mass ranged from 29% in the wastew ater wetland to 37-40% in the riverine wetlands, although differences in re tention varied widely by season and were not statistically significant amon g the wetlands. Retention efficiency was considerably lower in all three we tlands during floods; nitrate outflow was as much as 400% greater than infl ow during some flood events. We developed a simple Vollenweider-type model of nitrate retention based on seasonal temperature, hydraulic loading, and nitrate loading. The model is general enough to be useful in describing nit rate retention in both high and low-loaded wetlands and was calibrated and validated with extensive field data. The model was used to predict wetland nitrate removal efficiency as the hydrologic and nutrient conditions change . The ability to make such predictions could be valuable in the design, con struction, and management of wetlands for nutrient removal. (C) 2000 Elsevi er Science B.V. All rights reserved.