Constructed Wetlands Targeting Nitrogen Removal in Agricultural Drainage Discharge – a Subcatchment Scale Mitigation Strategy.

Despite substantial efforts, the leaching of nutrients from agricultural land is still a serious and costly environmental problem in Denmark and elsewhere. The quality goals of the European Water Framework Directive (WFD) for the aquatic environment require a substantial reduction of diffuse nutrient loads from farmland in Denmark. Tile drains and ditches connecting fields to receiving waters thus acting as subsurface highways for nitrogen (N) as well as other agricultural contaminants. Drainage losses of nutrients contribute to estimated 45-60% of total N losses. Hence, for a large number of recipients, drainage water nutrient loads have a major impact on water quality, and end-of-pipe drainage filter solution may offer the benefits of a targeted measure. This calls for a paradigm shift towards the development of new, cost-efficient technologies to mitigate site-specific nutrient losses in agricultural drainage water.

The Danish strategic research project “SUPREME-TECH” (www.supremetech.dk) and the innovation project “iDRAIN” (www.idrain.dk ) aims at providing the scientific basis for developing and implementing cost-effective drainage filter technologies targeting N-removal in agricultural subsurface drainage. These projects study different approaches of implementing site-specific drainage filter technologies including surface-flow constructed wetlands (SF-CW) and subsurface flow constructed wetlands (SSF-CW). A large number of constructed wetlands mitigating nutrient losses in drainage water have been constructed in Denmark since 2010. Presently, an evaluation of the Danish monitoring results have demonstrated average yearly N removal efficiencies ranging from 20 to 55% depending on CW type, seasonal variations in drainage discharge and N-loading rate. A sub catchment scale monitoring program combined with scenario analysis of variable mitigation strategies and cost-efficiency analysis reveals that even at low to moderate yearly N removal efficiencies (20-25% N removal efficiency) CWs targeting drainage water are highly efficient and cost-efficient measures. Thus, although challenges remain regarding site-specific documentations, CWs targeting drainage discharge has been included as new mitigation strategy in the Danish environmental regulation.