The utility of agricultural constructed wetlands


The Aquisafe project aims at mitigation of diffuse pollution from agricultural sources to protect surface water resources. The first project phase (2007-2009) focused on the review of available information and preliminary tests regarding (i) most relevant contaminants, (ii) system-analytical tools to assess sources and pathways of diffuse agricultural pollution, (iii) the potential of mitigation zones, such as wetlands or riparian buffers, to reduce diffuse agricultural pollution of surface waters and (iv) experimental setups to simulate mitigation zones under controlled conditions. The present report deals with (iii), providing a review of the potential of constructed wetlands to protect surface waters from diffuse agricultural pollution. Population growth and industrialization have lead to the demise of large majorities of natural wetland systems. Recent research continues to suggest the importance of these often saturated areas in the natural remediation of pollutants in water, as well as being aesthetically pleasing and acting as potential habitat for declining species. The drastic losses in wetland areas, combined with the realization of their importance, have stimulated recent attempts at wetland restoration and even construction of wetlands where they would not have naturally occurred. In terms of substance remediation, constructed wetlands were traditionally used for the treatment of point sources, such as urban or industrial waste water. Recently they have also become increasingly popular for the treatment of diffuse pollution from agriculture and urban storm runoff. Constructed wetlands have been shown to be efficient in the treatment of nutrients, organic matter and heavy metals. Few studies also show their potential against trace organics, such as pesticides and pharmaceutical residues and against pathogens. Retention efficiencies vary significantly among case studies. In agricultural settings the following design criteria should be considered: (i) Water residence time in wetlands is critical. Some studies concerning nutrient removal suggest that a constructed wetland should be about 5 % of the watershed area and assure water residence time of 7 days. (ii) Vegetation is important to slow down flow and increase sedimentation. Regular cutting and removal of plants is controversially discussed, since it may reduce their beneficial effect on wetland hydrology. (iii) Constant redox conditions are important to avoid release of sedimented or adsorbed pollutants. (iv) A combination of constructed wetlands with buffer strips showed very positive results.

Kompetenzzentrum Wasser Berlin gGmbH