Constructed wetlands are simply areas of wet or flooded ground that have been engineered in a manner that allows the water within to remain separate from local groundwater. These can be shallow ponds or lakes used to attenuate flood water or prevent pollution entering a watercourse, boggy swamp like structures with waterlogged soil or reed beds constructed over gravel.
What they all have in common except attenuation ponds (where this is not always required) is that to function properly as parts of a water treatment system they must be contained within a non permeable structure.
In the case of small reed beds this could be made of concrete or lined with a plastic or geosynthetic membrane whereas larger areas can be either puddled clay or lined with non permeable synthetic membranes or geosynthetic clay liners (gcl) either way they must be capable of containing the water within and resisting the efforts of plant roots to escape. Any holes or tears could result in contamination of surrounding watercourses.
The way in which these wetlands are constructed is always dependant on the site conditions and no two are likely to be the same. These wetlands can be a haven for wildlife and insects and can improve the appearance of areas that may be unused due to topography or access and it is often possible to build them in places which seem unsuitable for cultivation or construction