All of the conventional materials used to line lagoons and ponds have been used successfully for constructed wetlands, and the basic construction and installation procedures are the same. Membrane liners have been used for both FWS and SSF wetland systems. Both 30-mil polyvinyl chloride (PVC) and high-density polyethylene (HDP) have been successful, as well as 45-mil ethylene propylene diene monomer (EPDM) for smaller systems. Larger systems construct the liner in place but again use conventional procedures for assembly, joint bonding, and anchoring. Puncture of the liner must be prevented during placement and subsequent construction activity. Some currently used membrane liners require protection from ultraviolet solar radiation. Conventional procedures can again be used for this purpose (WEF, 2001). Clay liners have included locally available clay soils and commercially available products such as bentonite. Bentonite is typically mixed with the in situ native soils and then graded and compacted. Bentonite liners in the form of pads or blankets are also available; these are laid on a prepared surface and covered with a shallow layer of soil or sand. Clay liners typically have to be a foot or more in depth to provide the necessary hydraulic barrier. In the case of FWS wetlands, the surface of this clay layer should be well compacted to discourage root penetration by the emergent vegetation over the long term.
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