Salt tracer experiments in constructed wetland ponds with emergent vegetation: laboratory study on the formation of density layers and its influence on breakthrough curve analysis

Constructed wetlands are a rapidly expanding and intensively studied wastewater treatment system. One of the main types in use is the free water surface (FWS) wetland or wetland pond. In studies on these ponds, salt tracer experiments are a convenient tool to determine travel time distributions, which are, in turn, related to hydraulic and sedimentation (trapping) as well as nutrient removal efficiencies. Typically, flows encountered in constructed wetland ponds are characterized by low Reynolds numbers, at times even within the laminar flow regime. In such conditions the injection of salt may cause strong density effects, thereby threatening the usefulness of the recorded breakthrough curves. The processes and mechanisms governing the formation of density stratification due to salt tracer injections into wetland ponds with emergent vegetation were studied in the laboratory. The results reported are expected to be useful in the planning of future field tracer experiments.