Mixing in deep zones within constructed treatment wetlands

Transverse deep zones are often included in treatment wetlands due to an expectation that they can improve contaminant removal by these natural systems. This paper explores the hydraulic behavior of deep zones to provide a better understanding of their potential benefit. First, we use a bench-scale model to show that recirculation within a deep zone is likely under field conditions. Recirculation is believed to provide a mechanism for mixing. We then use a mathematical model to explore the effect on the outlet concentration of replacing a section of a wetland marsh with a deep zone that provides mixing. We observe that even when deep zones have a lower volumetric removal rate than the marsh areas they replace, the presence of a deep zone may decrease the outlet concentration and hence improve the performance of the wetland segment.