Current rational design methods for subsurface flow wetlands idealise these systems as plug flow reactors. This assumption ignores the existence of longitudinal dispersion, short-circuiting, stagnant zones and so on. This paper reports on a series of three tracer experiments performed on a gravel bed wetland with a design hydraulic retention time of four days in order to study its hydraulic characteristics. The results present the hydraulic retention time distributions. The first tracer run using a single point inlet source had a mean time of 2.71 days. The second and third had the influent distributed across their width using an inlet manifold and yielded mean retention times of 3.47 and 3.41 days respectively. Further interpretation of these results indicates that the current plug flow assumption used in design can result in over estimation of treatment efficiency.

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