Use of tracers and mathematical modelling to evaluate of hydraulic characteristics of constructed wetlands is presented for a duckweed pond in Mniów, Poland. Instantaneously injected bromide was used to obtain residence time distribution (RTD) of wastewater in that wetland. Flow components were identified and their hydraulic characteristics were derived from tracer concentration curve measured in the outflow by partial fitting of the analytical solution of one-dimensional advection-dispersion equation to the experimental data. The modelling has shown that the wastewater flows along three different flow-paths to the exit. The mean weighted transit time obtained from modelling of tracer data combined with measured mean flow rate of waste water yields the volume of wastewater in the pond.