Model evaluation of simultaneous injection of inert and reactive tracers in treatment wetlands

Treatment wetlands have shown to play an important role in reducing the nutrient content of recipient waters. The overall objective of this study is to evaluate simultaneous tracer experiment in Ekeby treatment wetland in Eskilstuna, Sweden, with tritiated water, nitrogen (N-15) and phosphorus (P-32). Particular modelling tools are essential to be able to understand the complexity of the wetland processes and to reveal the role underlying mechanisms. The model evaluation is based on a 2D physical description of stationary water flow and an uncoupled 1D formulation of the transport and transformation of nitrogen (N) and phosphorus (P). The overall N- and P- response of the wetland is obtained from averaging responses over the distribution of flow pathways. The flow modelling, described in a companion presentation, is thus a prerequisite for the interpretation of the P- and N-processes. Both P and N are affected by retention in the wetland, which is clear from comparing the slope of the tails of the breakthrough of P-32 and N-15 with tritium. The lower slopes of the reactive tracers compared to tritium indicate a slow wash-out for N and P. Nitrogen was also partly removed due to denitrification, which is evident from a larger degree of removal than phosphorus. A first-order loss rate coefficient, k, for N averaged for the entire wetland is about k = 0.12 1/days. The rate coefficient was defined from the expression Meff/Min=Exp(-k <T>), where <T> is the mean residence time, the effluent N-mass is Meff and the influent N-mass is Min. However, laboratory experiments indicate that the micro-biological activity in the bed sediment is generally much higher than the average for the wetland. Therefore, a consistent model interpretation of the denitrification process required that we accounted for both a rate-limited exchange with the hyporheic zone (sediments) and the first-order loss of N-2 from the hyporheic zone to the atmosphere.