| Constructed wetlands are increasingly being used worldwide to facilitate nutrient removal, in particular, phosphorus in Florida, from agricultural runoff or conventional wastewater treatment plant effluents. Two-dimensional (2-D) flow dynamics, solute transport, and phosphorus dynamics models are developed to simulate spatio-temporal variations of flow and/or phosphorus dynamics in constructed wetlands. The MIKE 21 hydrodynamics (HD), advection-dispersion (AD), and ECO Lab models were adopted as the basic framework with modifications and enhancement of phosphorus kinetic pathways to incorporate ecosystem dynamics among water column, floc and upper soil layers, and vegetative communities including emergent aquatic vegetation (EAV), submerged aquatic vegetation (SAV), and periphyton. The models were calibrated and validated for two constructed wetland systems in Florida: the Orlando Easterly Wetland (OEW) Cell 7 and the Stormwater Treatment Area (STA) 5 northern flow-way. The spatio-temporal water level fluctuations, tracer (bromide/chloride), and phosphorus concentration profiles were reasonably simulated through linkages between the HD and the AD or ECO Lab model, and key model parameters were estimated.;The OEW modeling study is focused on impacts of topographic and vegetative heterogeneity on short-circuiting flow through sensitivity analysis deduced during model calibration on a bromide breakthrough curve. The short-term simulation results show that relic ditches or other ditch-shaped landforms and the associated sparse vegetation along the main flow direction intensify the short-circuiting pattern, considerably reducing hydraulic efficiency.;In the northern flow-way of STA 5, the HD model calibration for six long-term monitoring sites accurately represented observed annual variations in hydroperiod. On average, the root-mean-square error (RMSE) for predicting daily water level was less than 0.10 m. Manning's roughness coefficients for dense EAV and SAV areas, which were estimated as a function of vegetation type and density, ranged from 0.67 to 1.0 s/m1/3 and 0.12 to 0.15 s/m 1/3, respectively. The AD model calibration for four long-term monitoring sites agreed very well with the measured annual variations in chloride concentration with the average RMSE of 13.48 mg/L. Longitudinal dispersivity was estimated to be 2 m and was over an order of magnitude higher than the transverse one. Results of conservative phosphorus transport simulation confirm the findings of recent STA studies that the EAV systems are less efficient for phosphorus retention compared to the SAV systems and current STA system is not sufficient for reducing dissolved organic phosphorus (DOP) concentration to very low levels. Linked with the HD model, the calibrated ECO Lab phosphorus dynamics model better simulated observed annual variations in soluble reactive phosphorus (SRP) and DOP level than particulate phosphorus (PP) level, which is primarily due to uncertainty of model parameters on spatio-temporal variations of mass transfer mechanisms between water column and floc layer.;Several limitations of this modeling study are addressed, followed by recommendations for future study to develop a more robust scientific and management modeling tool for constructed wetlands, overcoming the shortcomings of traditional treatment wetland modeling approaches. |
