A decision support system to design event driven constructed wetlands for treating stormwater runoff

Stormwater runoff is unfiltered rainwater that did not percolate into the ground. If the stormwater is left untreated, it may carry pollutants and contaminate the surface water body it flows into. Constructed wetlands are proposed to treat stormwater runoff as they offer significant cost advantages and a wide range of ecological benefits. The main goal of this study was to generate a mathematical modeling based decision support system (DSS) for designing wetlands to treat stormwater runoff. The developed model was applied to study the utility of such constructed wetlands in the Arroyo Colorado River watershed in the Rio Grande Valley area of Texas along the rapidly urbanizing US-Mexico border. The free water surface (FWS) wetland was modeled using the tanks-in-series reactor theory assuming unsteady-state flow and transport conditions. The model considered major runoff pollutants including biochemical oxygen demand, particulate organic nitrogen, ammonia nitrogen, nitrate nitrogen, total suspended solids, and total phosphorus. The simulation model was then used to construct an optimization model for sizing the wetland. The results of the study indicate that the assumption of steady-state flow and transport that is commonly used to design constructed wetlands leads to a significantly larger wetland than the unsteady-state flow and transport model that realistically represents the stormwater runoff event. The modeling results also indicate that with increased urbanization, the contaminant of concern affecting the size shifts from nitrogen compounds to biochemical oxygen demand. As such, a multi-species modeling framework is vital for proper sizing of the stormwater wetland.