Nutrient Dynamics in Constructed Subsurface Wetlands: Model Development and Performance Assessment

Abstract Submission: Constructed wetlands are designed to replicate the water treatment functions of natural wetlands. Numerical models are essential for understanding the processes involved in nutrient retention and removal within these ecosystems. In this study, we developed a novel cell-averaged subsurface flow numerical model to simulate water flux and fate and transport of nutrients in a vegetated highly conductive, porous bed. We applied the model to a constructed vegetated wetland in Ohio, wherein water depth, flow, ammonia, nitrate, total organic nitrogen, dissolved phosphorus, total phosphorus, and suspended sediment were monitored from 2014 to 2017. The subsurface flow model is based on the unconfined groundwater flow equation, modified using the Darcy-Forchheimer formulation for post-Darcy’s flow regime and a controlled discharge at the outlet. The primary focus here is on the water quality module, which simulates nutrients and sediments within completely-mixed cells. The module incorporates mass balance differential equations for ammonia, nitrate, organic nitrogen, suspended sediment, organic and inorganic phosphorus, oxygen, and plant growth. The Euler method is implemented to solve a coupled system of first-order ordinary differential equations. Models’ performances were assessed against observed data. Parameter sensitivity analysis and predictive uncertainty estimation were conducted using the Kolmogorov-Smirnov and the Generalized Likelihood Uncertainty Estimation methods, respectively. Although model simulations of the water depth and nutrient concentrations showed promising results, the analyses however revealed gaps in both the flow and water quality data. The developed model has the potential to capture key processes of constructed wetlands, making it a valuable tool for improved wetland design.

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