A model was developed using contemporary wetland theory to predict the fate of nitrogen runoff in a constructed wetland. The model utilizes nitrogen concentrations of influent water as system inputs. The model is three-dimensional, one dimensional in time, and two dimensional in space. The physical domain of the model incorporates a flat emergent marsh and deep pool and includes the water body and underlying sediment. Solutions for concentration of sediment-bound organic nitrogen are obtained for the water body and the sediment-water interface, while solutions for concentration of ammonium and nitrate are obtained for the entire physical domain. Physical conditions are considered along the system boundaries, and a jump condition is modeled for nutrient diffusion through the sediment-water interface.

A hyperbolic advection-settling equation models the transport and deposition of sediment-bound organic nitrogen; mineralization of deposited nitrogen is modeled. A parabolic advection-diffusion equation is used to model the movement of dissolved ammonium and nitrate through the wetland water body; the equation is modified for both ammonium and nitrate to model diffusion and transformation in the sediment layer. Spatial variation of sediment layer aerobic and anaerobic regions is considered, as are temperature and pH effects on transformation rates. Numerical solutions are obtained using divided differences.

Constructed wetlands for use in NPS pollution control are a new concept; there is no data currently available to use for model validation. The model was shown to be consistent with qualitative theoretical considerations, based on simulations of different scenarios.