Modeling variably saturated water flow and multicomponent reactive transport in constructed wetlands

Constructed wetlands (CWs) are becoming increasingly popular worldwide for removing organic matter (OM), nutrients, trace elements, pathogens, or other pollutants from wastewater and/or runoff water. We present a multicomponent reactive transport model CW2D (i.e., Constructed Wetlands 2D), as an extension of the HYDRUS-2D variably saturated water flow and solute transport software package. CW2D was developed to model the biochemical transformation and degradation processes in subsurface-flow CWs. Such wetlands involve a complex mixture of water, substrate, plants, litter, and a variety of microorganisms to provide optimal conditions for improving water quality. The water flow regime in subsurface-flow CWs can be highly dynamic and requires the use of a transient variably saturated flow model. The biochemical components defined in CW2D include dissolved oxygen (DO), three fractions of OM (readily and slowly biodegradable, and inert), four N compounds (ammonium, nitrite, nitrate, and dinitrogen), inorganic P, and heterotrophic and autotrophic microorganisms. Organic N and organic P were modeled as part of the OM. The biochemical degradation and transformation processes were based on Monod-type rate expressions. All process rates and diffusion coefficients were assumed to be temperature dependent. Heterotrophic bacteria were assumed to be responsible for hydrolysis, mineralization of OM (aerobic growth), and denitrification (anoxic growth). Autotrophic bacteria were assumed to be responsible for nitrification, which was modeled as a two-step process. Lysis was considered to be the sum of all decay and sink processes. We demonstrate the performance of the model for one- and two-stage subsurface vertical flow CWs. Model simulations of water flow, tracer transport, and selected biochemical compounds are compared with experimental observations. Limitations of the model are discussed, and needs for model improvements are summarized.