Development of a Phosphorus Removal Model for Bioretention Systems

Bioretention is a relatively new stormwater management practice that relies on physical, chemical and biological processes within a terrestrial ecosystem to provide stormwater retention and treatment. Field monitoring and laboratory testing performed to date have demonstrated the ability of the systems to significantly decrease runoff flows and to efficiently reduce a number of pollutant loads. However, large discrepancies in phosphorus removal have been reported from the field monitoring of bioretention systems. Two bioretention cells on the University of Maryland campus, monitored by Davis (2007), achieved 79% and 77% total phosphorus mean mass removals, respectively, over 12 storm events. Conversely, Hunt et al. (2006) noted an increase of 240% in total phosphorus on a mass basis in the outflow of a bioretention cell in North Carolina over a 12-month monitoring period.

Current bioretention design guidelines focus mainly on peak flow reduction, which may not provide adequate treatment in areas sensitive to pollutant loading. In particular, most freshwater environments are sensitive to high phosphorus loadings, which can lead to the eutrophication of receiving water bodies. The aim of this study was to build a simple and user-friendly tool for designers to predict phosphorus removal in a bioretention system. This tool will also assist researchers in understanding the large phosphorus removal discrepancies observed in the field.

An event-based one-dimensional finite difference model has been developed to simulate total phosphorus removal in bioretention systems. The model comprises four completely mixed layers of user-defined depth: the ponding water layer, the mulch layer (optional in bioretention systems), the soil root zone, and the deep soil zone. Model subcomponents estimate water volumes and phosphorus masses within the model layers. Total phosphorus is divided in the model into particulate and soluble phosphorus. The model requires input time series for rainfall and runoff inflows, as well as soluble and particulate phosphorus inflow concentrations. Processes modeled inside the bioretention unit consist of evapotranspiration; precipitation of soluble phosphorus to particulate phosphorus; dissolution of particulate phosphorus to soluble phosphorus; ponding water layer overflow of water, particulate and soluble phosphorus; sedimentation of particulate phosphorus; infiltration of water and soluble phosphorus from one layer to the next; vegetative uptake of soluble phosphorus; mulch and soil sorption and desorption of soluble phosphorus; exfiltration of water and soluble phosphorus from the system to the surrounding native soils; as well as underdrain discharge of water and soluble phosphorus. Model processes, input parameter requirements and model limitations will be discussed in detail in this presentation.