Phosphorus retention in a bioretention cell: Insights from process-based reaction-transport modelling

Bioretention cells (Bio-C) have emerged as a low impact development (LID) option to reduce peak discharge and nutrient export from urban areas. Despite growing implementation globally, understanding of P cycling and retention mechanisms in Bio-C is limited. Here we present a novel numerical reactive transport model to simulate the fate and transport of P in a Bio-C system in the greater Toronto metropolitan area. Unlike existing Bio-C models, our model incorporates a detailed representation of the biogeochemical reaction network that control P cycling and retention within the cell. We used this model as a diagnostic tool to determine the relative importance of different P removal processes and their contributions to the P accumulation trajectory within the Bio-C over 8 years of operation. Model results were validated against time series flow data, plus water chemistry and soil filter media P concentration depth profiles measured between 2012 and 2019. A sequential extraction method was also applied to soil cores collected in 2019 to validate the model-derived P pools profiles. The simulations reproduce the TP and SRP outflow loads, the TP accumulation rate in the soil filter media and the partitioning of P between different soil chemical pools. Results indicate that groundwater recharge is the dominant mechanism responsible for decreasing the surface water discharge from the Bio-C (63% runoff reduction), but that accumulation in the soil filter media is the predominant P removal mechanism (57% of TP influx). Of P retained within the soil filter media, 48% is highly stable, 41% potentially remobilizable, and 11% easily remobilizable. In addition to elucidating P cycling, our model can help assess the impact of Bio-C design choices on P retention efficiency and the stability of the retained P.