Behaviour of soluble reactive phosphorus within field-scale bioretention systems

Abstract

Soluble reactive phosphorus (SRP) in urban runoff can contribute to eutrophication of downstream water bodies. Bioretention systems, a popular low impact development (LID) practice, have the potential to reduce SRP loads in urban runoff. In this study, six field-scale bioretention systems located in southern Ontario, Canada, with different soil media compositions were monitored to evaluate their performance with respect to SRP retention and to identify the geochemical processes influencing SRP behaviour within the systems. The physical and chemical characteristics of the soil media were analysed to compare the systems as well as to assess whether the installed soil media compositions met the LID bioretention design guidelines. To examine how SRP evolves as stormwater infiltrates through the different media, porewater was collected from multiple depths within the bioretention systems. The geochemical processes controlling SRP retention-release were identified by chemical analysis of the porewater and selective solid phase extractions. The geochemical processes governing SRP behaviour differed between the bioretention systems with processes validated via one-dimensional reactive transport simulations conducted in PHREEQC. Results indicate: 1) Soil media characteristics are highly variable between systems despite the systems having the same overall engineering design specifications; 2) Porewater SRP does not evolve uniformly with depth indicating the importance of overall system design including redox conditions, infiltration rate, and vertical layering of different materials (e.g., surface cover layer and bioretention media); 3) SRP retention-release processes often occur at shallow depth (<0.1 m) and more attention is needed to assess the impact of different surface cover layers used in field installations; 4) Hydraulic conductivity controls the residence time of water in the systems which in turn influences SRP retention-release; 5) SRP adsorption onto Al- and/or Fe-oxides is the main retention mechanism in systems with high hydraulic conductivity while Ca phosphate mineral precipitation may be important in systems with low hydraulic conductivity. The study findings provide insights needed to improve the field performance of bioretention systems in retaining SRP including the testing and application of soil media amendments for enhancing SRP retention.