Modeling multi-year phosphorus dynamics in a bioretention cell: Phosphorus partitioning, accumulation, and export

Area of Science:

• Environmental Engineering
• Water Quality Management
• Biogeochemistry

Background:

• Urban stormwater runoff is a significant source of phosphorus (P) pollution, leading to eutrophication.
• Bioretention cells, a type of Low Impact Development (LID), are increasingly implemented to mitigate urban runoff and nutrient export.
• A predictive understanding of bioretention cell efficiency in reducing phosphorus loadings is limited.

Purpose of the Study:

• To develop and apply a reaction-transport model to simulate phosphorus fate and transport in a bioretention cell.
• To identify the key processes responsible for phosphorus immobilization within the bioretention cell.
• To evaluate the long-term phosphorus reduction efficiency and retention capacity of the bioretention cell.

Main Methods:

• A reaction-transport model incorporating biogeochemical reactions was developed to simulate P cycling.
• Model predictions were validated against multi-year observational data (2012-2017) including outflow loads (TP, SRP) and P depth profiles.
• Sequential chemical P extractions were performed on filter media core samples to assess P forms.

Main Results:

• Exfiltration to native soil was the primary driver of runoff reduction (63%).
• The bioretention cell demonstrated high P removal efficiency, retaining 57% of total P inflow load, primarily through media accumulation (48% stable, 41% mobilizable P forms) and plant uptake (21%).
• Outflow loads of TP and SRP were minimal (1% and 2% of inflow loads, respectively), and the cell showed no signs of saturation after seven years.

Conclusions:

• Bioretention cells are highly effective at reducing urban phosphorus export, with accumulation in filter media being the dominant retention mechanism.
• The developed reactive transport modeling approach is adaptable for assessing P reduction in various bioretention designs and hydrological conditions.
• These findings support the continued implementation of bioretention cells as a sustainable solution for managing urban water quality.