A conventionally-designed bioretention cell, consisting of very sandy media meant to achieve high infiltration rates, was studied for its hydrologic and treatment performance over two monitoring periods: 2013–2014 (immediately post-construction) and 2017–2018 (4–5 years post-construction). Given the limited literature on mature performance of bioretention, the purpose of this study was to determine how the effluent from the bioretention cell and the hydrology changed over 5 years. The hydrologic performance was maintained 5 years post-construction, with median volume reductions of 100% in both monitoring periods, despite an underutilization of the soil volume. Using censored data analysis and a 95% confidence level, the results revealed that the effluent water quality in 2017–2018 was improved compared to 2013–2014 for some parameters, e.g., dissolved solids and nitrogen species, and was maintained for phosphorus, metals, and suspended solids. These results suggest that for a reliable assessment of bioretention cell treatment performance, it is recommended to wait for soil and plant establishment, e.g., 2 years after construction. Between the inlet and outlet of the bioretention cell (2017–2018 data only), concentration decreased for nitrogen species and suspended solids, but did not significantly increase or decrease for alkalinity, hardness, dissolved solids, phosphorus and metals. Mass removal of all contaminants was very high, largely due to high volume reductions. Despite sustained hydrologic performance up to 5 years post-construction, there is a need for targeted bioretention design for enhanced treatment performance of dissolved contaminants.
High hydrologic and contaminant mass removal was maintained over 5 years.
Dissolved contaminants’ data revealed poor to moderate treatment performance.
Improved treatment would require targeted design for dissolved contaminant removal.
Monitoring >2 yrs. post-construction gives reliable treatment performance evaluation.
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