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Abstract
Bioretention is a common stormwater management practice that mimics natural hydrology. Thisstudy evaluated the long-term performance of a bioretention basin in South Downtown Atlanta,
Georgia, treating highway runoff over five years. Between November 2024 and April 2025, 14
storm events were monitored. Removal efficiencies were 94% for total nitrogen, 96% for total
Kjeldahl nitrogen, 95% for orthophosphate, 95% for total solids, and 88% for nitrate-nitrogen.
Compared to year one (82% TN, 88% TKN, 86% orthophosphate, 100% TSS), results show
sustained or improved pollutant removal despite gradual media changes. High treatment
performance was maintained through biological uptake and denitrification. These results support
the long-term effectiveness of bioretention and highlight the importance of ongoing monitoring
for optimizing urban green infrastructure design and maintenance. Keywords: stormwater runoff, nitrogen, phosphorus, pollutant, long-term performance.