Long-term hydrological performance of bioretention systems: Do they age well?

Bioretention is a widely adopted green infrastructure practice for urban stormwater management, yet its long-term hydrological performance remains poorly understood. Concerns exist that physical clogging of sediments will impair infiltration capacity over time, potentially leading to system failure. This review synthesizes results from over 150 peer-reviewed studies to evaluate how bioretention's hydrological performance evolves over operational timescales of 5-20+ years. Across field studies of systems aged 5-22 years, most well-maintained systems maintained or exceeded design hydraulic conductivity thresholds (typically 25-50 mm/h), with a minority of studies reporting declining performance attributable to maintenance deficiencies or design failures. We identify a key observation: accelerated laboratory column experiments consistently show declines in saturated hydraulic conductivity (Ksat), while field surveys of mature bioretention reveal that most well-maintained systems appear to maintain their infiltration capacity over time, though performance is context-dependent and conditioned on adequate design and maintenance. We attribute this pattern to biological processes (root growth, macrofauna, wetting-and-drying, pedogenesis) that appear to mitigate physical clogging in field systems but are absent from most laboratory experiments. The review addresses four interconnected topics: (1) hydraulic performance and clogging, (2) soil media evolution through pedogenesis and pollutant accumulation, (3) vegetation maturation and ecohydrological feedbacks, and (4) current models and their capacity to simulate long-term behavior. We propose a conceptual three-phase framework for the typical lifecycle of bioretention systems: establishment (0-3 years), maturation (3-10 years), and steady state (>10 years). The available evidence suggests that well-designed and maintained bioretention can function as a living ecosystem whose biological inhabitants help sustain hydrological function, rather than a static engineered filter that inevitably clogs; however, this outcome is not universal and depends critically on vegetation establishment, maintenance regime, and design quality. Critical knowledge gaps remain regarding performance beyond 20 years, transferability across climate zones, and quantitative representation of biological feedbacks in predictive models.