Bioretention is among the most frequently implemented green stormwater infrastructure practices. Beyond basic maintenance checks, however, little monitoring is done, despite functioning to clean stormwater for years. This seminar will focus on a field research study aimed at understanding pollutant accumulation and soil development in older bioretention cells. We visited 29 bioretention facilities in Ohio, Michigan, and Kentucky and sampled soils in the forebay (if present), and near the inlet, middle, and outlet of each bioretention cell. Further, three soil depths (0-2, 4-6, and 14-20 inches) were sampled to understand how pollutants accumulate spatially and with depth, with the ultimate goal of informing maintenance. The outcomes of this study include an understanding of phosphorus, metals, microplastics, PAHs, PCBs, alkylphenols, phthalates, and PFAS accumulation in older bioretention media. Further, we evaluate hydraulic function of the media using quick infiltration tests and relate the hydraulics to soil physical properties, including the development of an organic horizon on top of the filter media. Practical recommendations will be provided from the study including when and where to maintain field-scale bioretention systems.