Hydrologic and water quality performance of an 8-year old bioretention cell treating residential runoff.

Bioretention cells (BRCs) are among the most popular low-impact development techniques in the United States. They effectively mitigate the runoff hydrograph and excessive pollutant loadings affected by widespread urbanization. Overwhelmingly, the literature focuses on the effectiveness of newly constructed BRCs and does not account for the variety of designs found in BRCs installed less recently. The objective of this study was to evaluate the efficacy of a BRC in mitigating runoff quantity, reducing peak flow rate, and improving water quality parameters (i.e., sediment, nutrients, heavy metals, and indicator bacteria) 8 years post-construction. Total runoff volume reduction was substantial (83%), accredited to mature vegetation promoting evapotranspiration, canopy interception, and hydraulic properties of the mature BRC soil. Additionally, the BRC provided similar peak flow mitigation (median 93%) to newly constructed BRCs in the literature. Significant reductions in event mean concentrations and loads of sediments (>88%), particulate-bound nutrients (>60%), and heavy metals (>70%) were observed. In many cases, the BRC had better - or at least similar - pollutant removal efficiencies than younger BRCs. The excellent function of this BRC provides evidence that these systems may improve over time, given proper long-term maintenance.