Nutrient leaching dynamics of bioretention systems: The impacts of design parameters and their interactions

Nutrient leaching of bioretention systems raises environmental concerns. This paper monitored 24 cells configured with different design parameters including three media types (two sandy-based media types (media 40 and 70) and clay-loam mixed with wood chips (CL)), three vegetation types (woody, herbaceous, and turfgrass) and two impervious-to-pervious (IP) ratios (15 and 30) during 2017-2021. The design parameters affected the degree and temporal evolution of nutrient leaching. The media CL exhibited the lowest nutrient leaching, with the highest average pollutant removal rates (PRR) of, e.g., -5 % for reactive phosphorus (RP) and nitrate (NO₃). Woody vegetation was most effective in mitigating nutrient leaching, e.g., with overall PRR of RP approximately 2-3 times higher than those observed for the other two vegetation types across all media types. The IP ratio also influenced nutrient leaching, though less prominently. Phosphorus (P) leaching patterns varied more distinctly over time across the design configurations than nitrogen (N) leaching, as reflected in inter-annual variation. The design parameters and their interactions had a more consistent and pronounced effect on P leaching than N leaching. Among the design parameters, media type was most influential for P leaching, explaining 58 %-67 % of the variations in outflow RP concentrations across stages. In contrast, the influence of design parameters on N leaching was minimal initially but increased over time, explaining 24-43 % of NO₃ variation in later stages, largely by media and vegetation types. These findings underscore the dynamic nature of nutrient leaching in bioretention systems and highlight the importance of design considerations to mitigate nutrient leaching.