Routine maintenance and proper construction oversight both need to occur during and after bioretention cell construction, in order to ensure proper functionality. Two sets of bioretention cells of varying media depths (0.6-m and 0.9-m) will have been monitored for two, 12-month periods, in Nashville, NC. They treat runoff from an impervious asphalt parking lot. These bioretention cells are unique in that during the first monitoring period, the bioretention cells were (1) clogged with sediment from construction and (2) were severely undersized. Complete drawdown of the surface storage took 48 hours or more, as compared to the recommended 12 hours. Initially, the surface storage volumes for the 0.6-m and 0.9-m media depth cells were only 28 percent and 35 percent of the design storage volume, respectively. The design event for the bioretention cells at this site was 2.5-cm, but the system was overwhelmed frequently and overflow occurred for events as small as 0.9 cm. Overflow was occurring three times more frequently than intended. After one year of monitoring, a contractor removed the fines layer that was present in the top 7.6-cm. Through removal of this layer and expanding the surface area, the surface storage volume was increased by 89 percent for both cells. With the increase in surface storage volume, more runoff was treated, and fewer events had overflow. The smallest event with overflow was 1.9 cm, and some events up to 2.8 cm were fully captured in the bowl. Overflow volume was reduced to approximately one-third of the volume during the first monitoring period. Moreover, removal of the fines layer increased the surface drawdown rate by a factor exceeding 10. Pollutant load reductions increased for nitrogen species and total suspended solids because more runoff was treated. The results of this study highlight the reduced performance associated with improperly constructed and maintained bioretention cells. Even a small construction error in setting the base elevation for the bottom of the bowl or the emergency overflow structure can drastically reduce the bowl storage volume.
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