Urban stormwater treatment using bioretention

Urban stormwater has negative environmental and ecological effects. Bioretention systems are starting to be used in efforts to mitigate these effects. A bioretention system receiving water from a light industrial catchment and a busy road was designed, built and monitored for changes in soil physics as well as hydrological and hydrochemical efficiency. The soils in the bioretention system were designed to have high metal removal potential and high permeability to compensate for undersized bioretention volume. The inflow hydrograph was a series of sharp peaks with little baseflow, typical of runoff from impervious surfaces. The bioretention system smoothed the hydrograph by reducing peak flow and volume for all 12 events monitored in detail. Overflow occurred in 10 events indicating the increased permeability did not fully compensate for the undersized volume. Runoff was heavily polluted with sediment and heavy metals, in particular zinc. The majority of the zinc, lead and Total Suspended Sediments were removed from the stormwater that flowed through the bioretention system, with TSS and total zinc concentrations reducing by orders of magnitude. Despite high removal efficiency, median concentrations of zinc exiting the bioretention system still exceeded ecosystem health guidelines and the bioretention system was both a source and sink of copper.