Metal treatment in stormwater bioretention systems with high hydraulic conductivity – Designed for more efficient runoff volume capture and cold climates

Bioretention systems are used to treat stormwater. Using coarser filter media than commonly recommended with high saturated hydraulic conductivities may increase annual runoff volume capture, facilitate smaller filters, less overflow and adaptation to cold winters. However, this may affect water quality treatment negatively. Therefore, we investigated total and dissolved metal treatment at three full scale bioretention systems with a coarse filter material and saturated hydraulic conductivities >1500 mm/h in Malmö/Sweden. One bioretention system was designed with a coarse sand-based filter medium, another with coarse sand-based filter medium and a submerged zone and the third with a 50:50 mixture of coarse sand and pumice as filter medium. The study included 19 rain events, partly during winter season when road salt was applied. The results suggest that also filter media with high hydraulic conductivity can be an effective option when metal treatment is targeted. The two systems with coarse sand filter media treated total metals effectively with median removals >80 % for Cu, Pb and Zn and median removals >35 % for Ni and Cr. Dissolved metal treatment was variable reaching from effective treatment for dissolved Cu, Pb and Zn with median removals >60 % to overall leaching of dissolved Cd, Ni and Cr. Applying a submerged zone did not showed benefits for total or dissolved metal removal. Further, treatment of total and partly dissolved metals was significantly impaired due to pumice addition of the filter media, discouraging pumice as a filter media amendment. Coarser filter materials could be recommended for sites with space limitations or when frozen ground is expected in winter. Further, they can generally reduce clogging risks and untreated overflows.