Seepage metal concentrations beneath long-term operated bioretention systems

Bioretention systems maintain the natural water cycle and help to mitigate climatic extremes impact on urban areas by retarding, storing, and evaporating stormwater runoff. Although bioretention systems have been operated for more than 25 years, systematic investigations on the hydrological functionality and pollutant retention performance of older systems are rare. We employed laboratory and field experiments to investigate three long-term operated bioretention systems in Germany with the following objectives: (i) physico-chemical substrate characterisation; (ii) an event-based influent and effluent trace metal concentration monitoring covering 22 months and (iii) the calculation of metal retention rates. Regarding the pollution status, we found significantly increased trace metal contents in the soil substrate mainly as a function of the drainage area type and the inflow regime. Nonetheless, all measured metal seepage concentrations fall below the German legislative trigger values. Our current findings demonstrate no risk of groundwater degradation even for old bioretention systems suggesting bioretention as a powerful and sustainable tool for stormwater management. Further research requires the handling of soil substrates modified by stormwater infiltration showing enhanced trace metal contents and a certain amount of technogenic sediments.