Accumulation and Pollution Hazard of Heavy Metals in Bioretention Systems Media

Many studies have found that bioretention systems are very effective in reducing heavy metal content. However, their long-term treatment performance, as well as the heavy metal variations in the systems are rarely explored. This paper reports a field study that the accumulation of chromium (Cr), nickel (Ni), arsenic (As), copper (Cu), lead (Pb) and zinc (Zn) in bioretention systems with different season, media depth and catchment characteristics. Two sampling events were conducted in 2017/6 and 2017/12 located in University of Maryland. Media samples were analyzed using XRF spectrometer to investigate the spatial and temporal accumulation of these metals within 10 bioretention systems. The results showed that: Heavy metal concentrations in the subsurface (below 30cm) were generally lower than that in the surface(0-10cm), and their concentrations varied with seasons (lower relatively in winter). The values of all heavy metals but Zn were lower than the reference values recommended by USEPA. However, compared with the control soil background values, Cr, Ni, Cu, Zn, Pb occured different degree of accumulation, and Cr had relatively serious accumulation. Ni, Cr and As concentrations exceed USEPA soil heavy metal residential soil screening levels, and Cu, Zn and Pb content were unlikely to exceed the level. According to geoaccumulation indexes and potential ecological hazard index, most bioretention systems which operation for about 10-20 years were uncontaminated or light contaminated and had a low ecological hazard. This study could provide managers with estimable data for bioretention systems to monitor and improve the accumulation of heavy metals in media