Vertical distribution and speciation of heavy metals in stormwater infiltration facilities: possible heavy metals release to groundwater

The infiltration inlet facilities on the side of the road along with the sewage system have been constructed two decades ago in highly urbanized residential area in Tokyo. Possible release of heavy metals from the infiltration inlet was studied by analyzing sediment samples in different vertical depth. Seven heavy metals (Cr, Mn, Co, Ni, Cu, Zn and Pb) were measured. The heavy metal contents in sediment decreased with depth. The low content of heavy metals at the bottom sediment compared to the top indicated possible release of heavy metals from the inlet sediment. The heavy metals speciation study showed that the order of the extractability/mobility in the sediment in acid exchangeable fraction was Zn and Mn>Co>Ni>Cu >Cr and Pb. The mobility order in road dust also followed the similar pattern. In reducible fraction (metal oxide bound) Pb was the most mobile in the sediment while the other metals mobility order was not similar. In oxidizable fraction (organic and sulfide bound) the order was different for the sediment and road dusts. The residual fraction contained 18 to 83% heavy metals. The presence of heavy metals in acid exchangeable, reducible and oxidizable fractions indicated a future possibility of their release to the underlying soil and the groundwater.