Enrichment, inter-relationship, and fractionation of heavy metals in road-deposited sediments of Sydney, Australia

Abstract

Urban road-deposited sediments (RDS) are potential sources of heavy metal pollution of both terrestrial and aquatic environments. We determined the heavy metals enrichments, their possible sources, and potential bioavailability and mobility in RDS from nine sites along major motorways of Sydney, the largest city with highest road traffic density in Australia. Mean total concentrations of metals in the RDS decreased in the order: Fe > Mn > Zn > Cu > Cr > Pb > Ni > Cd. The corresponding order in the background soils (minimally contaminated from roads) was: Fe > Mn > Zn ~ Ni > Cu ~ Pb > Cr > Cd. Both the pollution index (PI) and metal enrichment factor (EF), which are comparative measures between contaminated and uncontaminated sites, were highest for Cu and Zn. Inputs of Cu and Zn to RDS were likely to be mainly the result of brake and tyre wear, respectively. Cluster and correlation analyses showed that while the concentrations of these two metals were related in the soil, they were not correlated in RDS. Low PI and EF values as well as the close inter-relationships of Fe, Mn, Cr, and Ni in both RDS and soils suggest that these metals were derived mainly from natural sources. Metal fractionation data showed 50–95% of Cr and Fe in RDS to be present in the immobile and bio-unavailable residual fraction, whereas 15–65% of Zn was contained in the exchangeable fraction, which is considered to be mobile and bioavailable.