3. Metal Transformation and Stormwater Quality

A detailed understanding of the metal transformation processes and consequent stormwater quality impacts provides a platform for the implementation of effective stormwater pollution mitigation strategies. This chapter discusses the geochemical behaviour of common metals associated with road dust and the processes they undergo during dry weather periods, in order to understand their mobility characteristics and potential stormwater quality impacts. Among the analysed metals, Zn and Cd have the greatest affinity for the readily available exchangeable geochemical fraction, whilst Cu and Pb are likely to associate with the reducible and oxidisable geochemical fractions. The characteristic affinity of Ni and Cr for the residual geochemical fraction indicates their low mobility. With increasing residence time, weakly bound metals gradually transform into more stable chemical forms. The transformation rate of metals follow the order: Pb > Cu > Zn > Cd, which is inversely opposite to the order of their mobility. Based on the transformation rate and mobility order, metals such as Cd and Zn can adversely influence stormwater quality due to their readily available nature.