Toxic Metals in Urban Runoff: Physico-Chemical Mobility Assessment Using Speciation Schemes

Physico-chemical characterization of lead, zinc and cadmium has been carried out on eight samples from both separate and combined sewers. Dissolved and paniculate total metal levels have been determined. A speciation scheme has been used to further divide these phases in two dissolved fractions, bioavailable and stable, and five paniculate fractions, ion-exchangeable, acid-soluble, reducible, oxidizable and residual. Total dissolved lead concentrations were found to be largely below European Community (EC) directives for drinking water production. Lead was represented by stable forms while zinc and cadmium were mainly bioavailable. Paniculate concentrations were higher than natural levels found in the Seine-Normandie basin. Zinc was characteristic for municipal effluents, and lead and cadmium were more concentrated in separate sewers. Correlations have been observed between paniculate heavy metals and volatile matter on the ring highway, or with iron at the Savigny site. The paniculate metal speciation scheme has demonstrated the important dependence of heavy metal distribution on the sampling site, as well as the potentially more mobile nature of zinc and cadmium, compared to that of lead. The structural modifications undergone by solids during their transport along the sewer networks had a significant impact on paniculate metal mobility. This has been clearly noted for lead, whose mobility decreased from upstream to downstream in the studied system.