Lead isotope ratios in urban road runoff

Lead isotopic analyses of road runoff and airborne particulate matter have been carried out to elucidate sources of lead pollution at urban and suburban sites. While lead is often observed in road runoff in suspended form, suspended particle size had no relation to the lead isotopic distribution, as a result of comparison between runoff samples with total suspended solids and those with minute particles passed through a 75 microm sieve. Lead isotope ratios in airborne particulate matter in urban areas fell within a wider range than those in road runoff. Since there was little difference of the ratios between a heavy traffic-flow site and residential sites, airborne lead derived from vehicle exhaust was found to make little contribution to the contamination of road runoff. On the other hand, the ratios in road runoff at a suburban site showed the same range as those at an urban site. Lead in road runoff was therefore suggested to be produced on site by traffic related substances, such as tire wear, other than vehicle exhaust.