Lead contamination and isotope signatures in the urban environment of Hong Kong

The source and the extent of Pb pollution in the urban environment of Hong Kong were investigated at five different urban settings selected on the basis of their annual average daily traffic (AADT) varying from less than 100 to 61,700. In addition, a small distant island without any traffic was selected to establish the possible baseline values. The surface environmental samples studied consisted of street and tunnel dusts, gully sediments, and a limited number of roadside topsoils. The analytical results clearly indicated variable degrees of Pb contamination in these urban settings. However, the level of contamination varied significantly among different types of samples collected at the same location. Pb concentrations of roadside topsoils (79+/-22 micrograms/g) and gully sediments (278+/-88 micrograms/g) were lower than those of the corresponding road dusts (327+/-54 micrograms/g). The Pb isotope compositions in different urban settings varied considerably. The bedrock in the small island had the lowest Pb concentration (12 micrograms/g) but with the highest 206Pb/207Pb ratio (1.2206), whereas the tunnel ceiling dusts with the highest level of Pb (1410 micrograms/g) had the lowest 206Pb/207Pb ratio (1.1062). Despite the significant differences in vehicle types and traffic volumes, and the presence of several different petroleum retailers in Hong Kong, the Pb isotope ratios of road dusts (206Pb/207Pb: 1.1553+/-0.0043, 208Pb/207Pb: 2.4408+/-0.0084) varied within a relatively narrow range among all the five urban sampling sites. On the other hand, the Pb isotopic compositions of gully sediments (206Pb/207Pb: 1.1515+/-0.0145, 208Pb/207Pb: 2.4322+/-0.0198) varied noticeably within the same setting, but were reasonably comparable across the different study sites. In general, the 206Pb/207Pb ratios of road dusts can be used to estimate the direct contribution from automobile emissions, whereas those of gully sediments might reflect the effects of the mixing of different anthropogenic sources. The Pb isotope signatures in the urban environment of Hong Kong clearly suggested that anthropogenic Pb in the environment originated from Pb ore with a low 206Pb/207Pb ratio (such as the Australian Pb ore and similar sources in Southeast Asia) were significantly different from those of the anthropogenic Pb present in the neighboring Pearl River Delta (PRD) region.