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

Abstract

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 μg/g) and gully sediments (278±88 μg/g) were lower than those of the corresponding road dusts (327±54 μg/g). The Pb isotope compositions in different urban settings varied considerably. The bedrock in the small island had the lowest Pb concentration (12 μg/g) but with the highest ²⁰⁶Pb/²⁰⁷Pb ratio (1.2206), whereas the tunnel ceiling dusts with the highest level of Pb (1410 μg/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.

Introduction

Most lead compounds, particularly alkyllead used as an antiknock additive in petrol, are important toxic pollutants and impose significant health hazards in urban environments Nriagu, 1990, Nriagu et al., 1996a, Nriagu et al., 1996b. The largest source of anthropogenic Pb is derived from the combustion of leaded petrol, which accounts for approximately 90% of the atmospheric Pb input in urban environments (Nageotte and Day, 1998). Other important sources of Pb include industrial discharges and coal burning Mukai et al., 1994, Mukai et al., 2001. The source and the extent of Pb pollution have been investigated extensively in the last three decades Chow et al., 1975, Bollhofer and Rosman, 2000, Bollhofer and Rosman, 2001. The isotopic compositions of Pb are useful indicators of their different sources, as they are not affected to a large extent by physical and/or chemical fractionation processes in the surface environments Bollhofer and Rosman, 2000, Zhu et al., 2001. Different ore bodies have distinguished isotopic compositions depending on their age and initial Th and U contents of the source rocks. The isotopic compositions of alkyllead and its compounds, emitted from burning of leaded petrol, can reflect the composition of the Pb ore(s) used in their production (Chow et al., 1975). These differences can be used to trace the sources of contamination. However, multiple sources of industrial Pb and the presence of natural ²³²Th-rich bedrocks, which are enriched in ²⁰⁸Pb, create a wide range of isotopic signatures in the urban environment.

In recent years, human exposure to Pb either through direct inhalation or from ingestion has been progressively reduced in many countries where stringent regulations are in place to control the use of leaded petrol. Similarly, the Hong Kong Government has implemented several pieces of legislation to reduce Pb emissions from motor vehicles, and began to progressively phase out leaded petrol from 1986. Since 1999, the use of leaded petrol has been totally banned in the territory. Most other Asian countries, however, still consume leaded petrol and/or use coal as the main fuel, and consequently contribute to Pb pollution not only locally, but also on a regional scale Bollhofer and Rosman, 2000, Mukai et al., 2001.

Street dusts and roadside soils are considered as good indicators of heavy metal contaminations in urban environments Harrison et al., 1981, Nageotte and Day, 1998 and often contain high concentrations of Pb. Several studies have focused on heavy metal contamination of street dusts and roadside soils in Hong Kong Wong and Tam, 1978, Tam et al., 1987, Yim and Nau, 1987, Wong et al., 1996, Chen et al., 1997, Wong and Mak, 1997, Wang et al., 1998, Li et al., 2001. Because of its environmental and health significance, Pb contamination has always been the prime concern of these investigations. However, none of the previous studies investigated heavy metal concentrations of roadside gully sediments or the tunnel deposits, despite their potential significance in improving our understanding of the behavior of potentially toxic elements in the urban environment. Furthermore, an increasing number of studies have focused on Pb isotope ratios in the region, particularly in the Pearl River Delta Zhu, 1995, Mukai et al., 2001, Zhu et al., 2001, Wong et al., 2002, Wong et al., 2003, providing invaluable information on the regional patterns of Pb isotope compositions and potential sources. However, possible sources of Pb pollution in the urban environment of Hong Kong have not been fully investigated. The primary objectives of the present study are to: (a) determine the Pb isotope signatures in the urban environment of Hong Kong and (b) evaluate the potential sources of Pb contamination. In this study, the urban environment of Hong Kong was represented by the street and tunnel dusts, gully sediments and roadside topsoils.