Pollutants in Hong Kong soils: Polycyclic aromatic hydrocarbons
Introduction
Located at the south-eastern tip of the mainland China (22°18′N, 114°10′E), Hong Kong is a city with a population about 6.8 million and a total land area of 1102 km2 (Survey and Mapping Office, 2003). The Pearl River Delta (PRD), at which Hong Kong is located, is the largest delta in Southern China. The rapid economic and population growth of this region in the last two decades has posed a significant environmental impact to the ambient environment, which has adversely affected the air and water quality in the PRD (Wong et al., 1995, Fu et al., 1997, Yang et al., 1997).
One of the most notorious and ubiquitous pollutants are polycyclic aromatic hydrocarbons (PAHs). It is also one of the Persistent Toxic Substances (PTS). PTS typically share the major characteristics of 12 Persistent Organic Pollutants (POPs) in the Stockholm Convention (United Nations Environment Programme, 2005). They are characterized by their exceptional toxicities towards many living organisms, reluctance in degradations and high lipophilicity, making them a class of very dangerous compounds. Some PAHs were found to be very toxic and 16 of them have been identified by US EPA as being toxic, partially mutagenic and carcinogenic “priority pollutants”. They represent most of the adverse effects that are caused primarily as a result of anthropogenic activities (Keith and Telliard, 1979).
Soil, comprising of mainly mineral particles and organic matters, is a major reservoir and sink for urban pollutants because of their quantity and holding capacity for pollutants (Wild and Jones, 1995). Atmospheric deposition is the most common source of pollution in soil (Agency for Toxic Substances and Disease Registry, 1995, Li et al., 2001) and many hydrophobic contaminants adsorb/absorb strongly to the particles and organic fractions and thereby retain in the soil matrix for a long time. Subsequently, leaching of pollutants and soil erosion can cause significant contamination to rivers and sediments in the nearby areas.
Soil is also a good indicator of pollution and environmental risks. It is continuously subject to pollution because of its open-system nature. Together with its capability to accumulate various pollutants, soil serves as a medium to integrate historical and recent pollution status indirectly. Fine soil particles, together with the adsorbed pollutants can enter human body via various pathways through direct ingestion and inhalation of the dust (Office of Emergency and Remedial Response, 1996) or indirectly from food in contaminated areas (Abdullah, 1995). In addition, toxic effects to soil organisms such as earthworms when in contact with PAH contaminated soils have been observed (Sverdrup et al., 2002).
A number of law-abiding guidelines on soil quality are set to protect human and ecological health in developed countries, but the significance of soil protection is often neglected in Asian countries. In light of this, a large-scale survey of soil pollutants has been initiated. The major objective of this paper was to (1) characterize the spatial distribution of soil PAHs, (2) identify the soruces of PAHs, and (3) assess the potential risk of current edaphic PAH levels in Hong Kong. It is hoped that the experience gained in Hong Kong could be used as a model to reflect the typical soil pollution levels of other densely populated cities. The results will also provide valuable information for regulatory purpose concerning PAH pollutions in soils.
Section snippets
Site description
Hong Kong can be broadly divided into four parts geographically: Hong Kong Island, Kowloon Peninsula, New Territories and outlying islands (Fig. 1). The population density varied, from 16 000 people km−2 for Hong Kong Island, 43 000 people km−2 for Kowloon Peninsula to 3500 people km−2 in the New Territories (Census and Statistics Department, 2004).
Definition of land uses
Ten land use classes had been designated to reveal the pollution impacts from various human activities. Greening area consisted of any locations with
Spatial pattern of PAHs
Detail descriptive statistics of individual PAHs in each land use can be found in Table 1. In general, the mean concentrations of were decreased in the following order of land uses: Nearby highway > urban park > industrial area > restored landfill > greening area > rural area > orchard farm > agricultural farmland > crematorium > country park. The difference between maximum and minimum values of can be classified into (1) land use with difference greater than 10 000 μg kg−1 (urban park), (2) greater
Conclusions
It was found that the pollution of was generally low at around 140 μg kg−1 in Hong Kong. The PAHs are typically originated from vehicular emissions and cross border atmospheric deposition of PAHs may also be a significant source. Samples with very high concentration of PAHs were found sporadically (∼2% of collected samples), and anthropogenic cause such as improper disposal of used motor oils is believed to be their main source. No adverse effects would be imposed to nearby
Acknowledgements
The authors are grateful to Mr. Y.Y. Chin from LCSD (HKSAR) for providing technical assistance. Financial support from the Strategic Research Fund from the Science Faculty, HKBU is also gratefully acknowledged.
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