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1、原原 文：文：Causes of Soil Contamination in the Urban EnvironmentDust DepositionExtensive dust deposition is mainly caused by industrial emission. It is deposited in dry conditions and as suspended particulate matter. The size of particulate matter varies. The fraction of less than 10 m designated as res
2、pirable particulate matter is most dangerous for human health. A portion of cement dust and fly ash may exceed that value, but there are some kinds of particulate matter with a general size distribution of less than 10 m like asbestos dust and smoke derived from oil-fired power stations. It should b
3、e borne in mind that the smaller the suspended matter is the higher the contaminant concentration is due to the enhanced sorption capacity. Contaminated particulate matter can be transported from outdoors into rooms. There, a dust accumulation occurs, particularly if windows remain opened during day
4、time. Of interest is the comparison between garden soil and house dust concentration. In England it has been found that the Cd, Cu, Pb and Zn values were larger in house dust than in the associated gardens (Thornton 1991). Dust formation in urban areas may play an important role, in particular in ar
5、id and semi-arid regions, where dry conditions dominates, but also in humid climates dust is of importance for soil formation. For instance, in the city of Hanover (Germany) with a population of 520,000, an enormous accumulation of 58 cm within 50 years was observed (Burghardt and Hke 2005). When du
6、st development and deposition occurred without any filter technique systems, the deposited layer can reach enormous thickness as observed in vicinity of a coal processing factory in Halle (Germany) with its 230,000 inhabitants (Fig. 3.2). One important feature of urban areas is the often exposed lan
7、d surface you can never discover in woodland and pasture and over long periods percentages of bare soils are private gardens and allotments in wintertime, playing fields, cemeteries, demolition and building sites, derelict and disused, mostly industrial land, waste heaps, railway embankments, and st
8、orage sites, where permanent dust deposition occurs (Thornton 1991). In urban and industrial areas sealed sites are influenced as well, since dust may easily penetrate into gaps between pavement stones, cobbles as well as railway embankments, constantly filling them up. It is supposed that dust will
9、 be laterally transported on the pavement stones and ultimately concentrated in gaps downslope (Burghardt and Hke 2005). It is logical to expect that urban soils show higher contamination levels than the rural areas because of their proximity to a number of potential pollution sources. Big cities li
10、ke New York (USA) with 23,200,000 inhabitants are affected by several contamination sources, for instance simultaneous industrial emission, impact of traffic, deposits of technogenic substrates, etc. Consequently, a decline in e.g. Cu, Ni and Pb concentrations was found with increasing distance from
11、 the city centre (Manhattan) into the rural district outside of the city. While in Manhattan Pb topsoil values of more than 130 mg kg1 were measured, at a distance of 5060 km the values decreased to about 40 mg kg1, and at a distance of 120130 km to about 30 mg kg1 (Pierzynski et al. 2005). The urba
12、n-to-rural gradient has frequently been found in developed countries of the northern hemisphere. Figure 3.3 presents the lead concentrations of five relatively small towns in the United States. The contamination level of the urban lawn is comparably high, as would be expected by the vicinity and exp
13、osure to high levels of air pollution in urban environments. The air pollution is related to metallic aerosols from heavy industry as well as combustion of fossil fuel. The investigations referred to lawns close to houses and in parks. The high level has not to be restricted to the upper horizons an
14、d forest floors. The activity of earthworms and ants (bioturbation) may play a role in the long-term mixing of the humic topsoil and the mineral subsoil, causing translocation of contaminants like Pb (Craul 1992) (see Section 6.3). A city suburb gradient has been confirmed by the soil investigations
15、 of the upper 5 cm in Marrakech (Morocco) with 1,200,000 inhabitants (El Khalil et al. 2008). They collected material from nine sites according to a gradient from suburban (No. 1) to urban zones (No. 9) (Fig. 3.4ac). It is obvious that the Cd, Cu, Ni and Zn values tend to increase the shorter the di
16、stance to the city centre is. However, other factors as well as the expected dust deposition close to the city influence the situation. With increasing distance to the historic city centre the anthropogenic disturbance of the soil profiles showed distinct fingerprints as well. The technogenic fraction in the upper soil layer reaches 14% at site No. 9, indicating the huge disturbance. The coarse technogenic fraction revealed similar values at a distance of approximately 500 m f
