Water Air Soil Pollut DOI 10.1007/s11270-010-0372-6
Trace Elements in Soils of Urban Areas Franco Ajmone-Marsan & Mattia Biasioli
Revised: 11 November 2009 / Accepted: 1 February 2010 # Springer Science+Business Media B.V. 2010
Abstract Urban soils are an essential element of the city environment. However, studies on urban soils are scattered in terms of geographical distribution, sampling pattern, analytical dataset, etc. One of the major issues arising from the studies on this ecosystem is the diffusion of its contamination. In cities, in fact, the proximity to humans may cause a serious danger for citizens. In the present study, results from the literature about trace elements in urban soils are presented to compare methodologies and results and to offer a basis for the harmonization of investigation approaches and establishment of remediation thresholds. A total of 153 studies on the urban ecosystem published in the last 10 years were collected and data on trace elements in soils of 94 world cities were compared and discussed. Data highlights the discrepancies among different studies (sampling strategies, analytical procedures) and the extreme variability of urban soils. Most cities are contaminated by one or more trace elements, revealing the environmental relevance of the urban soil system. While Pb is still one of the major concerns in many locations, new contaminants are on the rise and would deserve more attention from the researchers. While in fact some contaminants are almost ubiquitous in world cities and could be used as tracers for urban contamiF. Ajmone-Marsan : M. Biasioli (*) DI.VA.P.R.A.—Chimica Agraria, Università degli Studi di Torino, Via Leonardo da Vinci 44, 10095 Grugliasco, Turin, Italy e-mail:
[email protected]
nation, some traffic-related elements such as platinum, rhodium, and palladium, whose reactivity and toxicity is still unknown, are becoming of concern. Collation of literature data highlights the need for the harmonization of sampling, analytical, and rendering procedures for regulatory purposes and provides a useful dataset for environmental scientists dealing with the urban ecosystem and for city planners. A sampling design adapted to local urban patterns, a prescribed sampling depth, and a minimum set of elements that deserve to be measured could be the core of a common methodology. Keywords Urban soils . Trace elements . Contamination . Cities
1 Introduction In 2005, 49% of humankind, 3.2 thousand million people, lived in an urban area and, by 2070, roughly 70% is expected to be urban (United Nations 2008). In the most developed regions, more than half of the population already lived in cities in 1950 (Fig. 1) and is projected to reach nearly 80% by the year 2030. A recent report by the European Environment Agency (2006) has highlighted the effects of the expanding urban areas on the environment. Soil is an essential compartment of the urban ecosystem, contributing directly or indirectly to the general quality of life (de Hollander and Staatsen 2003; van Kamp et al. 2003). In urban areas, it
Water Air Soil Pollut Fig. 1 Proportion (in percent) of urban population on the total population (United Nations 2008)
100 90
Percentage of urban population
80 70 60 50 40 30 20 10 0 1940
1960
Percentage urban
acquires additional functions with respect to natural or agricultural soils. As an element of the landscape, it has esthetical and recreational functions in parks and gardens and contributes to the preservation of biodiversity. The urban soil often undergoes rapid use changes which often end up with extensive sealing, which alters its relationships with the other ecosystem compartments, air, water, and biota. Its substrate can be mixed with other anthropogenic materials that may modify its functioning (De Kimpe and Morel 2000; Morel et al. 2005). Anthropic activities, such as industry, traffic, fuel combustion, and waste disposal, often result in soil pollution. In other terms, most of the major threats to soil conservation listed by the European Commission (2006), viz., erosion, salinization, compaction, sealing, and contamination, are active in urban environments. There is a substantial body of literature dealing with urban soils contamination. Sources like traffic, heating, industry, and waste disposal have often caused contamination of the soils with organic and inorganic pollutants. Among others, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, dioxins, metals, and metalloids are the main causes of concern. The typical diffuse pattern of this contamination and the proximity of soils to humans enhance the risk for citizens. When investigating soils of cities, the traditional approach to soil survey and investigation that are used in open, non-urban areas cannot be transferred as
1980
2000
more developed regions
2020
2040
2060
less developed regions
such. In fact a number of problems, listed below, complicate the inference procedures commonly used for rural areas. High Spatial Variability of Chemical, Physical, and Biological Properties Recent research (Madrid et al. 2006; Wei and Yang 2009) indicates that urban soils have a very high variability, even in the short range. This concerns not only the usual soil quality indicators such as pH or cation exchange capacity but also the pollutants. This is due in part to the natural soil spatial variability but, in a city environment, the spatial heterogeneity is greatly intensified by human activities. In fact, excavation, redistribution, and mixing of the soil matrix and addition of extraneous materials are frequent in the built environment as a consequence of the intensive use of the territory and the rapid land use changes. Soil-forming processes are deeply modified or interrupted when sealing of the surface occurs and are resumed when built areas are dismantled. Soil features are then the result of complex processes that might be very far from the natural ones. Fragmented Distribution As a large portion of the surface of an urban area is sealed by constructions or roads, the areas where the soil is exposed have very variable sizes and usually are randomly distributed. This would add a constraint to the sampling design which is then limited to the exposed surfaces
Water Air Soil Pollut
Unknown Accessibility During the phase of soil survey preparation, maps or aerial photographs are usually used to identify areas of exposed soil so to be able to plan the sampling operation. However, when field operations are carried out, many places may be inaccessible or accessible only by permission for various reasons: private property, construction areas, no-traffic areas. The sampling coverage can then be further limited. Rapid and Unpredictable Land Use Change In cities, land use change is often rapid. As a result, surveyed locations might disappear and new sites are exposed that would require new sampling. The traditional approach to sampling design is then not directly transferable to urban areas and remote sensing- and geographic information system-supported techniques become indispensable. This paper reviews studies conducted in the last 10 years of trace elements in the soils of cities throughout the world in order to compare methodologies and results and to offer a basis for the harmonization of investigation approaches and establishment of remediation thresholds.
2 Trace Elements in Urban Soils The concentration of energy and matter in a city brings about the accumulation of trace elements (Wong et al. 2006). In fact, when collectively compared with the outlying areas (Higgs et al. 1999; Birke and Rauch 2000; Gbadegesin and Olabode 2000; Thuy et al. 2000; Zhai et al. 2003; Biasioli et al. 2006), urban soils appear to be definitely more contaminated than their agricultural or natural surroundings (Johnson and Ander 2008). For example, garden soils contain twice the concentration of metals (Cd, Pb, and Zn) as corresponding agricultural soils (Schwartz et al. 1995). The risk from high concentrations of trace elements is usually related to their likelihood to leach to groundwater or to enter the food chain through plant uptake. In city environments, however, due to the proximity of the soil to humans, it is mostly through dermal contact with the soil, direct ingestion, and inhalation of particles that trace elements exert their toxicity (Abrahams 2002; Poggio et al. 2009). Children and elderly people are the social
categories physiologically more concerned by this kind of contamination (Ren et al. 2006; De Miguel et al. 2007). Sources of trace elements are manifold and, as an example, the amounts of metals that can be emitted yearly by traffic in the Netherlands are reported in Table 1. The accumulation of trace elements under various forms in the urban environments has been estimated for some cities. In Stockholm, Sweden, the stock of Cd resulted to be 0.2 kg per capita together with 8 kg Cr, 170 kg Cu, 0.01 kg Hg, 4 kg Ni, 73 kg Pb, and 40 kg Zn (Sörme et al. 2001). In the city of Vienna, the stock of Pb in goods was calculated to be 230 kg per capita and as much as 1,840 kg of lead per square meter of floorspace (Obernosterer and Brunner 2001). More recently, Drakonakis et al. (2007) reported an average 238 kg Cu per capita in the USA. In studying metal stocks in the urban regions of Australia, van Beers and Graedel (2007) estimated Cu and Zn stock spatial densities to be more than 100 times greater than in rural areas. In Table 2, data are collated of the studies on urban areas (their geographical distribution is presented in Fig. 2) published in the last 10 years showing soil uses or location, number of samples, and sampling depth. The soil use in sampling locations is very diverse, reflecting the variety of city landscapes, although parks and roadsides are the most common locations. Although an effort was made to select studies about diffuse contamination, in some cases, it was not possible to separate the effect of a specific source of pollution. The number of samples ranges from a minimum of three to a maximum of 2,182, allowing for very different consistency in the extrapolation of soil properties to the entire city. The same variability is observed for sampling depth which ranges from 1 to 25 cm. It is evident that the dilution effect of surfacedeposited contaminants is very different and comparisons are not immediate. However, a general picture can be obtained for many trace elements. Some interesting data can be derived on the methods used for the extraction of elements from soil samples. Of a total amount of almost 12,000 samples (94 studies), in the majority of cases (25), aqua regia (HCl:HNO3, 3:1 ratio) was used as extractant (2,272 samples in total), followed by HF in combination with other strong acids (13 studies, 1,002 samples in total) and by a mixture HNO3:HClO4 (nine studies, 1,043
Water Air Soil Pollut Table 1 Calculated emissions of trace elements (in tons per year) by car traffic per emission source (based on 6×106 vehicles; modified from Van Bohemen and Van de Laak 2003)
Contamination
Exhaust
Oil leaks
Tires
Brakes 0.004
Radiator
Total
Arsenic
0.17
0.015
0.013
Cadmium
1.2
0.002
0.73
Chromium
1.7
0.014
2.6
0.518
Copper
0.25
0.061
3.65
9.072
50.9
63.9
Lead
240
1.96
0.022
0.072
242
Nickel
1.7
0.007
2.48
0.285
0.192
4.7
Zinc
2.3
1.49
175
0.117
0.168
179
samples in total) or HNO3:H2O2 (seven studies, 995 samples). Other studies used HNO 3 in various concentrations (12 studies, 1,826 samples) or other extraction methods (e.g., H2SO4 + KMnO4; sum of fractions extracted using sequential extraction methods, etc.). A nondestructive detection method (X-ray fluorescence) has also been employed on a large number of soil samples, given its quick response (seven studies, 3,389 samples analyzed). The variability in the extraction methods used in the literature can be a serious constraint in the comparison of published data. In fact, although the term “total” is often used in the studies, data can be assumed to have a 20–30% of variation from the others due to the different extraction techniques adopted. 2.1 Lead Lead has been used as an antiknock agent in gasoline since 1920 and has been one of the major sources of pollution in cities together with lead used in paints. Other sources of Pb are car batteries, glass, radiation shields, and soldering. In recent years, electronic products, e-wastes, have grown as a considerable Pb source (Terazono et al. 2006; Lincoln et al. 2007). Its manifest toxic effects (Järup 2003; Nevin 2007) have prompted a generalized reduction of its use. Unleaded gasoline is now in use in the vast majority of countries (IPIECA—International Petroleum Industry Environmental Conservation Association 2003) and Pb has been banned from paint since 1978. However, its long use and persistence in the environment has concentrated Pb in urban areas and numerous studies have been carried out in cities to investigate the possible toxicological consequences or to simply monitor the content of the soils. The cities for which data are available for this element are shown in Table 3, together with the
0.20 1.9 4.8
relevant reference. Grouping was done according to the limits of 100, 200, and 500 mg kg−1 that represent common limits for the evaluation of soil Pb contamination in Europe (Environment Agency 2002; Ministero dell'Ambiente e della Tutela del Territorio e del Mare 2006; VROM—The Ministry of Housing, Spatial Planning and Environment 2000). In Europe, the lowest concentrations of Pb are reported for Aveiro and Jakobstad that both average 20 mg kg−1. These are small cities (population 53,000 and 175,000, respectively) where traffic does not appear to be a prominent source of pollution. Other cities with a population below 200,000 like Osnabrück, Uppsala, Mieres, Galway, Celje, and Aberdeen show an average Pb content in the topsoil 500 mg kg−1
X X X X X X X X X X X X X X X X X X X
Chicago
USA
Cincinnati
USA
X
Da Nang
Vietnam
Dalnegorsk
Russia
Damascus
Syria
Danang
Vietnam
Detroit
USA
Ermoupolis
Greece
Falun
Sweden
Fuhis
Jordan
X
Gaborone
Botswana
X
Gainesville
USA
X
Galway
Ireland
X
Gibraltar
UK
Glasgow
UK
Hong Kong
China
X
X
Honolulu
USA
X
X
Ibadan
Nigeria
X
Izmit
Turkey
X
Jakobstad
Finland
X
Kampala City
Uganda
Koyang
Korea
La Coruña
Spain
Lagos
Nigeria
Ljubljana
Slovenia
X X X X X X
X X X X
X X
X
X X X X X
Water Air Soil Pollut Table 3 (continued) City
Country
Pb≤100 mg kg−1
Lubbock
USA
X
Madrid
Spain
Manila
The Philippines
Mexico City
Mexico
Miami
USA
Mieres
Spain
Montevideo
Uruguay
Moscow
Russia
Nanjing
China
Naples
Italy
New Orleans
USA
100200 mg kg−1
Pb range≤500 mg kg−1
Pb range>500 mg kg−1
X X X X X X X X X X
Newcastle u.T.
UK
Oslo
Norway
Osnabrück
Germany
X
Ottawa
Canada
X
Palermo
Italy
Pforzheim
Germany
Pueblo
USA
X
Raipur
India
X
Richmond u.T.
UK
Rome
Italy
Salamanca
Spain
X X X
X X
X X X
Sao Paulo
Brasil
X
Seville
Spain
X
Shanghai
China
Shenyang
China
Stockholm
Sweden
Syracuse
USA
X
Tallinn
Estonia
X
Turin
Italy
Turku
Finland
X X X
X X
Uijeongbu
Korea
X
Uppsala
Sweden
X
Valladolid
Spain
Wallsend
UK
X
Warsaw
Poland
Wolverhampton
UK
Wroclaw
Poland
Xi'an
China
Xuzhou
China
X
Zagreb
Croatia
X
X X X X X
Water Air Soil Pollut
the chemical fractionation revealed a strong fixation of Pb by Fe and Mn oxides (Yang et al. 2006). Similarly, the reducible fraction of Pb was observed to be dominant in Hong Kong soils (Wong and Li 2004). Lead was also found to be mainly residual (56.8%) and associated to Fe and Mn oxide (30.9%) in the soils of Nanjing (Lu et al. 2003). El Khalil et al. (2008) have shown that, for samples collected in the city of Marrakech, metals were mainly associated to the fine fraction but the coarse fraction, including technologic material, contained significant amounts of extractable metals. 2.2 Copper This element is widely used for manufacturing and electrical wiring. Electronic equipment is also emerging as a source of Cu (Lincoln et al. 2007; Wong et al. 2007). Therefore, copper tends to accumulate in urban areas. In fact, Drakonakis et al. (2007) have calculated a stock of 144 kg per capita of Cu in the city of New Haven, USA. In Sydney central city, van Beers and Graedel (2007) estimated the copper stock to be 520 kg/capita. Copper, together with Zn, was reported to dominate (14 t/year) the transfer of metals to the biosphere and sewage sludge in Stockholm, Sweden (Bergbäck et al. 2001). Spatari et al. (2005) reported that, in 1999, about 2,790 Gg of Cu were placed in landfills in North America, and according to Bertram et al. (2002), 2 kg of copper waste per capita are produced every year in Europe. The average world soil content is 30 mg kg−1 (Adriano 2001). The majority of cities show a mean or median soil content above this limit (Fig. 3). The cities of Nigeria, Ibadan, and Abuja have very low average content and the range of Cu in Lagos is 0.1– 2.9 mg kg−1. The highest average concentrations were recorded in Turin (90 mg kg−1), Honolulu (136 mg kg − 1 ), and Newcastle upon Tyne (233 mg kg−1). High levels of Cu were measured in the soils of Bradford, UK (range, 35–173 mg kg−1), Moscow, Russia (99–197 mg kg−1), and Montreal (32– 640 mg kg−1). Forty-seven cities have a maximum content which is above 120 mg kg−1, the limit for remediation of residential areas in Italy (Ministero dell'Ambiente e della Tutela del Territorio e del Mare 2006). In particular, extremely high values were observed in Avilés (1,040 mg kg−1), Richmond upon Thames (1,130 mg kg−1), Stockholm (1,315 mg kg−1),
Osnabrück (1,570 mg kg−1), Berlin (1,840 mg kg−1), Jakobstad (2,612 mg kg−1), Wolverhampton (2,750 mg kg−1), Newcastle upon Tyne (12,107 mg kg−1), and Gibraltar (12,500 mg kg−1). Street dusts appear to be important carriers of Cu as 350 mg kg−1 were measured in Amman, Jordan (Al-Khashman 2007) and 1,071 mg kg−1 in Xi'an. In fact, Cu was observed to accumulate in the 30 mg kg−1, in green are cities with average or median Cu 140 mg kg−1, in green are cities with average or median Zn
