Introduction

Trace elements are ubiquitous in our environment, as a result of natural and anthropogenic activities, and humans are exposed to them through various pathways such as inhalation of dust (Christoforidis and Stamatis 2009), food chain (Harmanescu et al. 2011), etc.

Some trace elements such as Zn and Cu are necessary due to their role as metalloenzymes (Harmanescu et al. 2011). According to Food and Drug Administration (FDA) these metals are known as cofactors of a number of enzymes (FDA 2001); however, recommended dietary allowances (RDA) of Zn and Cu for adults are 8–11 and 0.9 mg day−1, respectively, and beyond this range deficiency and toxic effects are observed (FDA 2001; Singh and Garg 2006). Cu surplus had been associated with liver damage (FDA 2001). Furthermore, metal fumes cause fever with flu-like symptoms; also, hair and skin discoloration may be caused by fumes of this trace element, although dermatitis has not been reported. Systematically as well, copper dust and fumes may cause irritation of the upper respiratory tract, a metallic taste in the nausea and mouth (Broyer et al. 1972).

The clinical signs of zinc toxicities have been reported as vomiting, diarrhea, bloody urine, icterus, liver failure, kidney failure and anemia (Fosmire 1990). Also, Zn reduces immune function and the levels of high density lipoproteins (FDA 2001).

Lead is considered to be the most important toxin of trace elements which can be absorbed through ingestion by food and water, and inhalation (Duruibe et al. 2007). Lead poisoning may cause inhibition of the synthesis of hemoglobin; joints and reproductive systems, dysfunctions in the kidneys, cardiovascular system and acute and chronic damage to the central nervous system as well as peripheral nervous system (Ogwuegbu and Muhanga 2005). Based on the previous reports lead poisoning is also associated with damage to the gastrointestinal tract and urinary tract which result in bloody urine, neurological disorder and can cause severe and permanent brain damage (INECAR 2000). Poor development of the grey matter of the children’s brain is caused by lead thereby resulting in poor intelligence quotient (IQ) (Udedi 2003). Acute and chronic effects of lead result in psychosis (Duruibe et al. 2007).

Chromium is also one of the known environmental toxic pollutants in the world. Despite a lot of research on the role of chromium in biological systems, it is still a controversial issue and no significant conclusions can be arrived with the available data. Chromium (VI) enters into the cells readily and is reduced to form stable chromium (III) complexes which react slowly and were considered non-toxic. Nevertheless, recent studies suggest that chromium (III) indeed damages cellular organelles, DNA and proteins and can be lethal to organisms and their offspring (Natesan and Balachandran Unni 2008).

Chromium exposure can cause skin rashes, respiratory problems, kidney and liver damage, upset stomach, ulcers, alteration of genetic material, weakened immune systems, lung cancer and ultimately death (Kabata-Pendias and Pendias 2011; McGrath and Smith 1990).

In spite of our comprehensive knowledge of pernicious and detrimental effects of trace elements on human health, during the last decades, contamination of urban soils by trace elements has gradually become one of the most significant concerns in metropolitan areas (Giannis et al. 2010) and also impose a long-term burden on the biogeochemical cycles in the urban ecosystem (Papa et al. 2010). Extensive use of sewage sludge and waste water for irrigation, pesticide, vehicle exhausts’ emissions, mining and rapid development of industries without effectual control has led to a large accumulation of trace elements in soil (Shi et al. 2009). Street dusts and top roadside soils in urban areas considered as a sign of trace element pollution from atmospheric deposition (Christoforidis and Stamatis 2009). Although leaded gasoline is the main source of Pb, other trace elements such as Cu, Zn and Cd are coming from car components, tire abrasion, lubricants, industrial and incinerator emissions (Markus and McBratney 1996; Wilcke et al. 1998). It is proved that source of Cr in the street dust is corrosion of cars (Ferguson and Kim 1991; Akhter and Madany 1993) and plating of some motor vehicle parts (Al-Shayep and Seaward 2001), respectively.

Comparing the trace element concentration with guideline or quantifying of an accumulation factor (pollution index, PI) with respect to the background concentration is used to evaluate the degree of contamination in the urban areas (Jung 2001).

Despite the presence of numerous industrial factories as well as rapid growth of population in Isfahan, a few studies associated with trace elements contamination in urban topsoil have been carried out. Urban topsoil, as well as agricultural soils, is being threatened by trace elements as a result of rapid industrialization and urbanization during the last two decades. The current study, however, is focused on soil pollution by trace elements in Isfahan, which is attributed to vehicle emissions, industrial discharges, domestic heating and other anthropogenic activities. The aim of the present study is to assess the effects of different land-use as well as human activities on topsoil trace element pollution and also identify the potential risk to human health in Isfahan.

Materials and methods

Study site

The study site is located in the urban area of Isfahan City relatively situated at the center of Iran (Fig. 1) which is the capital of Isfahan Province and an important social-economic center of Iran. The soil of Isfahan is calcareous and its mean annual temperature and average annual precipitation are 14.5 °C and 140 mm, respectively, with a population of 1.6 million. Zayanderood River divides the city into two, northern and southern parts.

Fig. 1
figure 1

Studied area, sampling points and land-use map of Isfahan

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Sampling

Sampling points were chosen from different parts of Isfahan extending from 51°31′25″E to 51°45′32″E longitude and 32°35′N to 32°47′45″N latitude. For this topsoil investigation, uppermost layer of the soil profile (0–1 cm), where industrial dust and emissions from vehicles are deposited, was collected. This indicates that these topsoil textures are characterized by the predominance of finest particle size. Also the background soil samples were collected from 60 cm depth (Morton-Bermea et al. 2009). Sampling points were selected from different land-use to cover the different zones such as parks, green-lands, roadside, crowded places and industrial areas of the city in order to specify and clarify the effects of green spaces or urbanized points on polluting the surface and comparing several points with different usage.

A total of 95 samples were collected in spring 2011, the top 1 cm layer of the soil profile was taken with a plastic trowel, stored in a plastic bag and transferred to laboratory immediately. The coordinates of the sample locations were recorded with a GPS, and the sampling points locations are given in Fig. 1.

Laboratory analysis

Samples were air-dried. About 20 g portions were ground until fine particles (sieved by 200 μm nylon mesh) were obtained (Morton-Bermea et al. 2009). In order to measure the trace elements’ concentrations (Pb, Zn, Cu, Cr), 2 g of each sample were digested by nitric acid (4 M) in water bath at 80 °C (Sposito et al. 1982). Afterward, trace elements’ concentrations were determined by atomic absorption spectrophotometer Perkin-Elmer 3030.

Statistical analysis and pollution index calculation

Descriptive statistics including the mean, standard deviation, minimum, maximum, median and range were determined and PI was defined as trace element concentration in the sample (topsoil)/background median value. Analysis of the data was done with SAS statistical software v 9.1.

Geo-statistical analysis based on GIS

Spatial interpolation and GIS mapping (kriging) techniques were employed to produce spatial distribution maps for the four observed trace elements by ArcGIS v.10. (Tao 1995; Cattle et al. 2002; Yasrebi et al. 2009). Regarded as mirror of pollution rate in surface soil compared to background, pollution index (PI) was used for depicting geochemical maps. The PI was defined as the value of the pollution index of each element, calculated by the use of the ratio of the trace element concentration in topsoil to the background concentration of the corresponding trace element as the following formulation (Lu et al. 2008; Faiz et al. 2009; Wei et al. 2009; Morton-Bermea et al. 2009)

$$ {\text{PI }} = \, C_{i} /B_{i} $$

where C i is the concentration of element in topsoil, B i is the background value.

Results and discussion

Land-use study

As shown in Fig. 1, most of the green-lands are located in west, southwest, east and also river bank, while the gigantic highways which connect the north of Iran to south are located in northwest, north and south. The northwest of the city is also considered as the greatest industrial area of Isfahan Province. Most of the urban fabrics, traffic load and commercial centers are seen in central parts of Isfahan.

Statistical study

Concentration of analyzed trace elements (Pb, Zn, Cu and Cr) and their geographical coordinates in urban topsoils of the studied zone are shown in Table 1.

Table 1 Concentration of trace elements in the analyzed soil samples
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Table 2 represents minimum, maximum, median and standard deviations for each trace element and intervention guideline, dictated by Canadian Council of Ministers of the Environment (CCME 2007).

Table 2 Minimum, maximum, median, mean, standard deviations and maximum acceptable concentration (MAC) of each analyzed trace elements in samples and background
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Based on the obtained results, maximum values of Zn, Pb, Cu and Cr were 651.6, 215.4, 144 and 110 mg kg−1, respectively, while minimums were 38.4, 17.4, 20.8 and 29.5 mg kg−1. Maximum values of all studied trace elements in samples were higher than the selected standards. With respect to Zn, Pb, Cu and Cr concentrations, about 36.84, 15.79, 38.95 and 40 % of data were larger than maximum acceptable concentration (MAC). None of the means, however, was larger than MAC. Mean concentrations of the analyzed trace elements in urban soils were 196.7, 84.2, 59.9 and 61.1 mg kg−1, respectively, for Zn, Pb, Cu and Cr, which exceed their background values in all sites. Concentration ranges of Zn, Pb, Cu and Cr were observed to be 613.2, 198, 123.2 and 80.5 mg kg−1, respectively (Table 2). Zinc showed the highest range of concentration while the lowest range belonged to chromium. Medians as well as mean values for the all trace elements, were more than background. Wei and Yang (2010) reported that concentration of trace elements in urban soils of all studied cities in China exceed their background values. It can be attributed to the role of human activities in increasing trace element concentrations (Morton-Bermea et al. 2009).

Pollution index and geochemical maps

Figures 2 and 3 represent the values and limitations of four evaluated trace elements in several sampling points based on the PI values and concentration of them. The PI was classified as: PI ≤ 1 low level of pollution; 1 < PI ≤ 5 moderate level of pollution and 5 < PI high level of pollution (Morton-Bermea et al. 2009).

Fig. 2
figure 2

a Pollution index (PI) values for Pb and Zn in the studied zone. PI values were calculated as ratio between sample metal concentration and background median. b Pollution index (PI) values for Cu and Cr in the studied zone. PI values were calculated as ratio between sample metal concentration and background median

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Fig. 3
figure 3

Spatial distribution of trace elements (mg kg−1) in the studied zone

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In comparison with the other trace elements, Pb allocated the largest value of PI to itself, varied from 0.9 to 10.8. All sampling points with high level of pollution (PI > 5) are located in the center, south and northwest of the city, where the highest amount of vehicle traffic load and industrial activities can be seen. Other scientists had also reported that maximum amounts of lead were measured in central parts of cities (Hernández-Álvarez 2001; STV Gobierno del Distrito Federal 2008; Morton-Bermea et al. 2009). Incontrovertibly, lead has been partly derived from tetraethyl lead, used as an additive to gasoline, released by vehicles into the environments so that more urban areas in the vicinity of highways have a critical lead concentration in topsoil. As shown in Fig. 3 the highest concentration of Pb was also measured in northwest, north and central part of the city where human health is threatened by severe symptoms including anemia, a decrease handgrip strength, pale skin, abdominal pain, nausea and paralysis of the wrist joint, etc. On the other hand, continued exposure can lead to decreased fertility and/or increased risk of miscarriage or birth defects (Zheljazkov and Jekov 1994).

It is obvious that minimum amounts of PI and concentration of Pb were found in the west and south west of Isfahan. Based on Fig. 1 these parts of the city belong to green-lands and river bank with strict traffic rules. Studying some of China’s urban topsoil and road dusts, Shi et al. (2008) reported that traffic contaminations are one of the major sources of trace elements in urban environment.

PI values for Zn in different urban topsoil varied from 0.6 to 10.2. Maximum values of PI for Zn are found to be in south, north and northwest. Although the number of PIs which is classified in a high level of pollution (PI > 5) is less than those for lead, comparison between Figs 1 and 3 revealed that the maximum concentration of Zn was measured in the vicinity of industrial factories located in north and northwest, and also the north and south entrance highways of Isfahan. Lower Zn concentration in center compared to the north and northwest suggests that concentration of Zn in topsoil is more relative to the industrial activity than vehicle traffic load. Nevertheless, concentrations of zinc in the central parts of Isfahan with high traffic load were higher than its concentration in the west and southwest (dense green-lands), which can prove the role of vehicles’ emissions or abrasion to increase concentration of Zn. Despite the fact that Zn is an essential element for plants, animals and human, prolonged exposure to this metal in the contaminated part of the city may cause vomiting, icterus, bloody urine, kidney failure, anemia and liver failure, etc. (Fosmire 1990). Similarly to lead, lowest concentrations of Zn were also found in the west and southwest of Isfahan, which reminds the role of green-lands and restrictive rules of traffic. Many scientists believe that the sources of Pb, Zn, Cu and Cr in urban areas are mainly derived from industrial activities and traffic emission (Ferguson and Kim 1991; Akhter and Madany 1993; Markus and McBratney 1996; Wilcke et al. 1998).

PI values obtained for Cu and Cr are more stable with the ranges of 0.9–6.8 and 1.4–5.2, respectively. Indeed, most of the selected points have a moderate PI (1 < PI < 5) for Cu. There were just a few places in industrial and over-crowded parts of the city (north, northwest and center) that showed acute values of Cu PIs. Among the analyzed samples, only one point was found for PI < 1 for Cu which, as well as other trace elements, is situated in the western green-land regarded as the lung of Isfahan. Figure 3 indicated that the pattern of distribution of Cu in the urban topsoil of Isfahan was approximately similar to that of Zn, which means risk of the liver damage (FDA 2001), hair and skin discoloration, irritation of the upper respiratory tract, etc. (Broyer et al. 1972), are higher in north and northwest, and also north and south entrance highways of Isfahan than in the central parts and green places.

As can be seen in Fig. 2, there is just one point with high PI for Cr in downtown, while none of the sampling points showed low PI (PI < 1). According to Fig. 3 most of the central, north and northwest areas have a concentration between 62.10 and 68.22 mg kg−1 which are near the MAC (64 mg kg−1) and may cause respiratory problems, alteration of genetic material, weakened immune systems, etc., for humans (Kabata-Pendias and Pendias 2011; McGrath and Smith 1990). Lowest concentrations of Cr were measured in northeast, south and southwest where there are no significant vehicle traffic load and industrial activities as well.

Conclusion

Based on the obtained PIs, approximately all concentrations of the determined trace elements in the urban soils of Isfahan are higher than their background values and the maximum concentrations of Zn, Pb, Cu, and Cr in soil samples exceed the MAC. Finding the lowest quantities of analyzed trace elements in green-lands and parks with strict vehicle traffic rules on one hand and the highest amounts in highways, industrial areas and city center with highest traffic load on the other hand emphasizes the negative role of vehicles’ traffic load and industrial activities in polluting the urban areas. In other words, the poor management of industrial development, urban construction and traffic load in Isfahan has seriously endangered the public health in most of the residential areas. It is suggested that effective monitoring of industrial activities and fuel quality as well as adoption of laws to reduce the traffic load may decrease the quantity of pollutants and trace elements’ poisoning in studied areas.