Bull Environ Contam Toxicol (2014) 93:710–715 DOI 10.1007/s00128-014-1408-1
Spatial Distribution of Mercury in the Surface Soils of the Urban Areas, Arak, Iran Eisa Solgi • Abbas Esmaili-Sari • Alireza Riyahi-Bakhtiari
Received: 13 January 2014 / Accepted: 17 October 2014 / Published online: 26 October 2014 Ó Springer Science+Business Media New York 2014
Abstract This study assessed the baseline concentrations and spatial distribution of total mercury (Hg) in urban soils of the city of Arak, Iran. Concentrations of Hg were determined in soil collected from urban areas, and the spatial distribution was analyzed using the semivariogram approach in geostatistical technology. Mercury in soil ranged from 66.3 to 581 lg/kg. The experimental variogram of soil mercury concentrations was best-fitted by a spherical model. A spatial distribution map revealed that Hg concentration showed decreasing trends from south to north, west to east and center to suburb. Overall, the results showed that Hg concentrations in urban soils of Arak may be considered medium or low. Keywords Total mercury Urban soils Spatial distribution Geostatistics Arak City It is important to establish contaminant levels that are normally present in soils to provide baseline data for pollution studies. Mercury (Hg) and its compounds are potent environmental pollutants which are widely recognized as highly toxic to all of the components of the biosphere. Mercury has been considered as a global pollutant due to its high toxicity, persistency in the environment, and ability to undergo long distance transportation in the atmosphere
E. Solgi (&) Department of Environment, Faculty of Natural Resources and Environment, Malayer University, P.O. Box 65719-95863, Malayer, Hamedan, Iran e-mail:
[email protected];
[email protected] A. Esmaili-Sari A. Riyahi-Bakhtiari Department of Environment, Faculty of Natural Resources and Marine Science, Tarbiat Modares University, P.O. Box 46414-356, Noor, Mazandaran, Iran
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(Liu et al. 2012). Mercury is a ubiquitous pollutant, which enters the environment from both natural and anthropogenic sources (Hojdova et al. 2008). Natural sources include volcanoes, soil erosion and oceans, whereas anthropogenic sources are various and have changed with time (Grangeon et al. 2012). The main anthropogenic Hg contamination sources are fuel combustion, waste incineration, wastewater discharges and industrial processes such as chlor-alkali plants. Historically, and still in developing countries, the chlor-alkali industry is a significant source of atmospheric Hg emissions and direct releases into aquatic systems (Remy et al. 2003). Soils are very important pools in the global biogeochemical cycle of Hg, acting both as source and sink (Gillis and Miler 2000). Globally, Hg concentrations in the soil range between 0.01 and 0.2 mg/kg, with an average of 0.03 mg/kg (Santos-France´s et al. 2011). The results of a study by Wang et al. (2003) demonstrated significant positive correlations between atmospheric and soil Hg concentrations in urban and suburban districts in China, with soil Hg concentrations showing a trend of decreasing concentration with increasing distance from emission sources. Soils in urban environments differ from natural and semi-natural ones because they are strongly influenced by humans. Anthropogenic activities are reflected by an alteration of soil formation processes, by a variation of their physical–chemical properties and by the presence of contaminants. Urban soils are important indicators of the urban environment and a primary sink for contaminants such as Hg, because anthropogenic metals and organic contaminants are usually deposited on topsoils. The accumulation of contaminants in urban topsoils not only degrades soil quality, but can also pose a health risk to humans and the ecosystem (Lu et al. 2009). Therefore, it is necessary to better understand the level of contamination
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Fig. 1 Location of the study area and sampling point distribution
and distribution of Hg in urban area (Chen et al. 2010). Several studies have reported on Hg contamination in urban soils (Fang et al. 2004, 2011; Rodrigues et al. 2006a; Chen et al. 2010). To date, no work has been undertaken in the city of Arak to examine the Hg contamination in its soils. The objectives of the present study were to provide a basic understanding and of Hg concentrations in urban soils of Arak and, to investigate its spatial distribution in the city.
Materials and Methods The study area, Arak, Iran (N 34°030 49.3500 –34°070 50.2700 and E 49°390 12.1600 –49°460 35.4300 ), is located in the Markazi Province, and is one of the industrial centers of the country. Arak is a city with about 160 years of history, and has a population of about five thousand. The climate of this region is cold and semiarid, with an annual mean temperature of 13.8°C and annual mean precipitation of 320 mm,
according to Emberger Climate classification (Vafaei et al. 2007). A total of 62 topsoil samples (depth = 0–20 cm) were collected from an area of about 67 sq km within the city of Arak in the spring of 2011 (Fig. 1). Soil samples were obtained from urban soils of five different land use categories: public parks (n = 28), green space (n = 12), garden/agriculture (n = 13), roadside (n = 6) and squares (n = 3). At each sampling point, 5–9 subsamples were randomly taken and then combined into a composite sample for the site (Fig. 1). The soil samples were stored in polyethylene bags and coordinates of sampling site locations were recorded using a portable global positioning system instrument. Prior to analysis, soil samples were airdried at room temperature (25°C) and stones or other debris were removed. The dried samples were sieved using a 2 mm mesh sieve. In the next stage, soil samples were ground and sieved through a 0.149-mm sieve. Soil pH was determined using a portable pH meter in a 1:5 soil–water suspension.
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The analysis of total Hg concentrations was performed using a LECO AMA 254 Advanced Mercury Analyzer (St. Joseph, MI, USA) according to ASTM standard method no. D-6722. No sample digestion was involved in this process. Analyses were performed directly on soil samples. This technique has particular advantages for soil analysis, as determinations are performed directly on solid samples without sample pre-treatment or digestion, avoiding mercury losses and contamination during digestion. It is also free from matrix interferences (Rodrigues et al. 2006b). In order to test the accuracy of the system, three standard reference materials of National Institute of Standard and Technology (NIST, 1633b coal fly ash, 2709 San Joaquin soil, and 2711 Montana soil) were used. The obtained recovery was between 99.1 % and 105 %. The detection limit was 0.001 mg/kg in dry weight. Statistical analyses were conducted by use of SPSS 17.0 software (IBM, Armonk, NY, USA). Descriptive data analysis, including mean and median contents, standard deviation, minimum and maximum concentrations, skewness, etc., were carried out. Data were tested for normality using the Kolmogorov–Smirnov test. The concentrations of Hg in topsoils were tested for land use differences using a Kruskall–Wallis sum rank test. The Pearson coefficient correlation test was used to analyze the relationship between mercury and pH. Geostatistics was applied in investigating and mapping soil metals (Liu et al. 2006). Geostatistics is based on the theory of a regionalized variable, which is distributed in space (with spatial coordinates) and shows spatial auto correlation such that samples close together in space are more alike than those that are further apart. Geostatistics uses the variogram (or semivariogram) technique to measure the spatial variability of a regionalized variable, and provides the input parameters for the spatial interpolation of kriging (Webster and Oliver 2001). The variogram function is expressed as: 1 X fZðxiÞ Zðxi þ hÞg2 2NðhÞ j¼1 NðhÞ
c ð hÞ ¼
ð1Þ
where c (h) is the semivariance; N(h) is the number of experimental pairs separated by a distance h; Z(xi) is the measured sample value at point i and Z(xi ? h) is the measured sample value at point i ? h. Information generated through the variogram was used to calculate sample weighting for spatial interpolation by a kriging procedure, using the nearest 16 sampling points and a maximum searching distance equal to the range distance of the variable (Lark and Ferguson 2004). Variogram plots were acquired by calculating variograms at different lags. A spherical model was selected in order to acquire information about the spatial structure as well as the input parameters for kriging interpolation.
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The spherical function is: 8 s > < C þ C 3h h ; 0 2a 2as cðhÞ ¼ > : C þ C;
ð2Þ
0
where C0 is the nugget variance (h = 0), which represents the experimental error and field variation within the minimum sampling spacing. Typically, the variogram increases with increasing lag distance to attain or approach a maximum value or sill (C0 ? C) almost equivalent to the population variance, i.e., priori variance. C is the structural variance and a is the spatial range across which the data exhibit spatial correlation. All geostatistical analyses were carried out using GS ? geostatistical software, version 5.0 (gamma Design Software, LLC, Plainwell, MI, USA) The distribution map was produced with GIS software (ArcGIS 9.3 by ESRI).
Results and Discussion A summary of the descriptive statistics for the data sets of Hg concentrations in the soil samples is given in Table 1. The Hg concentration ranged from 66.3 to 581 lg/kg with a mean value of 102 lg/kg. Mercury concentrations in urban soil had obvious differences: the maximum value was 8.7 times the minimum value. The median Hg concentration was 88.7 lg/kg. The pH values ranged from 6.35 to 8.96 (average: 7.98) which suggests neutral to subalkaline conditions for all the soil samples. Correlation analysis showed that there was no significance relationship between total Hg and pH. The lowest Hg concentration (66.3 lg/kg) was observed in the north region of Arak, while the highest concentration (581 lg/kg) was recorded in the west. Shazand Petrochemical Complex (SPC) is a well known chemical complex located 30 km southwest from Arak City. This complex includes several chemical industries. As winds from the south and southwest directions are common, it might be expected that Hg levels in Arak would increase due to these industrial activities. Table 1 Total Hg concentration in urban soils in Arak Hg concentration (lg/kg) Min
Max
Mean
Total
66.3
581
102
88.7
64
7.14
Public park
75.1
136
94
85.6
17
1.32
Green space
66.3
118
Garden/ agriculture
88.1
581
84.8 140
Median
82.8 109
SD
14.1 133
Roadside
68.7
115
91.1
91.2
15.3
Square
74.8
113
95.1
96.9
19.5
Skewness
1.31 3.66 0.18 -0.4
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Mercury can be transported and deposited to remote places even 1,000 km away from the sources (Johansson et al. 2001). The normal range of mercury concentration in soils is 10–500 lg/kg (Alloway 1995). Using 4,090 soil samples, Wu et al. (1991) established a Hg level of 300 lg/kg as the threshold value at which toxicity symptoms may occur. In this study, Hg concentrations in samples were not as high as this value, except for one agricultural soil sample. The mean value of Hg concentrations found in Arak urban soils is lower than the USEPA soil screening level for ingestion of inorganic Hg of 23 mg/kg (USEPA 1996), as well as the Canadian soil quality guideline for agricultural soils (6.6 mg/kg) and the UK maximum permissible concentration of Hg in agricultural soil of 1.0 mg/ kg (Appleton et al. 2006). In comparison with other cities around the world (Table 2), the median Hg concentration of this study was lower than median/mean concentrations in Amursk, Russia (Kot and Matyushkina 2002), Aveiro, Portugal (Rodrigues et al. 2006a, b), Berlin, Germany (Birke and Rauch, 2000), Xuzhou, China (Wang and Qin 2005), Beijing, China (Chen et al. 2010) and Jakobstad, Finland (Peltola and Astrom 2003). Meanwhile, it was higher than that in Palermo, Italy (Manta et al. 2002), Khabarovsk, Russia (Kot and Matyushkina 2002) and Oslo, Norway (Tijhuis et al. 2002). The median Hg concentration in this study (0.088 mg/kg) was higher than the worldwide median of 0.05 mg/kg (Reimann and Caritat 1998). Therefore Hg concentration in urban soils of Arak may be considered medium or low. With regard to the obtained results from Table 1, Hg concentrations in five land use classes followed the sequence: garden/agriculture [ squares [ roadside [ public parks [ green space. The mean Hg levels in the garden/ agriculture category (140 lg/kg) was greater than levels in the other four types of land use (84.8–102 lg/kg). The Kolmogorov–Smirnov (K–S) test showed that results for Hg Table 2 Comparison of mercury concentrations in urban soils in different cities (mg/kg)
did not have a normal distribution. The Kruskal–Wallis test was applied to the data for mean comparisons in order to statistically evaluate the variations observed in five different land uses. Because the Kruskal–Wallis test was significant (p \ 0.05), a Mann–Whitney U test was performed. The results showed that the differences of Hg concentration between the garden/agriculture and public parks and green spaces were statistically significant (p \ 0.05), while there were no significant differences among the garden/agriculture, squares and roadside. The difference between garden/ agriculture and public parks was statistically significant (p \ 0.05), as well as the difference between garden/agriculture and green spaces. The high levels of Hg in garden/ agriculture soils is probably due to the use of Hg in pesticides such as fungicides or mildewcides. Also the application of phosphorus fertilizers may result in an increase in soil Hg levels (FIFA 2006) due to the presence of Hg as an impurity. In all of the land uses surveyed, the collected samples from the west and southwest had higher Hg concentrations than other samples. With the prevailing winds arriving from the direction of the SPC, it was speculated that this complex contributed to the higher Hg concentrations in the soil here. Geostatistics was used for mapping soil Hg. The attributes of the semivariogram for Hg are summarized in Table 3. Based on the results of variography, soil Hg data were fitted with a spherical model. The values of r2 showed that the semivariogram models gave good descriptions of spatial structure of soil Hg. The ratio of nugget to sill Table 3 Geostatiscal model parameters for soil mercury in the study area Model
Spherical
Range (m)
6,980
Nugget (C0)
0.10
Sill (C0 ? C)
1.49
C0/(C0 ? C)
0.06
Prediction errors r2
RSS
0.97
0.04
RSS residual sum of squares
City
Median/mean
Range
References
Soil world median
0.05
–
Reimann and Caritat (1998)
Beijing, China
0.30
0.022–9.4
Chen et al. (2010)
Palermo, Italy
0.68
0.04–6.96
Manta et al. (2002)
Changchun, China
–
0.139–0.479
Fang et al. (2004)
Khabarovsk, Russia
0.080
0.011–0.950
Kot and Matyushkina (2002)
Amursk, Russia
0.175
0.004–0.464
Kot and Matyushkina (2002)
Aveiro, Portugal
0.091
0.015–0.50
Rodrigues et al. (2006a, b)
Berlin, Germany Jakobstad, Finland
0.19 0.093
– 0.011–0.093
Birke and Rauch (2000) Peltola and Astrom (2003)
Oslo, Norway
0.06
0.010–2.30
Tijhuis et al. (2002)
Xuzhou, China
0.18
0.02–1.3
Wang and Qin (2005)
Arak, Iran
0.088
0.066–0.581
This study
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Fig. 2 Spatial distribution map of mercury in urban soils of Arak
(RNS) can be used to express the extent of spatial autocorrelations of soil metal concentrations. The ratios of \0.25, 0.25–0.75, and [0.75 were used to describe the proportion of the spatial structure that showed strong, moderate, and weak spatial autocorrelation, respectively (Cambardella et al. 1994). As observed in Table 3, a low RNS (\0.25) indicates the strong spatial autocorrelations of soil mercury contents. To some extent, this indicator reflects predominant factors which impact the spatial variability of soil metals between intrinsic factors (natural factors, such as soil parent materials) and extrinsic factors (anthropogenic factors, such as agricultural practices). In general, weak spatial dependence of soil metals can be ascribed to extrinsic factors, and strong spatial dependence can be ascribed to intrinsic factors (Cambardella et al. 1994). The spatial distribution patterns for Hg levels in urban soils of Arak City (Fig. 2) showed a decreasing trend from the center of the city to the suburbs, which is in agreement with a study by Chen et al. (2010). Also regularly, higher concentrations of Hg in city centers were mentioned for other urban areas (Linde et al. 2001; Tijhuis et al. 2002). As an example, the gathered soil samples from the city center of Stockholm presented a higher Hg concentration with a mean of 860 ± 960 ng/g. Concentrations in the center of Stockholm were approximately 50 times higher than in the rural arable soils surrounding the city (Linde et al. 2001). In Oslo, Norway (Tijhuis et al. 2002) it was observed that the median Hg level in soil from the center of the city was 480 lg/g, 8 times higher than the rest of the city. Several studies have suggested that automobiles are possible sources of atmospheric Hg. Sediment samples
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collected in water bodies throughout the US showed a positive correlation between proximity to vehicular traffic sources and elevated Hg concentrations (Rice 1999; Callender and Rice 2000). Although the amount of Hg from automobiles is not as great as that from coal-fired power plants or various types of waste incinerators, they are still a source of Hg emissions (Won et al. 2007). This atmospheric deposition may be a source of environmental contaminants such as trace metals and toxic organic compounds that are finally deposited on soil surface. Field investigations and experimental simulations were carried out for understanding the accumulation and transformation of Hg in soil in relation to the deposition of atmospheric mercury. A positive correlation between the atmospheric Hg concentration and the content of Hg in soil was observed in the field investigation (Wang et al. 2003). Thus, most of the mercury burden in urban, rural and remote landscapes is due to atmospheric transport and deposition of anthropogenic emissions (Miller et al. 2005). The hotspots existed in the west and center of Arak. The results obtained in this work increase our knowledge of Hg contents and their possible sources in the urban soils of Arak. Values of Hg in the study area had obvious spatial differences with the maximum concentration being about nine times greater than the minimum concentration. In general, the concentration of Hg in urban soils of Arak was considered to be in the mid-range compared with other cities around the world. Mercury concentrations with different types of land use followed the sequence: garden/ agriculture [ squares [ roadside [ public parks [ green space. Geostatistical analysis was applied to understand the Hg pollution and spatial relationships. Soil Hg concentrations showed a decreasing trend from south to north, from
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west to east, and from center to suburb. Based on our results, SPC appears to have a significant impact on soil Hg concentrations. The Hg baseline concentrations and the median value of the regionally distributed soils are slightly higher than reported for soil Hg worldwide, suggesting long-term atmospheric input or more recent local atmospheric input from industrial activities. Acknowledgments The authors gratefully acknowledge funding provided for this study by the Tarbiat Modares University of Iran. Also the authors are grateful to Dr M. Solgi (Head of Department of Horticulture at Arak University) for his support and assistance in this research.
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