Document Type : Research Paper
1- Assistant Professor in Department of Soil Science, Ahvaz Branch, Islamic Azad University, Ahvaz, Iran
Environmental investigation of heavy metals concentration in Ahvaz city street dust, by using Geographical Information Systems (GIS)
Rapid urbanization and continuous demand for land for infrastructural development in urban areas have placed great stress on the local environment. Consequently, varieties of environmental problems have emerged, which among them toxic metal pollution is a major issue, especially in urban soil and street's dust. Street's dust receives varying inputs of heavy metals, mineral constituents, organic matter (humus), living organism, air, and water; the anthropogenic materials are vehicle exhaust particles, lubricating oil residues, tire wear particles etc…; and the natural biogenic materials are tree leaves and other plant matters. To certain degree, street's dust is a more pertinent indicator to urban environ-mental quality than single compartmental monitoring of air, water and soil, because it reflects pollutants from the different sources. Heavy metals may come from many different sources in urbanized areas, including vehicle emissions, industrial discharges and other activities. It is important to identify the origin and distribution of heavy metals in street dust, and estimate population from heavy metal exposure via street's dust in smelting district. There were many recent investigations on Heavy metals from many different sources in urbanized areas, including vehicle emissions, industrial discharges and other activities. Ahvaz, a metropolis city located in Southwest Iran, with a population of over 1.2 million, has experienced a rapid urbanization and industrialization in the last few decades. Industrial growth along with expansion of population and increase in number of vehicles in Ahvaz caused increase of heavy metals accumulation in airborne particles and urban soils. Ahvaz city is considered as one of the heavily polluted cities in the world. However, the spatial distribution patterns and contamination levels of heavy metals in road dust in the area is still not clear.
The aim of this paper is to: 1) identifying the patterns of spatial distribution of Cd, Cr, Cu, Pb, As and Zn; 2) assess contamination levels of these metals by integrated pollution index (IPI) and Nemerow integrated pollution index (NIPI). Multivariate statistical methods and spatial analyses were used to achieve these goals. Geographic Information System (GIS) mapping was applied to evaluate the results by visualizing the spatial patterns.
Material and Methods
Soil sampling and analytical methods
A total of 115 street's dust samples were collected form urban area in July 2014 when it was dry season. The sampling compagain was chosen in driest month of the year to avoid rain-washing out the heavy metals. The weather condition was stable during the sampling period and no rain had occurred during one month prior to sample collection. The street dust samples were mainly collected by sweeping an area of about 1×1 m2 from road pavement using a clean plastic dustpan and brushes for each sampling site. The sampling points and background of samples locations are marked in Fig. 1. Geographical coordinates of samples collection locations were recorded at each sampling point with a GPS device. The streets' dust samples were analyzed for toxic metals by Atomic Absorption Spectrophotometer (AAS).
Fig.1. location of samples sites in Ahvaz city
Metal pollution index
PI was calculated for all the six elements under study and the minimum, maximum, and mean values of PI are given in Table 4. The pollution index IPI is defined as the mean value of the pollution index PI of an element. It is classified as. Non-pollution (PI≤1), low level pollution (1<PI≤2), moderate level of pollution (2<PI≤3) and high level of pollution (PI>3). The NIPI of the six metals for each sampling site was defined as follows:
The NIPI was classified as: non-pollution (NIPI<0.7); non pollution (NIPI≤0.7); warning line of pollution (0.7<NIPI≤1); low level pollution (1<NIPI≤2); moderate level of pollution (2<NIPI≤3) and high level of pollution (NIPI>3).
Results and Discussion
The mean concentration of Pb, Cu, Zn, Cr, As and Cd concentrations in the street dust samples were 179.75, 179.60, 150.15, 101, 67.27 and 5.60 mg/kg respectively, and they were 5.4, 12.7, 5.3, 1.1, 21.6 and 62.2 times as high as the background values in street dust samples. Table )1( compares the concentration of heavy metals measured in road dusts of metropolis city of Ahvaz with other metropolitan cities in the world. Concentrations of heavy metals in street dust particles vary considerably among cities depending on the density of industrial activities in the area and technologies employed. As summarized in Table 1, the mean concentration of Cu in Ahvaz street dust (present work) is lower than mean concentration of Guangzhou, Baoji, Ottawa, Calcutta, Luanda, Oslo and Nanjing cities and higher than Tehran city. Pb concentration in Ahvaz street dust is higher than Oslo and Nanjing and lower than Guangzhou, Baoji, Ottawa, Calcutta, and Tehran. The mean concentration of Zn in Ahvaz is higher than Tehran, Ottawa, Calcutta, Baoji and lower than Oslo, Guangzhou and Nanjing. The mean concentration of Cr is lower than Baoji and Nanjing higher than Tehran, Guangzhou, Ottawa, Calcutta, and Oslo. The mean concentration of As in street dust of Ahvaz is higher than other metropolitan cities. The mean concentration of Cd in street dust of Ahvaz is higher than other metropolitan cities except Tehran. The range of PI values for all the elements under consideration were determined, and their behavior was found to be as follows: 0.31 to 17.32 for Pb, 1.3 to 15.29 for Zn, 0.28 to 7.69 for Cu, 0.2 to 22.5 for As, 0.7 to 2.06 for Cr and 0.05 to 9.45 for Cd.
Spatial distribution map of PI indicated that there are several clear trends in the distribution of the PI values in the studied area. In the old urban area, most of the street dust samples collected were from the areas with high levels of pollution, which can significantly attributed to traffic emission and long-term accumulation of heavy metals. On the contrary, most of the streets dust samples collected with low levels of pollution were from the new urban areas and city suburban areas. Moreover, the areas closed to manufacturing companies were with high levels of pollution. These trends can be attributed to urbanization, distribution of industrial and commercial areas. The NIPI of all of the Samples collected varied between 0.71 to 59.01 with an average of 9.66. Assessment of geochemical data indicates that there were 24 soil samples collected (20.8% of samples) with an NIPI<0.7, while 26 soil samples collected (22.6%) of all soil samples had a NIPI between 0.7 and 1. About 16.5% of samples had a NIPI between 1 and 2 and about 16.5 of all samples had a NIPI between 2 and 3. Finally 28 soil samples (24.3%) had NIPI>3 with high level of pollution. Fig. 2 shows the spatial distribution of NIPIs in Ahvaz city. Overall, these findings suggest that the street dust of Ahvaz city has been polluted by anthropogenic emission. Soil samples with high and moderate pollutions were located in area with high dense pollution, high traffic volume, manufacturing industries such as smelting, chemical industry, industrial towns, and oil drilling activities.
Fig.2. Spatial distribution of NIPI in the study area
The present study examines the content of metals in the urban soils in Ahvaz city. The mean concentration of heavy metals were significantly higher than the other cities. The results of spatial distribution reflect the influence of urbanization and industrialization on the areas considered. The PI values indicate that a significant degree of metal pollution exist in some street dusts within the urban area with high population density, high traffic volume and also areas with high industrial activities such as oil drilling. Then NIPI values also indicate that Ahvaz street dusts have high degree of pollution. These finding indicate that more attention should be paid to metal pollution of the urban street dust and urban topsoil’s in Ahvaz.