Evaluation of Soil Contamination by Heavy Metals around Fasa-Darab Road

Automobiles are generally one of the major sources of heavy metal pollutants in roadside sediments that enter the environment as particles from the exhaust or other components of the vehicle and pollute the soil and vegetation of these areas (Van Bohemen, 2003). The most common heavy metals in vehicles on the road are Cd, Cu, Pb, Ni and Zn (Elik, 2003; Li, 2001). These heavy metals have a toxic effect on the human body and cause many complications such as blood, nervous and bone diseases (Sarkar, 2002). So, in this study, the amount of heavy metal changes in sediments of road margins Darab-Fasa road was investigated. Al-Chalabi and Hawker (2000) collected samples from three-site soil in Brisbane, Australia, and studied the distribution of Pb around roads. The results showed that vehicle traffic was the main source of pollution, and in areas with more stable climatic conditions, the distribution of Pb in soil was significantly reduced by increasing the distance from the road. The study also found that Pb is accumulated in the first five cms of soil depth. Fakayode and Owolabi-Olu (2003) studied the soil contamination of the streets of Osgbo, Nigeria. By selecting 39 sites with different traffic volumes, they measured the distribution of zinc, nickel, copper, cadmium and lead metals at distances of 5, 15, 30 and 50 meters from the streets. The results showed that with increasing distance from the streets (distance of 50 meters), the concentration of soil metals decreases. Given the importance of the subject, this study investigates the soil contamination to heavy metals in the margins of Darab-Fasa road. In this study, the effect of distance to gas station and its effect on soil pollution was investigated. For this purpose, index of mular and contamination factor (CF), ecological risk index (RI), tolerance contamination index (PLI) were calculated and measured.

The study area is located in the south of Fars province (Darab-Fasa road) at a geographical location of 28 ° 36΄-28 ° 54΄ N and 53 ° 36΄ - 54 ° 30΄ E. Fasa-Darab road with a length of about 100 km has long been considered as the main road connecting Shiraz (Iran's largest city in the southern half of the country) to Bandar Abbas (Iran's largest commercial port). This road has always been used by a variety of light and heavy cars. The cities of Fasa and Darab are the most important demographic centers in the southeast of Fars province, and have a long history and their route is known as one of the busiest roads in Fars province.

For this purpose, soil samples were collected from distance of 2.15 m from two depths of 0-15 cm in a randomized complete block design. Total and absorbable amounts of heavy metals were measured and read with atomic absorption spectrophotometer. Contamination factor (CF) was used to determine soil pollution to heavy elements. Based on this factor, the amount of elements can be measured in relation to its normal value and determine the degree of soil pollution. The contamination factor was calculated according to the following equation for all elements of the study (Hakanson, 1980).  (1) where CFmetal is the ratio of the concentration of each metal (Cmetal) to the concentration of the natural background of that metal (Cbackground). In order to assess the state of soil contamination with heavy metals, the PLI index for the 5 metals under study was calculated. The index, the number of times the concentration of heavy metals in soils and sediments has increased due to pollution and shows a summary of the toxicity of metals (Chan, ., Ng, , Davis, , Yim & Yeung. 2001). The PLI, which is used to study the environmental situation, is defined as the nth root of the multiple concentration of the various metals studied (Angulo, 1996): (2) where CFmetal represents the contamination factor of each metal and n is the number of heavy metals studied. The RI index has been used by various researchers such as Wang, Z., Wang, Y., Chen, L., Yan, C., Yan, Y., & Chi, Q.(2015). based on the toxicity of metals. According to Hakanson (1986), the toxicity factor for Cd, Zn, Pb, Cr and Zn metals is 30, 5, 5, 2 and 1, respectively. In this study, the ecological risk potential was calculated based on Eq. 3 and 4.  (3)  (4)   Where  is the potential ecological risk index,  and , are the measured value and the normal background values respectively and is the toxicity response factor of the metal.

The results showed that the mean concentrations of Ni, copper, Zn, cadmium, and Pb were 1.20, 2.86, 9.81, 10.08, and 84.17, respectively, which was higher than the local background to Pb, and Cd. The highest amount of heavy metals for the nearest road distance is 5 meters and it is 38.86 mg / kg which decreases with increasing distance from the road. The Igeo index calculated indicates no serious contamination of the area with copper, Zn and Ni metals and only Cd and Pb in the contaminated grade to moderate contamination, and high contamination to highly contaminated were affected. In fact, the results of the Igeo index show that these elements are naturally present in the soil, but anthropogenic activity has increased the concentration of these metals in the soil. Results of CF index showed that Ni, Cuand Zn values ​​were less than 1 and were in low pollution class. Whereas, Pb with values ​​range from 6-6 in high pollution class and Pb is with values ​​greater than 6 in high pollution class. PLI index was used to evaluate and determine the contaminated stations. PLI indicator’s results for the most polluted stations showed that stations 14 and 23 were classified as highly polluted and the rest of the stations were classified as non-polluting. According to RI results, stations 13, 15, 14, 22, 18, 38, 40, 35, 20, 41, 29, 30, 43, 16, 4, 48, 11, 12, 36, 39, 21, 8 have very high risk and other stations have medium risk. Kriging distribution map of RI index of pollution load index showed that the highest RI index is near the gas pump and near urban areas. Furthermore, the results of Kriging interpolating maps showed that Zn, Ni and Pb are found in the southern region (near Darab city), Cd is found in the north (near Fasa city) and Cuin is found in the northern, central and southern parts.

The results of this study showed that the mean concentrations of Pb, Ni, Zn, Cu and Cd in the roadside margins were higher. Statistically, there was a significant difference between the mean concentrations of Pb, Ni, Zn, Cu and Cd within 5 m of the roads and the values ​​(out-of-town locations and other pollutant sources) and in most observations of concentrations of Pb, Ni , Zn, Cuand Cd are higher than background values. This difference is related to the impact of traffic and transportation factors on the contamination of the roadside. Therefore, caution should be exercised in the application of these soils to create green space and even plant crops. The results also showed that Pb and Cd concentrations in areas close to gas pumps and urban areas were higher than other areas, indicating the impact of human activities.