Pollution Status of Some Heavy Metals in an Agricultural Land Along Arak-Qom Highway

In this study, the pollution status and horizontal distribution of heavy metals of Ni, Cd, Pb and Zn was investigated along the Arak-Qom highway in an agricultural field under alfalfa. Soil samples from depths of 0-10 and 10-20 cm were collected perpendicular to the road at the distances of 0, 50, 100, 150, 200, 250 and 300 m. The content of the total and available levels of these heavy metals was investigated as a function of distance from the road and soil depth. Results showed that the total and available concentration of heavy metals decreased by increasing distance from the road, indicating that accumulation of heavy metals in top soils was greatly influenced by traffic. Also, organic matter content and pH decreased with distance from the roadway, probably due to the presence of vegetation on the surface and accumulation of organic matter including debris such as paper, plastic and hydrocarbons (garbage out of vehicles). The concentration of measured heavy metals were higher than standard (FAO/WHO) and in layer of 0-10 cm were higher than 10-20 cm. The mean total concentration of heavy metals of Ni (109.60), Cd (47.11), Pb (553.41) and Zn (241.95) all in mg kg-1 followed the order of Pb> Zn> Ni> Cd. In terms of geo-accumulation index and contamination factor for Ni, Cd and Pb, the soil showed high levels of contamination, as the geo-accumulation index for Ni, Cd and Pb were 6.75, 6.71 and 4.21, and the contamination factors were 161.18, 157.02 and 27.67, respectively,which suggests care must be taken to control the level of heavy metals concentration on roadside soils as these soils are often cultivated to produce food crops.High accumulations of heavy metals were usually found on alfalfa and accumulation of the heavy metals was higher in shoot than root. Median values of Ni, Cd, Pb and Zn were 29.44, 15.64, 5.79 and 10.50 mg kg-1 for root and 47.98, 19.77, 7.14 and 13.53 mg kg-1 for shoot, respectively. Results show that it is necessary to have continuous studies on the accumulation of heavy metals and the potential ecological risk assessment to ensure effective protection and management of urban soil and other environmental media.