Quantifying the effect of traffic on lead accumulation in soils: a case study in Iran

Road transport is an ubiquitous source of lead contamination in the soils near highways with direct and indirect impacts on human health. Accumulation of traffic-induced lead in the soils depends on gasoline lead content, traffic volume, as well as meteorological conditions. To evaluate the effect of traffic on soil lead concentration, 113 samples from the topsoil (0-15 cm) were collected in a regularly spaced grid (70 x 70 m) at the vicinity of a busy highway (Tehran-Karaj) west of Tehran. Total lead (Pb) concentration in soil samples was extracted by 4 M HNO3. Three different kriging techniques (ordinary, universal and indicator kriging) were applied to investigate the spatial distribution of Pb. According to the measured values, the average concentration of Pb (48 mg kg -1) was very close to the upper limit of the Swiss guide value of 50 mg kg-1 with the maximum value (75 mg kg-1) exceeding this limit. The result also showed an accumulation of 5 to 25 mg kg-1 Pb in the soil based on the distance from the highway. The average concentration of Pb decreased exponentially with distance from the highway up to 200 m becoming relatively constant thereafter. A trend of spatial continuity of Pb concentration was in agreement with major wind direction. Comparing different kriging techniques indicated that UK was superior due to smaller error of estimation. A probability map of Pd concentration exceeding the Swiss guide value clearly shows the traffic to be the main cause of soil Pb contamination.