Traffic-related sources and factors influencing soil contamination by potentially toxic elements: a case study of roadside soils in Siena, Italy

A geochemical study was performed to determine the concentrations of 18 potentially toxic elements (PTEs) in roadside soils from the peri-urban area of Siena (Italy). The objectives were as follows: (1) to assess the soil natural background and contamination threshold of PTEs in the investigated peri-urban sectors; (2) to define the level and extent of soil contamination by PTEs; and (3) to identify the main traffic-related sources and factors controlling the distribution of PTEs in roadside soils. Fifty roadside soil samples were collected along two linear transects perpendicular to two heavily trafficked peri-urban roads, at various distances from the roadside (up to 20 m) and different depths along the soil profile (up to 60 cm). Concentrations of PTEs were determined by inductively coupled plasma-mass spectrometry (ICP-MS), and contamination levels were evaluated with the contamination factor (CF) and pollution load index (PLI). The results revealed that Cd, Cu, Ni, Pb, Pd, Pt, Sb, and Zn represent the contaminants in the investigated peri-urban soils. Soil contamination by these PTEs decreased with increasing distance from the roadside (up to 5 m) and with depth along the profile (up to 60 cm). The spatial distribution and accumulation patterns of Cd, Cu, Ni, Pb, Pd, Pt, Sb, and Zn in roadside soils reflect the combined effects of multiple traffic-related sources (e.g. fuel combustion, brake and tyre wear, and catalytic converter deterioration), as well as traffic-related factors such as road age, traffic density, and driving behaviour. Analytical data suggested that, although Pb mainly shows historical contamination from leaded petrol, it continues to act as a traffic-related contaminant due to ongoing non-exhaust emissions and fuel combustion. In contrast, Pd, Pt, and Sb were identified as more recent urban contaminants associated with particles produced by the abrasion and deterioration of catalytic converters (Pd and Pt) and modern brake linings (Sb). Similarly, the accumulation of Cu, Ni, and Zn in roadside soils was likely affected by recent and significant contributions from vehicular non-exhaust emissions.