Profile distribution of traffic-related contaminants in road-side soils of Leningrad highway in Moscow region

The study investigates the profile distribution of a complex of pollutants in roadside soils. The research was conducted in the Moscow Region, within the territory of the Educational and Experimental Soil and Ecological Center of Lomonosov Moscow State University "Chashnikovo". Soil samples were collected along a transect perpendicular to the Leningrad Highway at distances of 2 m (roadside), 7 m (a depression in the terrain before the forest belt), 50 m (a field behind the forest belt), and 175 m (the central part of the field) from the road surface, at depths of 0–5 cm, 15–20 cm, 30–35 cm, and 45–50 cm. The 2–7 m zone was characterized by technogenic soils, while the 50–175 m zone consisted of sod-podzolic soils used for agriculture. The contents of heavy metals (HMs), polycyclic aromatic hydrocarbons (PAHs), petroleum hydrocarbons (PHCs), as well as chlorides (Cl⁻)—key components of deicing agents—were determined. Additional analyses included particle size distribution, organic carbon content, soil pH, and electrical conductivity. It was found that the distribution of different pollutants in the soil profile was not uniform and depended on soil properties, pollutant characteristics, and the landscape design. The 2–7 m zone was characterized by intense contamination with all types of pollutants. PAHs showed a decreasing or bimodal vertical distribution (from top to bottom); HMs showed a decreasing pattern for Cu and Zn, while for Ni and Pb, an increase in concentration was observed in the lower layers or a uniform profile distribution was noted. PHCs showed a decreasing profile, and Cl- exhibited either a decreasing or bimodal distribution. Among soil properties, organic carbon content and particle size distribution were the main factors influencing the vertical differentiation of pollutant concentrations. Exceedances of the maximum allowable concentrations were observed for the mobile forms of Cu and Zn, as well as for petroleum hydrocarbons and PAHs. In the 50–175 m zone, the dynamics of soil contamination were different. Concentrations of all pollutants gradually decreased with depth, reaching background levels. Pollution was detected only in the surface soil layer for PAHs, the concentrations of which slightly (up to twofold) exceeded regional background values. The study highlighted the importance of investigating soil profile contamination near roads. Certain pollutants (Ni and Pb, PAHs, and Cl⁻) may show complex vertical distributions with increasing concentrations in deeper soil layers. It was also shown that the design of the roadside landscape played a crucial role in the distribution of pollutants. An artificially created depression a few meters from the Leningrad Highway can intercept runoff from the road surface, preventing pollutant migration to agricultural areas located upslope. Additionally, a living windbreak in the form of a forest belt reduced the airborne migration of pollutants. Intensive sedimentation of PAHs in front of the forest belt resulted in a nearly 20-fold decrease in their concentrations in soils behind it. Such landscape design can be an effective solution for protecting agricultural fields adjacent to highways. However, the study also emphasized the need for remediation measures to reduce extremely high pollutant levels in close proximity to the road, as surface dust can act as a secondary source of soil contamination near roads and also affect pedestrians and drivers