Atmospheric deposition of polycyclic aromatic hydrocarbons in southwestern German soils : sources, cancer risks and policy implications

Abstract

Background: Polycyclic aromatic hydrocarbons (PAHs) comprise a group of persistent carcinogenic pollutants whose transfer from air to soil via the atmospheric deposition represents a major chronic exposure pathway. Regulatory frameworks across Europe and beyond rely heavily on single compound indicators such as benzo[a]pyrene and focus predominantly on point source contamination, creating potential blind spots for contamination by deposition. This observational study quantified concentrations, identified emission sources and assessed the lifetime cancer risk of 20 PAHs in 97 topsoil samples from four locations in southwestern Germany. Results: The total concentrations of the 16 USEPA priority pollutants ranged from 52 to 3956 μg/kg, with median values between 214 μg/kg (Hunsrück) and 371 μg/kg (Saarland/Palatinate Forest). Most sites (76%) were classified as “not” or “weakly” contaminated, while 12% were “heavily contaminated”. PMF identified four major sources accounting for 97% of the total PAH mass: coal/coke combustion (34.5%), biomass combustion (9.8%), petrogenic sources (15.8%) and combustion of liquid fuels from vehicles and stationary sources (37.0%). A Monte Carlo based cancer risk assessment for ingestion, dermal contact, and inhalation indicated cumulative risks between 10-4 – 10-6, with ingestion and dermal contact as the dominant pathways. Localized hotspots with elevated risks were identified, indicating the need for site specific follow up. Conclusions: Liquid fuel combustion represents the dominant process influencing soil contamination by atmospherically deposited polycyclic aromatic hydrocarbons (PAHs) in the study area. While measured concentrations at most sites remain within current regulatory limits, the cumulative toxicity of PAH mixtures results in cancer risks that are not negligible. These findings point to a structural limitation in existing EU and national soil protection frameworks (e.g., the German BBodSchG/BBodSchV), which remain primarily oriented toward contamination from identifiable point sources and do not adequately account for diffuse deposition-driven inputs. Consequently, soils affected by atmospheric PAH deposition may fall outside current regulatory attention despite carrying a cumulative toxic burden of potential relevance for human health. Strengthening soil quality assessment approaches to incorporate cumulative toxicity metrics and deposition-driven contamination pathways would improve health protection and better align soil policy with contemporary emission patterns.