Spatial and temporal variations, sources, air-soil exchanges and health risks of PCDD/Fs in soils from a metropolitan...

In this study, soil samples were collected at monthly intervals between June 2022 and June 2023 from a semi-rural area, an urban area, and a wastewater treatment plant (WWTP) in Bursa, Türkiye. The spatial and temporal distribution, congener profiles, potential sources, and human health risks of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) were comprehensively evaluated. The mean ∑₁₇ PCDD/F concentration in soils was 32.34 ± 12.79 pg/g dry weight (dw), with a corresponding toxic equivalent (TEQ) of 1.97 ± 0.55 pg WHO-TEQ/g dw. Concentrations ranged from 17.17 to 70.20 pg/g dw. PCDD/Fs concentrations in the urban area were significantly higher than those in the semi-rural and WWTP sites (p < 0.001). A pronounced seasonal pattern was observed at all stations, with winter ∑₁₇ PCDD/F concentrations 1.6-1.9 times higher than those measured in summer. Pearson correlation analysis revealed strong negative associations with air temperature and boundary layer height, indicating that meteorological conditions influenced accumulation. Congener profiles were dominated by OCDD and OCDF, and the PCDF/PCDD ratio (0.84 ± 0.18) indicated multiple temporally variable source inputs. Air-soil exchange showed that soils acted predominantly as accumulation media throughout the year. Equilibrium was approached only in August and September, particularly for lower-chlorinated congeners. Principal Component Analysis (PCA) indicated source-related congener groupings consistent with mixed combustion, traffic-related, industrial sources, and possibly transported inputs at the overall study-area scale. Health risk assessment following USEPA methodology showed total carcinogenic risks on the order of 10^-8 across all age groups, below the acceptable threshold of 10^-6, and hazard quotients were well below 1. This study provides one of the first high-resolution monthly assessments of soil PCDD/F contamination in Türkiye.