Distribution characteristics, source analysis and probabilistic risk assessment of dioxins in soil at a decommissioned medical waste incineration site

A medical waste incineration disposal site that had been decommissioned after more than 10 years of operation was taken as the research object. The contamination level, spatial distribution and the main sources of dioxins (PCDD/Fs) in soil were studied through statistical analysis, interpolation simulation and traceability analysis. Additionally, the probabilistic risk of PCDD/Fs in soil was analyzed by Monte Carlo simulation. The results showed that the total toxic equivalent concentration of PCDD/Fs ranged from 0.18 to 1 300 ng/kg in all the soil samples in the study area, and the heaviest PCDD/Fs contaminated areas appeared in the maximum ground concentration location of the emission source, as well as downstream low-lying areas. The source tracing analysis indicated that the main sources of PCDD/Fs in the soil of the site were flue gas, fly ash, and slag produced from the incineration of medical waste. Under the second-type land exposure scenario, the probabilistic risk assessment results based on the uncertainty of the pollutant concentrations in the study area and the exposure parameters of the Chinese indigenous population showed that the carcinogenic risk of PCDD/Fs varied from 2.14×10^–7 to 2.53×10^–3 with a 95% percentile value of 1.71×10^–4. The non-carcinogenic hazard quotient was 1.38×10^–2 to 1.69×10² with a 95% percentile value of 11.4. Parameter sensitivity analyses showed that the sensitivity of each parameter to the total carcinogenic risk and the non-carcinogenic hazard quotient varied little. The concentration of PCDD/Fs in surface soil was the dominant factor, contributing 76.4% of the sensitivities, followed by the daily oral ingestion of soil by adults (47.6%), and the exposure duration of adults (33.2%). Moreover, the main exposure route for both carcinogenic and non-carcinogenic hazards of PCDD/Fs was through oral ingestion of soil, followed by skin exposure..