Research On Tiered Risk Assessment Method Of Contaminated Sites Based On Uncertainty Analysis

Contaminated sites have become an urgent environmental problem that threatens the environment and human health and restricts the safe redevelopment and utilization of soil resources,which has now attracted widespread attention.Risk assessment is considered to be the critical part of environmental management of contaminated sites.However,the uncertainty problem in the assessment process,especially the imprecision of exposure scenario construction,exposure model characterization,and exposure parameter selection,has usually led to the overestimation or underestimation of the actual health risk,thus affecting the effective risk management of contaminated sites.The tiered risk assessment is effective in balancing the uncertainty of the assessment process,the accuracy of the assessment results,and the limited resource allocation,which can provide a cost-effective risk management strategy for contaminated sites.This study addressed the principal sources of uncertainty in the risk assessment process and constructed the refined and tiered risk assessment technology system for contaminated sites from the perspectives of refined assignment of exposure parameters,comprehensive analysis of exposure scenarios,and optimization of exposure models.Based on this,a typical contaminated site of heavy metal and a typical contaminated site of VOCs were taken as research cases to confirm the practical application value of the layered approach to the risk assessment of contaminated sites,with the hope of providing a reference for the refined and tiered risk management of contaminated sites.(1)The refined assignment of risk assessment model parameters is the key foundation for carrying out the tiered risk assessment,and sensitivity analysis can provide an important reference for this purpose.Based on the literature research,the current guidelines(HJ 25.3-2019)contain 4 categories and 58 parameters,including soil parameters,pollutant physical and chemical properties and its toxicological parameters,population exposure parameters,and building parameters.According to the sensitivity analysis method and the availability of the parameters,combined with the time-varying and regionalized characteristics of the exposure parameters,taking arsenic,the main heavy metal pollutant,and benzene,the organic pollutant,as examples,we could focus on the common sensitive parameters such as the average time for carcinogenic effect,daily air inhalation rate,average body weight,and average height on the basis of the existing studies.(2)Combined with the case studies,the sensitive parameters for 2 types of typical pollutants(heavy metal arsenic and volatile organic compounds benzene)were clarified,and the corresponding key techniques for tiered risk assessment of contaminated sites were proposed around exposure scenarios and model optimization.The results show that for arsenic-contaminated sites,the sensitivity of absorption factor of oral ingestion to carcinogenic risk(86.34%)and non-carcinogenic risk(77.38%)is relatively high,and the literature analysis could conclude that it was necessary to incorporate the relative bioavailability of arsenic into risk assessment calculations;for benzenecontaminated sites,the sensitivity of soil parameters and building parameters such as soil bulk density,soil air content-soil filled foundation cracks was relatively high,and the application of the Biovapor model,which took into account the biodegradability of benzene in the risk assessment,was more accurate and reasonable in comparison.(3)Monte Carlo simulation,as an important complementary method for risk assessment,can provide environmental managers with more credible and quantitative expressions and a wider range of references.Based on the previous studies,the results of the risk assessment combined with Monte Carlo simulation showed that the health risk level after parameter correction and consideration of relative bioavailability(compared with the 95%quantile)for arsenic-contaminated sites was reduced by nearly one order of magnitude compared with the traditional risk assessment results,and the carcinogenic and non-carcinogenic risks were reduced by 42.00%and 43.11%,respectively;for VOCs-contaminated sites,the carcinogenic risk and hazard quotient after parameter correction and consideration of benzene biodegradability were respectively 28.83%and 26.50%of the results calculated by the traditional method.It can be concluded that tiered risk assessment methods based on the uncertainty analysis of exposure scenarios,models,and parameters are of practical significance in the risk management of contaminated risk.