Study On Risk Assessment Methods For Groundwater Contamination At Solidified/Stabilized Heavy Metal Sites

Heavy metal pollution of Chinese soil is becoming increasingly critical,and remediation of heavy metals in soils is a matter of urgency.Solidified/Stabilized（S/S）is cost-effective and generally applied in the remediation of actual heavy metal-contaminated soils.After remediation,The leaching concentration of soil heavy metals decreased,but the total amount of soil heavy metals did not decrease.There is still a possibility of re-release and migration into the groundwater with rainfall leaching,causing secondary pollution.How to scientifically and objectively assess the risk of groundwater contamination of S/S heavy metal sites,and provide security for it’s ecological environment,is a pressing issue to be solved.Based on this,this paper investigates the long-term release risk of stabilized heavy metals in the environment and the action rules under different influencing factors through soil column leaching and improved quantitative accelerated aging experiments.A groundwater contamination risk assessment system is established based on DRASTIC and the special vulnerability of S/S heavy metal sites,which provides a new method for the long-term risk of S/S soil heavy metals and the contamination risk assessment to groundwater.The method is simple and practical,with less workload,and can set priority levels for effective management of S/S heavy metal sites,provide references for the selection of S/S remediation agents,assessment of remediation effects,and also suggest better site selection and construction strategies for protecting the surrounding environment.The main findings are as follows:（1）By conducting soil column leaching experiments on stabilized As-contaminated soil,the As leaching concentration was inversely proportional to the p H of the leaching solution and directly proportional to the leaching amount.In the soil column leaching experiments of Pb-contaminated soil after stabilized,stabilization can reduce the environmental risk of Pb-contaminated soil,and the environmental risk of Ca O-stabilized soil is lower than that of KH₂PO₄-stabilized soil in 160 years of simulation.The quantitative acceleration experiment leaching concentration is higher than soil column leaching,and this assessment method may overestimate the actual site environmental risk,while the soil column experiment may underestimate the actual site environmental risk.（2）The environmental risk of stabilized heavy metal contaminated soil is a dynamic and increasing process,acid rain leaching converts the insoluble state of heavy metals to the soluble state.The freeze-thaw cycle destroys soil aggregates and accelerates the process of soil acidification,which further increases the environmental risk of stabilized heavy metals.SEM-EDX results show that acid rain leaching causes the dissolution of Pb-containing precipitates and the continuous dissolution of the residual state into other unstable forms or direct release leads to increased environmental risk of S/S soils.（3）Combining experimental findings and literature research summaries,18evaluation indexes were identified from 4 evaluation factors based on DRASTIC and the special vulnerability of S/S heavy metal contaminated sites to establish the S/S heavy metal site groundwater pollution risk assessment system.The comprehensive weights of the four influencing factors of site hazard,pollutant stability,aquifer vulnerability and natural conditions are 0.1894,0.3508,0.3508 and 0.1092,respectively.equal spacing method is used to classify the risk level into should five risk levels.（4）An actual site was selected to assess the environmental risk of groundwater at the site using numerical simulation and risk assessment models.Under direct landfill scenario,the As released from the upper S/S soil will all penetrate into the groundwater after 7.25 years with a concentration of 73.90μg/L,as the maximum adsorption capacity of the vadose zone is reached by the Hydrus software simulation.By GMS software,As exceeded the 10μg/L range up to 135 m at 100 years and started to pollute the groundwater at 109 years.Based on the spread of As,the risk level of groundwater contamination is assessed as high risk.In the case of direct landfill by the integrated index risk assessment model,The integrated contamination risk indices for As and Cd were 4.55 and 4.58,respectively,both of which were medium risk.After supplementary protection measures,the integrated groundwater contamination risk indices of As and Cd are 3.98 and 4.02,respectively,and the risk of As to groundwater is reduced to low risk,but Cd is still medium risk.The hydrogeological conditions of the original site are fragile,groundwater is vulnerable to contamination,and in-situ landfill of S/S soil is at risk.To further reduce the risk,the topographic slope was increased thereby improving the surface drainage capacity,which reduced the combined groundwater contamination risk index for As and Cd to 3.94 and 3.90,both of which are low risk.