Probabilistic Assessment Of Compacted Clay Liners In Municipal Solid Waste Landfills

The operation of municipal solid waste（MSW）landfills inevitably produce a large amount of leachate containing inorganic or organic pollutants,which have become a potential threat to the surrounding natural environment.Groundwater as an important source of human drinking and irrigation,a honrizontal liner system should be constracted at the bottom of the landfill to prevent contaminant migrating into the groundwater.Compacted clay liner（CCL）is one of the common low-permeability liner in landfills around the world,Chinese regulations require that its minimum thickness should not be less than 2 m and maximum hydraulic conductivity should not exceed 1.0×10^-9 m/s.Based on the specific analytical solution,numerical calculation methods are proposed to simulate and predict the transport process in CCL with Cd²⁺and toluene,two representative inorganic and organic contaminants.In addition,the reliability analysis in terms of the probabilistic assessment of CCLs and the parameters study are conducted based on the database summarized by previous researchers,and the specific situation of the change of the probability of failure with time during the use of the CCLs is investigated quantitatively,and according to the maximum concentration level（MCL）of China,the United States,Canada and the World Health Organization（WHO）,the service life of CCLs under different failure of probabilities is estimated.The main conclusions are as follows::（1）Compared with the Ogata and Banks analytical solution,the assumptions and results of the analytical solution of Rowe and Booker are more reasonable,and the Ogata and Banks analytical solution can also be regarded as a special case.The results show that the concentration of contaminant in the aquifer will increase to the maximum concentration C_max over time t_max,and then gradually decrease to zero concentration.In the Rowe and Booker analytical solution,the equivalent leachate height H_f and groundwater horizontal velocity of groundwater v_b have a significant effect on the maximum contaminant concentration C_max in the aquifer,and will also ralate to the sevice life of CCL corresponding to the MCL.（2）In the deterministic analysis,if the soil parameters of the compacted clay liner with a thickness of 2 m are set as the average number in the database established by Zhan et al.（2017）,the source concentrations of Cd²⁺and toluene are set as C₀=0.217 mg/L,3.847 mg/L,respectively.The leachate head is controlled properly at 0.3 m according to the Chinese national standard,then the time elapsed for the concentration of Cd²⁺in the aquifer to reach the Chinese healthy drinking water standard is 57 years,and toluene will never exceeds this limit.In this case,CCL with a thickness of 2 m can meet the general use requirements.（3）In the reliability analysis,if parameters are adjusted properly in the Rowe and Booker solution,the probability of failure obtained in this paper is very consistent with the result of Zhan et al.（2017）based on Ogata and Banks solution.When other conditions are same,with the increase of groundwater flow velocity or CCL thickness,the probability of failure of a CCL target service life will decrease.If the equivalent leachate height or leachate head is increased,the failure probability corresponding to the CCL target service life will result in a rapid rise.If the target service life of CCL is 50 years and the probability of failure is required within 5%according to the specification requirements,then only 5 m thick CCL can meet the design requirements.From this point of view,a single CCL is not an economic design option.（4）It can be seen from the contour map of probability of failure that,a probability of exceedance corresponds to a higher peak concentration and a shorter service life under a certain maximum concentration level.（5）As of 2022,the probability of failure of natural clay deposit for Cd²⁺in Chengdu Chang’an landfill will reach 94%,and the probability of failue for toluene will be 87%,respectively.