Study On The Natural Attenuation Of Landfill Leachate Pollutants In The Groundwater Environmennt Of Unconsolidated Porous Medium By Simulation Method

The main remediation method of the landfill contaminated site is the natural attenuation of the groundwater environment on the basis of minimizing the produced leachate by closed field governance in China.The main factors affecting of the natural attenuation include hydrogeological conditions,landfill scale,leachate composition and the amount of infiltration and other factors.So domestic and foreign research achievements on the natural attenuation of groundwater environment can't directly applied to different landfill site.Therefore,it is necessary to study the retention-release mechanism and efficiency in the unsaturated zone,and natural recovery process of the pollutants in the groundwater environment of landfill contaminated sites with typical hydrogeological characteristics.It provides a scientific basis for the protection and the remediation of groundwater environment.In view of the defects of the one-dimensional leaching column physical model is that the change of water content and solute concentration gradient were not considered,this study builds a three-dimensional physical model.Then,the mechanism of entrapment and release of contaminants in the unsaturated zone was quantitatively elucidated by physical simulation test.The capacity of pollutants interception in the unsaturated zone is limited,which is related to the media volume and the contaminant concentration of leachate.Interception capacity can be divided into"dynamic interception"and"stable interception"."Dynamic interception"refers to the"solute consumption"that caused by increasing the water content of unsaturated zone to construct infiltration channel.It is the main source of"secondary pollution"to the groundwater environment,because it is easily re-released from the unsaturated zone.“Stable interception”refers to the physical adsorption and biochemical conversion or decomposition of pollutants that no longer released in the aquifer.This phenomenon reflects the ability of the unsaturated zone to intercept contaminants.In aerobic conditions,the attenuation of pollutants in the unsaturated zone mainly depends on the spontaneous reaction of O₂ consumption.Such as the consumption of organic matter,NH₃^-N and NO₂^-be oxidized into NH₃^-N,S^2-be oxidized into SO₄^2-.The adsorption of COD and NH₃^-N in the unsaturated zone is obvious.The unit adsorption intensity of COD is 100.17¹99.06mg/kg,and the data of NH₃^-N is 28.93³9.08mg/kg.In addition,statistics display that the pore water volume increment is about 27³8%of the effective porosity,the Ds of the unsaturated zone is between 20.8²7.5m²/d and the a_s is between 1.96².53 m/d under the experimental conditions.In this paper,the uncertainties of numerical simulation of groundwater are studied from conceptual model and parameter uncertainty by multi-model analysis and numerical test method.It is that the aim is to improve the accuracy of numerical model.The Multi-Model analysis is method that consists of improved A-C sampling algorithm and AICc criterion which can identify the sensitivity of different factors and effectively to obtain the optimal parameter interval.Not only that it to complete the selection of the optimal concept model at the same time.When the A-M algorithm is used to collect 20 groups of parameter samples,it means that the number of parameter samples is 45000,the mean and variance of the parameters tend to be stable.The calculation model was established according to the difference of conceptual model and the parameters of the collected samples.The calculation model with the variance<2 accounted for 65%in 1#conceptual model,it is accounted for46%in 2#conceptual model.Base on removing the calculation model with Delta>10,only10 calculation models were retained in the 1#concept model groups which consider the terrain change caused by the landfill.The water level of the retained model is almost coincident with the observed value,and the cumulative posterior probability of it is 0.996.Number of 21 calculation models were retained in the 2#concept model groups,the cumulative posterior probability of the top 10 models is only 0.884.Based on the numerical model obtained by the uncertainty analysis,it was modified through a series of observational data and the results of contaminant retention-release studies in this paper.And then,It is applied to prediction of groundwater environmental natural attenuation.The following results are obtained through case study:the reliability of the model is based on the actual observation data.The simulation results of the correction model are closer to the actual observations.According to the gray relational analysis,the correlation between the time series output and the actual monitoring value can be improved from 0.4189⁰.5184 to 0.8054⁰.8256 through model correction.Based on the decay rate of leachate contaminant and groundwater environment recovery prediction results,the groundwater environment of the highly permeable porous media has been gradually restored before the landfill is fully stabilized.In this case,the main controlling factors of the rapid attenuation of groundwater environment are the mixing dilution of lateral supply and convection diffusion of solute rather than the biochemical conversion or decomposition of pollutants.Therefore,migration control of pollutants is the primary problem in the process of groundwater environmental restored in highly permeable porous media.Two points,the one is that there is a water with enough environmental capacity,and the other is that there is no sensitive area for the development and utilization of groundwater in the way of pollutant migration,are the main control factors of feasibility of natural attenuation method for groundwater environment.