Study On Antifouling Property Of Layered Heterogeneous Clay Soil And Construction Of Groundwater Pollution Control System For In-situ Disposal Of Solid Waste

Due to traditional and economic reasons,only a few of China's industrial and mining enterprises are located in the periphery of towns,most are located in suburban fields.The raw materials for production and waste dumping are basically arbitrary.Consequently,there have been serious damage to the surrounding water and soil which have become a major source of environmental pollution.Expensive off-site relocation and standardized landfill disposal work are weak for the case of spot-like distribution,and it is a more economical and effective choice to use the advantage of natural cohesive soil anti-fouling performance to achieve the in-situ treatment plan as much as possible.However,effective control of groundwater pollution from in-situ disposal of solid waste requires solving two scientific and engineering problems:(1)feasibility study to determine whether the heterogeneous cohesive soil layer under the contaminated site can effectively prevent pollutants from entering the groundwater,(2)standardizing groundwater pollution prevention and control system.Both problems require a systematically rigorous and practical application research.Zhehai Town situated in Huize County of Yunnan Province is an important lead and zinc mining area in China.Other major activities there also include metal smelting where large amounts of smelting waste residue(hereinafter referred to as large slag heap)constitutes one of the biggest pollution hazards in the area.In this paper,by introducing two indicators of pollution degree and antifouling factor,a numerical simulation method is used to quantify the antifouling performance of cohesive soil,study the antifouling mechanism and analyze the main influencing factors of layered heterogeneous cohesive soil with a relatively high permeability layer.Furthermore,geological and hydrogeological investigation of the contaminated site will improve our understanding of the status of groundwater and soil pollution as well as the hydrogeological conditions in the contaminated site of the slag heap.A series of lab and field experiments were used to determine the parameters required for the evaluation of anti-fouling performance of layered heterogeneous cohesive soil.These anti-fouling performance parameters were input into a numerical model to quantitatively calculate the anti-fouling of nonhomogeneous cohesive soil under the big slag heap.Following a critical analysis and review of the feasibility of in-situ disposal of large slag piles,results show:(1)Due to the sedimentation,a relatively high-permeability layer exists in the layered heterogeneous clay layer,and the layered heterogeneous clay layer can be generalized into B+A+C type structure,where a,b,c respectively representing cohesive soil of three permeability properties,and kA > kB > kC.(2)The antifouling mechanism of layered heterogeneous cohesive soil with relatively high permeability is summarized as: infiltration(penetration)? diversion(dilution)? infiltration(penetration)? obstruction ? re-diversion(dilution).The mechanism of it is stronger than that of the homogeneous cohesive soil of the infiltration(penetration)mechanism with the same permeability and adsorption performance.(3)The increase of the permeability coefficient of the relatively high permeability layer A has a greater influence on the antifouling performance than the thickness increases of the layered heterogeneous cohesive soil.(4)The stratum in the lower part of the large slag heap can be divided into two lowpermeability layers and two relative aquicludes: the upper low-permeability layer is mainly composed of Deluvial Material,and the Lacustrine Deposits or the brown-red clay of the middle Pleistocene Eluvial Deposit(Q2dl+el)constitutes the first relative aquicludes;the lower low-permeability layer is the weathered Permian basalt stone(P2?),and the fully weathered and strongly weathered layer underneath constitutes a second relative aquicludes.There are two stable and continuous relative aquicludes in the lower part of the large slag heap,which form the pore water level of different buried depths,that is,the two relatively high permeability layer with different permeability,and it has a layered heterogeneous structure,which is infiltration(penetration)? diversion(dilution)? infiltration(penetration)? obstruction ? re-diversion(dilution)antifouling mechanism,and it can excellently protect deep groundwater.(5)The pollution characteristics of soil profile: The characteristic pollutants of the large slag heap are Zn and Cd,which are mainly transported in the gravel-clay soil in the upper Deluvial Material,and the cohesive soil of the lower Eluvial Deposit can well block the vertical migration of pollutants,so the migration depth of characteristic pollutants Zn and Cd on the lower soil does not exceed 20 m.(6)Groundwater pollution characteristics: In the vertical direction,the characteristic pollutants are mainly concentrated in the shallow groundwater,and the deep groundwater is basically not polluted;in the horizontal direction,the influence of the large slag heap on the shallow groundwater is mainly concentrated in the downstream of the slag heap between the west and east gully,and the southern boundary is about 1km away from the large slag heap.(7)The heterogeneous cohesive soil structure underneath the large slag heap has antifouling properties close to or even exceeding 10 m compacted clay(K=1×10-7cm/s),which can be used as a natural liner structure to stop from heavy metal leachate pollutants enter the deep groundwater.Based on the analysis of anti-fouling performance of layered heterogeneous cohesive soil in large slag heap,geological,hydrogeological survey and water and soil pollution survey,a conceptual model of two-layer aquifer in the study area is established.Then,using the monitoring data of atmospheric rainfall,groundwater level and groundwater quality,as well as the input parameters of water flow model and solute transport model such as permeability coefficient and adsorption performance parameters,the numerical model of groundwater pollution migration in the study area is established and designed a groundwater pollution prevention and control system,the core are Controlling Source,Breaking Path,and Intercepting Flow projects(CSBPIF)for the in-situ disposal of large slag heap.It includes,First,adopt ecological engineering coverage measures at the top of the slag heap to reduce the amount of atmospheric precipitation that penetrates into the slag heap,to reduce the amount of leachate produced;the second is to use the physical measures for vertical partitioning in the lateral groundwater runoff zone of the slag heap to cut off the channel through;and the third is to use physical measures of vertical barrier in the southern of the slag heap,intercepts groundwater runoff,prevents pollutants from spreading downstream,and use hydraulic interception projects to extract and intercept contaminated groundwater.Finally,through the groundwater environment monitoring work around the large slag heap,the groundwater level,water quality and water volume changes around the slag heap before and after the implementation of the groundwater pollution prevention and control system were compared and analyzed.The pollution prevention and control effects of the system were analyzed and evaluated.The results show that:(1)Effect of top ecological cover projects: The “control source” effect of the entire top ecological cover project can reach 80% or even higher.(2)Hydraulic interception and vertical barrier projects in the southern part of the slag heap: It has successfully blocked the passage of leaching filtrate or contaminated groundwater to the south side.(3)Effect of vertical barrier projects on the east side of the slag heap: It has changed the runoff direction of the shallow groundwater in the east side,reducing the groundwater entering the slag heap in the east side,and also blocking part of the slag heap leachate leaks laterally.In this paper,a groundwater pollution prevention and control system for in-situ solid waste disposal of non-ferrous metal smelting waste slag was established in the typical contaminated site of Yunnan Zhehai.The "control source-interception-break diameter" groundwater pollution dynamics interception and runoff blocking technical system is proposed.The technical system includes a solid waste on-site disposal feasibility study discriminating technology based on the research on anti-fouling performance of layered heterogeneous cohesive soil,and a groundwater pollution prevention and control system construction technology comprehensive of geological,hydrogeological survey and water and soil pollution survey as a means to assist the numerical simulation method.The system can reduce the groundwater pollution source load by up to 80% and effectively control the migration and diffusion of groundwater pollutants.Those achievements has important theoretical and practical significance for the major special research on “the cause and treatment technology of soil pollution in the site scales” in China.