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Mục tiêu và đối tượng nghiên cứu của luận án - Mục tiêu nghiên cứu: Luận án nghiên cứu về đặc điểm ô nhiễm, nguồn ô nhiễm, xuất phát từ nước rỉ rác của bãi chôn lấp (BCL) chất thải rắn (CTR) hợp vệ sinh nhằm thấy rõ sự ảnh hưởng đến quá trình lan truyền chất ô nhiễm trong môi trường đất bãi chôn lấp. Đánh giá và dự báo được mức độ lan truyền ô nhiễm kim loại nặng (KLN) bằng phương pháp định lượng để xây dựng dữ liệu phục vụ công tác quản lý môi trường liên quan đến bãi chôn lấp. - Đối tượng nghiên cứu: Luận án tiến hành lựa chọn đối tượng nghiên cứu là ô nhiễm kim loại nặng trong môi trường đất bãi chôn lấp chất thải rắn hợp vệ sinh - ô chôn lấp đã đóng tại Kiêu Kỵ, huyện Gia lâm, Hà Nội. Nghiên cứu các yếu tố ảnh hưởng đến quá trình lan truyền chất ô nhiễm của bãi chôn lấp để đánh giá và dự báo mức độ ảnh hưởng ô nhiễm từ bãi chôn lấp đến môi trường đất xung quanh bãi chôn lấp theo không gian và thời gian. b) Các phương pháp nghiên cứu đã sử dụng Do tính chất của đối tượng nghiên cứu là ô nhiễm phát sinh từ nước rỉ rác tại BCL, luận án đã sử dụng các phương pháp nghiên cứu như nghiên cứu lý thuyết để tìm hiểu những dấu hiệu đặc thù, bản chất, cấu trúc của phương pháp số trong cách tính toán lan truyền ô nhiễm qua đó tiến hành phân tích nội suy khái niệm, xây dựng phương trình toán học mới xác định mức độ lan truyền ô nhiễm với điều kiện biên phức tạp; sử dụng phương pháp phần tử hữu hạn để tính toán lan truyền chất ô nhiễm. Nghiên cứu đã sử dụng phương pháp phần tử hữa hạn là phương pháp số gần đúng để giải các bài toán được mô tả bởi các phương trình vi phân đạo hàm riêng trên miền xác định có hình dạng và điều kiện biên bất kỳ mà nghiệm chính xác không thể tìm được bằng phương pháp giải tích, bằng phương pháp này có thể tính toán và mô phỏng sự thay đổi nồng độ của kim loại nặng theo thời gian trong không gian một cách định lượng và rõ ràng. c) Kết quả chính và kết luận - Kết quả chính: Luận án đã tổng hợp và đề xuất phương pháp tính toán nước rỉ rác phát sinh phù hợp điều kiện khí hậu khu vực và qui trình vận hành chôn lấp chất thải rắn hợp vệ sinh. Đánh giá và dự báo được mức độ ô nhiễm kim loại nặng trong môi trường đất từ khu vực ô chôn lấp đã đóng của bãi chôn lấp hợp vệ sinh với mô hình mô phỏng 3D. Bằng việc sử dụng các khái niệm về trường vectơ gradient ∇C (x, y, z, t), và các phép toán gradient có liên quan (∇⋅), ⊙, cơ chế khuếch tán - đối lưu của chất gây ô nhiễm trong nước rỉ rác được mô hình hóa một cách cụ thể và ngắn gọn, thấy được bản chất vật lý của quá trình lan truyền một cách trực quan hơn. Biến đổi phương trình lan truyền về dạng thức phương trình vi phân đạo hàm riêng, thuật toán PTHH đã được xây dựng để giải phương trình bằng phương pháp số một cách hiệu quả. Khai thác thuật toán và sử dụng phần mềm MATLAB để tính toán, dự báo được khả năng lan truyền nồng độ chất ô nhiễm theo thời gian và trong không gian lòng đất. - Kết luận: Từ các kết quả nghiên cứu lý thuyết và thực nghiệm luận án rút ra được các kết luận như sau: Các yếu tố ảnh hưởng đến quá trình lan truyền chất ô nhiễm trong môi trường bãi chôn lấp chất thải rắn tại Kiêu Kỵ Gia Lâm bao gồm: 1) nguồn gây ô nhiễm KLN từ nước rỉ rác, 2) nồng độ KLN As trong nước rỉ rác và hàm lượng KLN As trong đất bãi chôn lấp đều vượt quá qui định cho phép QCVN 40:2011/BTNMT và QCVN 03:2015/BTNMT, 3) thời gian tích lũy chất ô nhiễm KLN do bãi chôn lấp đã tồn tại hơn 10 năm. Các kim loại nặng khác được phát hiện trong nước rỉ rác là Cr, Pb, Hg, Cd…đều có hàm lượng cao hơn qui chuẩn cho phép do chất thải rắn được chôn lấp không phân loại tại nguồn. Nước rỉ rác là yếu tố gây ô nhiễm môi trường BCL rất nghiêm trọng. Nước rỉ rác được tính toán theo phương pháp đề xuất trong luận án bao gồm các yếu tố ảnh hưởng đến lượng phát sinh từ khi vận hành chôn lấp (lượng mưa, độ bốc hơi, độ ẩm của CTR, thiết bị đầm nén, qui trình vận hành…) đến khi đóng ô và thời gian sau đó (lớp che phủ không trồng cây, có trồng cây, lớp che phủ bị suy thoái…). Các yếu tố này là phù hợp với điều kiện thực tế để tính toán lượng nước rỉ rác khi vận hành BCL tại Việt Nam nói chung và tại BCL Kiêu Kỵ, Gia Lâm nói riêng. Vành đai an toàn đối với nguy cơ ô nhiễm KLN As đã được xác định dựa theo kết quả tính toán của mô hình 1D và 3D. Theo chiều sâu, kết quả mô hình 1D đã khẳng định cụ thể tại các vị trí dễ gây tổn thương tới tầng chứa nước là đáy ô chôn lấp (cách 1,5m đến tầng chứa nước), đáy hồ chứa nước rỉ rác (4,5m), thời gian lan truyền xảy ra rất nhanh nếu không tính đến khả năng hấp phụ đất đá thời gian tương ứng sẽ là 40 ngày và 105 ngày. Vành đai an toàn theo phương x và z tại bất kỳ vị trí nào xung quanh ô chôn lấp tính theo mô hình 3D là 3m và 3,5m sau 8 tháng nếu hàm lượng As ban đầu là không đổi, sau khoảng giá trị này, hàm lượng As tính toán được mới đạt được qui định cho phép của QCVN 03:2015/BTNMT. Kết quả trên 2 mô hình đều thấy kim loại nặng As có xu hướng di chuyển từ lớp đất trên xuống tầng chứa nước sâu hơn, tốc độ lan truyền theo phương z nhanh hơn phương x (mô hình 3D). Các kết quả nghiên cứu đạt được của luận án có thể được sử dụng như một bộ số liệu để làm cơ sở đánh giá và phát triển các giải pháp, chính sách hiệu quả trong việc phân loại, quản lý chất thải rắn nhằm giảm thiểu các tác động tiêu cực từ nguy cơ ô nhiễm kim loại nặng tới môi trường đất và ô nhiễm nước dưới đất tại các khu vực bãi chôn lấp CTR cũng như các trường hợp không kiểm soát chặt chẽ qui trình vận hành các bãi chôn lấp CTR.
0 MINISTRY OF EDUCATION AND TRAINING HANOI UNIVERSITY OF CIVIL ENGINEERING HOANG NGOC HA STUDY ON THE SPREAD CHARACTERISTICS OF HEAVY METALS FROM SANITARY LANDFILL AT KIEU KY, GIA LAM, HANOI Specialization: Environmental Technology for Solid Wastes Code: 9520320-1 SUMARY OF DISSERTATION Ha Noi – 2022 The dissertation was completed at: Hanoi University of Civil Engineering Scientific Advisor 1: Prof Nguyen Thi Kim Thai Scientific Advisor 2: Assoc.Prof Nguyen Van Hoang Reviewer : Assoc.Prof Nghiem Van Khanh Reviewer : Assoc.Prof Nguyen Manh Khai Reviewer : Assoc.Prof Nguyen Thi Anh Tuyet The dissertation will be defended before the University level Committee at Hanoi National University of Civil Engineering At hour day month 2022 The dissertation can be found at the libraries: - Hanoi National University of Civil Engineering - The National Library INTRODUCTION I Research rationale Research on the effects of leachate from landfills on the soil environment of the sanitary landfill area has received little attention and has almost not been carried out Therefore, within the research framework of the topic, the author mentions issues related to the spread of pollutants from sanitary landfill, factors affecting the spread of pollutants that generated from landfill and using mathematical models to predict the spread of pollutants from landfills to the soil environment The topic selected to study the characteristics of heavy metal spread from the sanitary landfill at Kieu Ky, Gia Lam, Hanoi is necessary to study the factors affecting the pollutant transmission process By doing the research to assess and forecast the level of pollution impacts from the landfill as a basis for proposing solutions to overcome and prevent pollution, and form a database for the solid waste management in particular as well as environmental management in general II Purpose and content of research Research and evaluate the characteristics of pollution - the source of pollution comes from leachate of the sanitary landfill in order to clearly see the influence on the pollutant spread process in the landfill soil environment Assess and forecast the spread of heavy metal pollution by quantitative methods to build data for environmental management related to landfills To achieve the above objectives, the thesis conducts research on the following contents: 1) Studying the factors affecting the formation of leachate quantity and properties, investigating the change of pollutant concentration in leachate and the factors affecting this process; 2) Building a model to calculate leachate suitable to the study object and conditions; 3) Research on the theory of pollutant spread by mathematical model Apply numerical methods to solve established equations to predict heavy metal concentrations in leachate spreading in landfill soil; 4) Studying the risks of soil environmental pollution arising from sanitary solid waste landfills and assessing the level of heavy metal pollution in the landfill soil environment III Object and scope of the study Research object: Heavy metal pollution in the soil environment of the sanitary solid waste landfill - the closured cell is located in Kieu Ky landfill, Gia Lam District, Hanoi Research scope: Sanitary solid waste landfill IV Scientific basis The field survey and assessment of Kieu Ky landfill in Gia Lam district, Hanoi city have been researched Studying the effects arising from landfills on the soil environment specifically, including quantifying the processes involved, the formation of pollutants in leachate over time under the influence of environmental landfill factors On the basis of mathematics, by means of finite element method, the subject builds simulation models and quantifies the spread components in the transmission mechanism for the risk of soil pollution following space and time of the study area V Methodology - Methods of synthesis, inheritance and theoretical research - Experimental research methods - Modeling and simulation methods, using finite element method to calculate pollutant spread VI New contributions of dissertation - The thesis has synthesized and proposed a method to calculate the generated leachate suitable to the regional climatic conditions and sanitary landfill operation process - Assess and forecast the level of heavy metal pollution in the soil environment from the closed landfill cell area of the sanitary landfill with 3D simulation model By using the concepts of the gradient vector field ∇C (x, y, z, t), and related gradient operations (∇⋅), and algorithm exploitation, using MATLAB software to calculate, predict the ability to spread pollutant concentrations over time and in underground space VII Scientific and practical significance of the dissertation Scientific significance: The dissertation has proposed a method to calculate leachate generated in accordance with the conditions of Vietnam, detailed calculation of leachate generated during operation and after closing the landfill The dissertation has calculated the risk of As pollution spread by 1D and 3D mathematical simulation models Simulation of the spread of heavy metals from solid waste landfills in the soil environment in 3D space Practical significance: The research results of the thesis are a reference source to help researchers and managers have orientations and solutions in the management of closed landfills as well as contribute in proposing design and build options of landfills suitable to the conditions of Vietnam Defining the safety belt of landfills over time, providing the information needed to reduce the spread of pollution arising from landfill as well as to improve and rehabilitate solid waste landfills in Vietnam Chapter LITERATURE REVIEW 1.1 Related Concepts - The concept of solid waste According to Decree 38/2015/ND-CP dated April 24, 2015 of the Government on waste and scrap management (Article 3), the terms are construed as follows: Solid waste is waste in solid or viscous form (also known as sludge) is discharged from production, business, service, daily life or other activities - The concept of landfill According to the provisions of TCVN 6696 - 2009, a sanitary solid waste landfill (hereinafter referred to as a landfill) is: A landfill that is planned in terms of location, structure and construction in accordance with regulations and the function for burial includes cells to bury normally solid waste generated from residential areas and industrial zones - The concept of leachate from landfill According to the provisions of TCVN 6696-2009, leachate from a landfill is defined as follows: All liquids, including components suspended in that liquid, that infiltrate through or flow out from the waste that buried in a landfill cell of a solid waste landfill 1.2 Sources of heavy metals in landfills The source of heavy metals in landfills is mainly due to simultaneous burying of industrial wastes, incinerator ash, mining waste and household waste containing hazardous ingredients such as batteries, paints, dyes, printing ink, …[58] Heavy metals can exist in solid waste at all stages of the product life cycle, a diagram showing the source of heavy metals in solid waste as illustrated in Figure Figure Source of heavy metal formation in solid waste Source: Vitalii Ishchenko, (2019) [111] Soil contamination by heavy metals source from waste landfills is a serious problem associated with urban and industrial development Soil layers are considered as the final storage place for heavy metals released into the environment and also for heavy metals existing in the soil Therefore, when considering the pollutants in the soil and leachate at the contaminated sites, the pollutant content needs to be measured directly and specifically, because determining the properties of the soil is necessary before recommending technical remediation solutions 5 1.3 Calculate the amount of leachate generation Calculation of leachate is important for landfill operation management There are many calculation methods, however, depending on the actual conditions of each locality, the appropriate method can be selected The fact that the layers in the landfill are not constructed at the same time and in the same season of the year, in addition to take advantage of the available sites for landfilling at the same area, and are often removed daily cover that not conform to the operational design requirements Table 1.Comparative summary of leachate calculation methods Method Water Balance Method Paramater WBM Hydrological Serial Water Evaluation Balance Landfill SWB Leachate - HELP General US Mexico Location Water Balance Software base on Water Balance Calculation Equation Water Balance Equation Method Equation Hydrogeological - Hydrogeological Physical Input data properties of requirements data (soil, weather and solid waste - Design data of climate) landfill Month Day Month Input requirements Run DOS and Excel Equipments Simple related computer equipment All types of All types of Operation Type of landfills landfills and every landfill landfill stages applied Forecast More than reality Less than Calculated WBM amount of leachate Considering the conditions of solid waste landfill activities in Vietnam, it is necessary to have a method to predict leachate more suitable with the nature of solid waste to be buried, weather and climate conditions, time and operation method… Therefore, it is necessary to study and propose a method to calculate leachate suitable to the current landfill operation and operation in Vietnam 6 1.4 The influence of leachate on the soil and underground water environment When studying the accumulation of heavy metals in soil, only considering the total content, it is not possible to properly assess their toxicity to plants as well as their tendency to change in the soil They can exist in many different forms, but mainly in the following forms: mobile, organic, carbonate, iron oxide, and manganese oxides [64] Heavy metals spread by routes such as inhalation, ingestion, skin contact and surface absorption through fish gills, microbial membranes, and roots and leaves The toxicity potential of heavy metals depends on many different factors such as: concentration, routes of entry, form of existence and duration of exposure In the environment it is necessary to determine the degree of harm to individuals or species, or to ecosystems 1.5 Literature review of heavy metal spread from landfills 1.5.1 Studies on the spread of heavy metals from landfills to the soil environment Sharma and Lewis [94] studied a mathematical model of leachate propagation that included the evaporation and dispersion of a contaminant Varank et al [109] investigated the migration of contaminants in landfill leachate through alternative composite liners Issues related to landfill hydraulic conductivity of different types of contaminating clay liners, when studying leachate migration from a landfill, dehumidification of synthetic geology clay liners was also evaluated by Hoor and Kerry Rowe [59] Some studies have focused on building leachate migration models and using digital to simulate the transport of pollutants from leachate into soil layers such as Abriola 1987 [ 17], Al-Niami ANS and Rushton KR (1997) [24], Bear J (1975) [29], Gao G, et al (2013) [55] , Mirbagheri S A (2009) [75], Reddy K, et al (2015) [84] Recently, several numerical methods have been developed to solve equations and simulate the transport of pollutants in space; and other specific mathematical approaches are also used such as Lai SH (1971) [71]; Moldrup P (1996) [77] AtaieAshtiani B (1996) [25] and Mirbagheri F (2013) [76] used finite difference (FD) and finite volume (FV) methods to calculate spread 7 Many studies in the world have long built a mathematical model to predict the spread of pollutants in the wastewater such as Ogata (1970) [79]; Ataie-Ashtiani B (1996) [25]; Thongmoon M (2006) [102]; Mirbagheri SA (2009) [75]; Patil S (2014) [81] researched and built a mathematical model to calculate the contaminant spread of leachate in the soil, but only in or dimensional models In a number of published studies, the general equation has been simplified by removing convection terms or using linear techniques to obtain analytical solutions under ideal conditions and the partial derivative equation have only been transformed and represented in one or two dimensions recently, such as Beidokhti MZ et al (2019) [30], Buaisha M et al (2020) [33] Thus, from the study of literature review from abroad, it is shown that building a mathematical model to predict the spread of heavy metals in leachate in 3D space is necessary and meaningful when evaluating solid waste landfill pollution 1.5.2 Literature review in Vietnam There are few domestic studies on heavy metal component in leachate and soil and groundwater contamination by heavy metals from leachate in Vietnam Most of the research on leachate focuses on organic compounds, nitrogen compounds, phosphorus and leachate treatment methods on these criteria From the literature review results in the world and in Vietnam, it can be seen that untreated leachate containing many heavy metals with high concentration is the most obvious direct source of surface water pollution Most of the studies have shown that the soil and groundwater at the solid waste landfill area and its vicinity have not been contaminated with heavy metals at a level greater than the allowable standard for agricultural land However, all studies show that solid waste and leachate play a pollutant role in increasing the concentration of heavy metals in soil and groundwater based on the analysis of spatial assessment and the heavy metal concentration in buffer capacity environment 1.6 Selected research direction The previous studies carried out to develop forecasting and calculation methods to determine heavy metal pollution but are not general and quite cumbersome, many studies only stop at 1D and 2D models The method of predicting the spread often uses available models for calculation, is costly and not suitable for the conditions of Vietnam Therefore, it is very necessary to conduct research on the risk of heavy metal pollution in the sanitary landfill, build a general mathematical model, apply and exploit it on modern tools to calculate and predict pollution in 3D is necessary and has practical significance in the solid waste management in Vietnam Chapter THEORY OF POLLUTION TRANSMISSION AND METHODOLOGY OF RESEARCH 2.1 Pollutants spread in the soil Pollutants in soil exist in many different forms (or phases) depending on the physical and chemical nature of the pollutant Pollutants can dissolve into groundwater and move through the pores of the soil On a large scale, this process can be modeled according to the flow and direction of groundwater, but on a narrow scale, this process is directly related to the particle size and porosity of the soil When moving in the soil, the pollutant (or in other words, the stream containing the pollutant) does not pass through the soil particles but through the voids in the soil 2.2 Mechanism of pollutant transmission in soil 2.2.1 Permeable flow rate Flow rate of groundwater in the soil can be estimated using the formula of Darcy law (2-1) Q=k i.A Q: flow (cm3/s); k: permeability coefficient (cm/s); i: hydraulic gradient; A: crosssectional area (cm2) 2.2.2 Diffusion coefficient of the pollutant When modeling the contaminant spread along the infiltration stream in the soil, it is necessary to determine an important parameter, which is the diffusion coefficient of the pollutant in the soil In fact, this diffusion coefficient depends on the pollutant and soil type, so it is necessary to conduct experiments to find the most accurate diffusion coefficient According to the study of Çamur et al [35], Rowe et al [89], Yang [116] the diffusion coefficient found by the authors can be used as a reference in calculating the dispersion of pollutants and heavy metals in the soil 2.3 Research methods to determine the spread of pollution 2.3.1 Method to determine the composition and charateristics of leachate Leachate is sampled and analyzed to determine the content of pollutants and heavy metals for the calculation and prediction of the spread of heavy metals in the landfill The leachate sample was taken at the leachate collection pit from the landfill cells before being pumped into the wastewater receiving lake Sampling location is shown in figure 2.3 Figure 2.3 Soil and leachate sampling locations 2.3.2 Methods to determine the composition and characteristics of the soil To determine the spread of pollutants, in the field, drilling methods are often used to take soil samples The holes are drilled in depth from 0.3m, 0.6m, 0.9m,… 6m Sampling location in Figure 2.3, soil samples were taken and analyzed to determine the concentration of heavy metals in the soil 2.3.3 Analytical solution – differential equations for contamination spread The spread equation of the pollutant in the soil can be solved through analytical solutions or by numerical methods (finite difference or finite element) Analytical methods usually only solve problems with simple boundary and initial conditions and the ground is considered homogeneous and isotropic To be able to give a unique solution to a differential equation, the initial and boundary conditions need to be clearly specified The first condition describes the starting values of some variables 10 in the problem, in this case the pollutant concentration Boundary conditions describe the relationship or interaction between the studied area and the outer areas 2.3.4 Finite element method The finite element method is an approximate numerical method for solving problems in engineering and mathematical physics The finite element method is a very general and effective method for numerical solutions of many different classes of engineering problems A system of linear equations is created from elemental equations by converting the coordinates from the local nodes of the subdomains to the global nodes of the domain This spatial transformation includes the appropriate orientation adjustments as applied relative to the coordinate reference system This process is usually performed by finite element method software using coordinate data generated from subdomains 2.3.5 Formulate the model of the contaminant transport in 3D To formulate equation, we consider a model of a landfill cell, in which, the source of leachate is on the horizontal surface The leachate stream containing heavy metal pollutants spreads vertically and horizontally to the surrounding environment During this process, pollutant concentrations change in all directions and over time Figure 2.4 The gradient field of the concentration function C (x, y, z, t) in the coordinate system Oxyz 11 Figure 1.5 The diagram to set up predictive model of heavy metal spread in soil Chaspter RESULTS AND DISCUSSTION 3.1 Kieu Ky landfill The selected research area is Kieu Ky landfill in Gia Lam district, Hanoi city This is a landfill operated in a sanitary manner, by the time of the research survey, there were landfill cell that had been closed for more than 10 years 3.2 Factors affecting the spread of pollutants from landfill leachate Set up a computational modeling method for leachate volume 12 Figure 3.1 Diagram describe leachate flow in landfill cell 3.3 Investigation of heavy metal content in leachate Among the various contaminant components in leachate, metals, especially heavy metals, need more attention because of their harmful effects on the environment and affect on biophysics treatment process There are many types of heavy metals in leachate, the insoluble metals in the waste are converted to soluble metals and then dissolved in leachate through physical and chemical reactions 13 Fe concentration in leachate mg/l Pb concentration in leachate mg/l 0.7 Concentration Pb Concentration Fe 10 0.6 0.5 0.4 0.3 0.2 0.1 M1 Fe M2 Column A M3 M4 M1 Column B Pb Cr concentration in leachate mg/l 1.5 0.5 M1 Cr (III) M2 Column A M3 M4 Column B Column A M3 M4 Column B As concentration in leachate mg/l Concentration As concentration Cr M2 0.3 0.25 0.2 0.24 0.2 0.16 0.13 0.15 0.1 0.05 M1 As M2 Column A M3 M4 Column B Figure 3.2 Concentration of heavy metals in leachate at Kieu Ky landfill 3.4 Survey of heavy metal pollution in the soil at Kieu Ky landfill The soil samples were analyzed for the content of heavy metals by the Inductively Coupled Plasma Mass Spectrometer (ICP-MS) of the AES analysis group, this method has high accuracy, can analyze the results up to limited to ppb or ppt (ie 1x10-12) 14 Variation of Cr content with borehole depth 350 300 250 200 150 100 50 Concentration Cr mg/kg Concentration Cr mg/kg Variation of Cr content with borehole depth 350 300 250 200 150 100 50 Depth, m Depth, m LK4 100 80 60 40 20 Depth, m LK5 LK4 LK4 Variation of Pb content with borehole depth Variation of Cu content with borehole depth Concentration Pb mg/kg Concentration Cu mg/kg LK5 LK5 150 100 50 Depth, m LK5 LK4 Figure 3.3 Variation of As, Cr, Cu, Pb with borehole depth L4 and L5 3.5 Evaluation of factors affecting the process of heavy metal spread from landfills - Pollution time - The amount of leachate generated: Calculation results of the amount of leachate generated from the landfill indicate that the pollution source still exists for a long time after the landfill cell is closed The amount of leachate in the last months of the 21st year generated from the landfill with an area of 10,084m2 is 14m3/day The calculation and forecast of the amount of leachate still generated from the closed landfill confirms that the risk of pollution from leachate persists - Concentrations of pollutants in leachate: Concentrations of pollutants collected exceeded the allowable standards comparing with QCVN 25:2009 and QCVN 40:2011 Depending on the time of sampling, a number of heavy metals such as Cr, As, Pb, Hg, Fe in leachate have higher concentrations than permitted standards comparing with QCVN 40:2011 - Contaminant content in soil: Survey results from boreholes in Kieu Ky landfill show that the soil is contaminated with heavy metals when compared with QCVN 03:2015 on the permissible concentration of heavy metals in agricultural land The 15 concentrations of heavy metals in the soil exceed the permitted standards as As, Cr, and Pb according to the depth and distance of the sample drilling compared to the closed landfill cell 3.6 Prediction of heavy metal concentrations spread in landfill environment by finite element method 3.6.1 Risks of soil pollution arising from sanitary landfills 3.6.2 Prediction and assessment of heavy metal concentrations spread by 1D Three simulation model domains for spreading heavy metals in the weakly permeable soil layer are selected: 1) Natural soil profile next to the landfill (from the ground to a depth of m, i.e to the aquifer, with a length of m); 2) The second zone is the cross section of the soil at the bottom of the lake that collects and stores leachate (from a depth of 1.5 m to the surface of the aquifer, 4.5 m long); 3) The third section is the section at the bottom of the landfill (from a depth of 4.5 m to the aquifer, 1.5 m depth) The first, second and third model domains are named domain 1, domain and domain 3, respectively The three model domains have different boundary conditions Figure 3.4 Heavy metal spread computational model domains 16 To ensure the accuracy of the model results, the time step t and the grid step x must satisfy the requirements of the Peclet number and the Courant number as follows [60]: Peclet: Pe Vx,i xi Dx ,i ; Courant: Cr Vx ,i t xi (3-9) 1 The finite element model program spreading dissolved substances in groundwater is used from the results of the application-oriented basic research project of Nguyen Van Hoang (2018) [7], and the accuracy has been determined tested by standard problems The program is linked with the least squares sum of squares algorithm to determine the parameters Heavy metal spread results with 1D As concentraion 0.20 0.18 Concentration (mg/L) 0.16 0.14 0.12 0.10 2,5m CTB 2,5m 0.08 0.06 0.04 0.02 0.00 0.5 1.5 2.5 3.5 Depth (m) 4.5 5.5 Figure 3.5 As concentration in water with R=1 and R=2 Discuss the results of spread according to 1D - The spread of heavy metal pollution from the bottom of the leachate reservoir to the aquifer takes place faster than the spread from the ground surface if calculated with the same delay coefficient R=1 Because the bottom of the leachate collection and storage reservoir is saturated cross section and vertical seepage occurs The time of transmission to the aquifer reaching the pollutant content as leachate is 3.5 months - When taking into account the adsorption capacity of soil and rock in the underground environment, it is difficult to accurately determine the processes that have occurred affecting the propagation capacity of heavy metals Calculation with 17 delay R factor = to simplify the actual condition because adsorption is a function of heavy metal concentration in groundwater that varies in space and time The forecast results if the heavy metal spreads in the MH1 domain to the aquifer, it will take twice as much time when taking into account the adsorption capacity with the hysteresis coefficient R Similar to the MH2 and MH3 domains when taking into account the delay factor R the propagation time will be times slower in the absence of adsorption It is very difficult to determine precisely the processes that have occurred and the adsorption capacity of soil and rock in the underground environment affecting the ability to spread of heavy metals In order to simplify the actual conditions, the 1D computational model using the delay factor = for the prediction results of heavy metal spread in the natural soil cross-section next to the landfill to the aquifer will need twice as much time when taking into account the adsorption capacity Similar to the soil cross-sections at the bottom of the leachate collection and storage reservoir, and the bottom section of the landfill when taking into account the propagation time delay factor, it will be times slower in the absence of adsorption 3.6.3 Set up of heavy metal spread evaluation equation with 3D model In this study, the author builds a mathematical equation to solve the pollution propagation equation according to the time variable t in the 3-dimensional space x, y, z with the assumption that the permeability and dispersion of the substance concentration pollution is similar in the two horizontal x and y directions To discretize the pollution propagation equation in 3D space consisting of finite elements, the finite difference method is used for both time and space variables x and z This method can also be called the finite element method with the grid nodes of each block element equally spaced at distances Δx and Δz The following FEM algorithm is built and Matlab programming syntaxes are used to implement the proposed algorithm FEM algorithm: Calculate concentration 𝐶 , at i, j grid and n time Input data: Initial concentration 𝐶 ; 𝐷𝑥 = 𝐷𝑦, 𝐷𝑧, 𝑢 = 𝑣, 𝑤; Time step 𝛥𝑡; Grid increment 𝛥𝑥 = 𝛥𝑦, 𝛥𝑧; Upper limit of increment 𝑛 , 𝑛 , 𝑛 ; Output: 𝐶 , 18 Simulation results of heavy metal spread in 3D Conducting research on arsenic spread at Kieu Ky landfill, Gia Lam includes main steps as follows: (1) Step 1: Determine the arsenic concentration in the closed landfill area in Kieu Ky landfill by experimental research and statistical methods (2) Step 2: The results in step are used as the initial value to perform the calculation algorithm to simulate research and forecast the spread of As in the soil To predict the spread of heavy metals, in this study, the initial concentration of As was chosen as C0≈30mg/kg, the experimental statistical value at the borehole L5 close to the wall of the landfill cell at the site of drill holes 0.3 – 0.6 m under the surface soil The boundary condition parameters are selected as follows: 𝐶 , = 0; 𝐶 , = 0; 𝐶 , = 𝐶 , with j from to Permeation velocity in selected directions: u=v=w = 0,037 m/day = 1,11 m/month Diffusion coefficient D= 0,00048 m2/day The selection of parameters for the computational model is based on some published research results, Rowe [101] Figure 3.42 is the result of calculation and simulation of the change of arsenic concentration in the vertical z (depth) and horizontal x direction at the 10th month Thus, it can be seen that pollutants spread with concentrations decreases in both vertical and horizontal directions over time Figure 3.42 Distribution of As concentration at 10th month in the X and Z 19 Figure 3.43 (a) Variation of arsenic concentration with Z and t (b) Variation of arsenic concentration with X and t (at depth Z = 2.0 m) Discussing the results of spread according to the 3D At any time, the 3D simulation model can be applied to calculate the heavy metal spread concentration The longer the time, the less likely the contamination will be if the initial concentration is constant Figure 3.42 on the left shows the spread of As pollution by color at the 10th month according to depth (z) and horizontal (x) with the unit m The concentration of heavy metals pollutant changes by color corresponding to the decrease in both directions Points on the same contour have the same As pollutant concentration The area outside the outermost contour has negligible pollutant concentrations From the image of contour lines as above, by the 10th month, As concentration is present at a depth of more than 4m in the ground; in horizontal direction, As also spreads over 4.5m from the center The results shown in Figure 3.42 also show that the polluted space in the x and z directions is divided into square grids, each square corresponds to a specific numerical value of the concentration in the direction horizontal x, the concentration decreases gradually from 27.3mg/kg to 21.65mg/kg from the initial location where the calculated pollution concentration will spread to a distance of 2.5m In the z direction, As content decreased from 27.3mg/kg to 16.75mg/kg from 0.5m to more than 3m depth In this spatial region, the predicted values of the As content are exceeding the allowable standards QCVN 03:2015 However, in fact, the risk of leachate leakage from the closed landfill is extremely large, then the leachate containing heavy metals is leaked, washed, spread to the environment and then permeated and diffused into the soil environment 20 The change of As content over space and time (different months) is shown in Figures 3.43a and 3.43b Figure 3.43a is the graph depicting the variation of As concentration by depth (z) corresponding to the 2nd, 4th, 6th, 8th months The calculated concentration at 3m position at the 8th month exceeds the regulation for Permits on As pollution in soil when compared with QCVN 03:2015/BTNMT This is also completely consistent with the survey results when drilling samples was carried out at the field in April 2016 to determine the risk of heavy metal pollution in the soil at Kieu Ky landfill, Gia Lam Thus, by using the concepts of gradient vector field ∇C (x, y, z, t), and related gradient operations (∇⋅), ⊙, the diffusion-convection mechanism of the substance Pollutants in leachate environments are clearly and concisely modeled This model partly shows the physical nature of the propagation process more clearly 3.7 Significance of the computational model of heavy metal spread from solid waste landfill 1D Model - The application of 1D computational model plays an important role when planning the location of landfill construction It will also be a necessary calculation method applied to closed landfill to determine the risk of heavy metal contamination spreading 3D Model - Algorithm based on finite element method (FEM) was built to solve digitized equations over time in 3D, effectively analyzing the variation of heavy metal pollutant content - The results on the graph of spreading content in underground space are easy to understand and compare when shown in color and numerical values of heavy metal content, by each grid cell (subdividing continuous into discrete subdomains) Therefore, it is possible to use the chart to know the value of heavy metal content spread at any location in the underground space 21 CONCLUSIONS Conclusions - Factors affecting the process of pollutant transmission in the solid waste landfill environment at Kieu Ky Gia Lam include: 1) the source of heavy metal pollution from leachate, 2) the concentration of heavy metal As in the solid waste landfill leachate and the content of heavy metals As in the soil of the landfill exceed the permitted regulations QCVN 40:20111/BTNMT and QCVN 03:2015/BTNMT, 3) accumulation time of heavy metal pollutants due to existing landfills more than 10 years In addition, other heavy metals detected in leachate are Cr, Pb, Hg, Cd all have higher concentrations than allowed standards because solid waste is buried without classification at the source - The safety belt for the risk of As pollution has been determined based on the calculation results of 1D and 3D models According to the depth, the 1D model results have confirmed specifically at the vulnerable locations to the aquifer is the bottom of the burial cell (1.5m away from the aquifer), the bottom of the leachate reservoir (4,5m), the spreading time occurs very quickly if not taking into account the soil adsorption capacity, the corresponding time will be 40 days and 105 days The spreading time needs to be doubled with adsorption capacity so the heavy metal concentration will be equal to in the leachate - The safety belt in the x and z directions at any position around the landfill cell according to the 3D model is 3m and 3.5m after months if the initial As content is constant, after this value range, the calculated As concentration only met the allowable regulations of QCVN 03:2015/BTNMT The simulation results of the change of heavy metal As concentration by depth (z), horizontal (x) and time in months show that the risk of comprehensive contamination of the environment surrounding the landfill is difficult avoid; and Arsenic spreading is the direct cause of pollution of groundwater and soil layers at the landfill area The results on both models show that heavy metal As tends to move from the upper soil layer to a deeper aquifer, the spread speed in the z direction is faster than the x direction (3D model) Recommendations Through consideration and research, we have some recommendations as follows: 22 - Segregation at source is a practical action that needs to be applied as soon as possible when conducting solid waste treatment by sanitary landfill technology, which has high significance in reducing heavy metals pollution in the landfills - It is necessary to carry out more specialized studies in mathematics as well as environmental geology to build a model of heavy metal spread with the influencing factors of different phases in the soil environment - It is necessary to continue to expand the research to different landfill sites with different natural conditions and conditions for receiving and operating the landfill (closured landfill) These studies will set up a complete scientific basis to help managers come up with technical regulations and standards on environmental quality control at landfills, minimizing environmental pollution from landfill activities LIST OF AUTHOR'S PUBLISHED ARTICLES RELATED TO THE DISSERTATION Hoang Ngoc Ha (2018), Heavy metal pollution from landfills to the soil environment: Kieu Ky landfill - Gia Lam - Hanoi, VNU Science Journal: Earth and Environmental Sciences, Volume 34, Issue (2018), pp 86-94 Hoang Ngoc Ha, Nguyen Thi Kim Thai, Nguyen Thu Huyen, Pham Anh Tu (2018), Assessment of the risk of landfill contamination by the LPI leachate pollution index, Journal of natural resources and environment, No.16 (294), part 2, August 2018, pp 31-33 Hoang Ngoc Ha, Nguyen Thi Kim Thai, Nguyen Van Hoang (2020) Modeling method for predicting leachate generated from municipal solid waste landfill Construction Magazine, September 2020, pp 44-47 Hoang Ngoc Ha, Anh My Chu, Kim Thai Thi Nguyen, and Chi Hieu Le (2021) A novel mathematical modelling for simulating the spread of heavy metals in solid waste landfills Environmental Engineering Research, 27(3) https://doi.org/10.4491/eer.2021.007 Nguyen Van Hoang, R Shakirov, Hoang Ngoc Ha, Trinh Hoai Thu, N Syrbub, and A Khokhlova (2021) Assessment of Soil and Groundwater Heavy Metal Contamination by Finite Element Modelling with Freundlich Isotherm Adsorption Parameters in Waste Landfill Kieu Ky in Hanoi, Vietnam Eurasian Soil Science, 2021, Vol 54, No 12, pp 1876–1887 https://doi.org/10.1134/S1064229321130020 ... Hanoi National University of Civil Engineering At hour day month 2022 The dissertation can be found at the libraries: - Hanoi National University of Civil Engineering - The National Library... solving problems in engineering and mathematical physics The finite element method is a very general and effective method for numerical solutions of many different classes of engineering problems...1 The dissertation was completed at: Hanoi University of Civil Engineering Scientific Advisor 1: Prof Nguyen Thi Kim Thai Scientific Advisor 2: Assoc.Prof Nguyen