A method for arranging monitoring wells and determining tracer injection boundaries for landfill leakage detection

Abstract

The present invention discloses a method for arranging monitoring wells and determining tracer injection boundaries for landfill leakage detection. The present invention aims to determine the injection amount of tracers in field tracer test injection wells and the arrangement of monitoring wells by using the migration law of tracers in landfill bodies and barriers. Considering the diffusion and convection of tracers in landfill bodies and barriers, a two-dimensional control equation for tracer migration is established in combination with Fick's law, Darcy's law and mass conservation equation, and the solution of the equation is obtained by Laplace-Fourier transform and Talbot numerical inversion. According to different characteristics of the landfill cover layer, garbage body and bottom liner, such as size, porosity and permeability characteristics, the injection amount of tracers and the arrangement method of barrier leakage monitoring wells are determined. The method proposed by the present invention is efficient and reliable in calculation, can be used for leakage detection and efficient positioning of barrier barriers in service landfills, and has important engineering value for the management of landfills.

Description

Technical Field

[0001] The invention belongs to the field of landfill leakage monitoring, and in particular relates to a monitoring well arrangement and a tracer injection boundary determination method for landfill leakage detection. Background Art

[0002] Landfills are composed of a cover layer, garbage and a bottom liner. The bottom liner and cover layer are used to reduce the pollution of landfills to groundwater, soil and atmosphere. However, due to factors such as construction, dry-wet cycles and climate change, the geomembranes that make up the cover layer and bottom liner of the landfill are prone to tearing, holes and clay cracking. This causes landfill gas to escape from the cover layer and leachate to leak from the bottom liner, polluting the atmosphere and groundwater. Assuming that the performance of other facilities in the landfill remains good, the leakage of leachate from a 25cm diameter hole in the bottom liner can reach 15kg / day, about 5.4 tons / year. The leakage of the landfill cover layer causes the landfill gas to escape, resulting in the greenhouse effect and odor nuisance. There are hundreds of odorous gases in the landfill gas, which will seriously pollute the environment and affect the health of surrounding residents once they leak.

[0003] The gas tracer test method can be used to determine many properties of the vadose zone system, such as moisture content, gas flow rate and tortuosity. It can also be used to test the gas flow pattern in the landfill and quantify the residence time of volatile pollutants. The main idea of ​​using tracers to detect leaks is to inject a harmless, low-content, inert and easy-to-monitor tracer gas (such as SF6) into the barrier system (cover system and bottom liner system) through the injection port to create a large-area tracer source. When there is a gap in the barrier, the tracer gas will diffuse into the adjacent medium through the gap, and the concentration of the tracer gas at different times will be measured in the soil gas of the sampling hole outside the barrier. The obtained relationship data of the tracer gas concentration over time (with the location of the sampling hole, the properties of the medium and the tracer gas source concentration) will be provided to the global optimization code. According to the relevant parameters input, the code iteration will find an optimal solution to accurately locate the leakage location. However, the migration patterns of tracers in landfill systems are not yet clear. In order to determine the injection volume of tracers in injection wells and determine the layout of monitoring wells, it is very necessary to understand their migration patterns in the landfill. Summary of the invention

[0004] The purpose of the present invention is to provide a method for determining the tracer injection amount and leakage location for landfill leakage detection. In view of the serious leakage problem of domestic landfills, leakage in landfills is detected in the post-operation stage of landfill closure. A method for calculating the tracer injection amount and a method for arranging monitoring wells for detection using tracers are provided, which is of great significance for the preliminary judgment of landfill leakage detection.

[0005] The present invention uses tracers to detect leakage of barriers such as bottom liners and cover layers in landfills, and uses the migration law of tracers in landfill bodies and barrier layers to determine the injection amount of tracers and the leakage location in the field tracer test injection well. Considering the diffusion and convection of tracers in landfill bodies and barrier layers, a two-dimensional control equation for tracer migration is established in combination with Fick's law, Darcy's law and the mass conservation equation, and the solution of the equation is obtained by Laplace-Fourier transform and Talbot numerical inversion. According to the different characteristics of the landfill cover layer, garbage body and bottom liner, such as size, porosity and permeability characteristics, the injection amount of tracers and the arrangement method of barrier leakage monitoring wells are determined. The method proposed by the present invention can be used for leakage detection and positioning of barrier layers in service landfills, and has important engineering value for the management of landfills.

[0006] The present invention is achieved through the following technical solutions:

[0007] The present invention first provides a method for arranging monitoring wells and determining tracer injection boundaries for landfill leakage detection, which is characterized by comprising the following steps:

[0008] Step 1): Constructing a physical model of gas phase tracer injection into landfill;

[0009] Step 2): For the above physical model, establish the control equation and solve it;

[0010] Step 3.1): Calculate the concentration profile and release flux of the tracer in the landfill system and plot its variation curve over time and space;

[0011] Step 3.2): Determine the location coordinates of the monitoring point based on the concentration profile drawn in step 3.1) and the detection limit of the tracer; on the calculated concentration profile, find the point where the concentration is equal to the detection limit of the tracer, and the location of this point and the point where the concentration is greater than this point is the location where the monitoring well can be arranged.

[0012] Step 4): Determine the release flux and concentration of the tracer at the monitoring point, and compare them with the detection limit. If the tracer concentration or release flux at the monitoring point is greater than the detection limit, the injection concentration boundary of the injection well in step 1) is determined to be the actual injection concentration; if the tracer concentration or release flux at the monitoring point is less than the detection limit, the tracer cannot be detected at the monitoring point, increase the injection boundary concentration or flux of the injection well, and repeat steps 3.1)-step 4).

[0013] As a preferred embodiment of the present invention, the step 1) specifically comprises the following steps:

[0014] Based on the construction scale of the closed landfill, the thickness of the garbage body is determined to be l1, the thickness of the cover layer is l2, and the thickness of the bottom liner is l3; the moisture content of the landfill cover layer, garbage body and bottom liner is measured on site, and the diffusion coefficient and convection rate of the gas phase tracer in the landfill cover layer, garbage body and bottom liner are obtained according to the diffusion coefficient calculation model (Formula 1) and Darcy's law (Formula 2).

[0015]

[0016]

[0017] Where D and D0 are the diffusion coefficients of the tracer in soil and air, respectively, θ a is the air-containing porosity of the soil, n is the porosity of the soil, v is the convection velocity of the tracer in the soil, k a is the permeability coefficient of the tracer in the soil, and dP / dz is the pressure gradient at both ends of the soil.

[0018] As a preferred embodiment of the present invention, the step 2) specifically comprises the following steps:

[0019] Step 2.1): Determine the governing equations and boundary conditions

[0020] A two-dimensional radial axisymmetric coordinate system is established with the middle point of the garbage body as the origin, and the two-dimensional control equation of tracer migration is established. The tracer injection condition is a constant concentration C0 injection, and the boundary conditions used in the model are determined;

[0021] Step 2.2): Obtain the migration law of the tracer in the landfill, specifically including:

[0022] Step 2.2.1): Calculate the tracer migration concentration profile in the Laplace domain;

[0023] Step 2.2.2): Calculate the tracer release flux on the overburden surface in the Laplace domain and calculate the tracer concentration on the bottom of the liner in the Laplace domain;

[0024] Step 2.2.3): Use Talbot numerical inversion to obtain the tracer concentration distribution profile and release flux.

[0025] As a preferred embodiment of the present invention, the step 2.2.1) is specifically as follows:

[0026] The injection well is injected at the fixed concentration boundary, and the tracer migration concentration in the landfill is

[0027]

[0028] Where C0 is the concentration of the tracer in the injection well, p is the time factor in the Laplace domain, μ n and ωn is the characteristic root, r is the radial coordinate, r w is the radius of the injection well, z is the vertical coordinate, and m2 is the migration parameter of the tracer in the garbage body, which can be calculated using the following formula:

[0029]

[0030] Where v is the convection velocity of the tracer in the garbage body; θ is the air-containing porosity of the garbage body; D z D is the diffusion coefficient of the tracer in the garbage along the axial direction; r is the radial diffusion coefficient of the tracer in the garbage body;

[0031] The concentration of the tracer in the overburden is

[0032]

[0033] Where m3 and m4 are the migration parameters of the tracer in the overburden layer, which can be calculated by the following formulas:

[0034]

[0035]

[0036] Where v u is the convection rate of the tracer in the overburden; D u is the diffusion coefficient of the tracer in the overburden along the axial direction;

[0037] The tracer concentration profile in the bottom liner is

[0038]

[0039] Where m5 and m6 are the migration parameters of the tracer in the bottom liner, which are solved by the following equations:

[0040]

[0041]

[0042] Where v l is the convection rate of the tracer in the bottom pad; D l is the diffusion coefficient of the tracer in the bottom liner along the axial direction.

[0043] As a preferred embodiment of the present invention, the step 2.2.2) is specifically as follows:

[0044] Calculate the tracer release flux on the overburden surface in the Laplace domain:

[0045]

[0046] Calculate the tracer concentration at the bottom of the pad on the Laplace domain:

[0047]

[0048] The step 2.2.3) is specifically:

[0049] The concentration distribution profile and release flux of the tracer are obtained by Talbot numerical inversion. The calculation formula is as follows:

[0050]

[0051] Where M is the superposition constant, which is taken as 64, Re is the real part of the complex number, γ i , δ i is the Laplace inversion constant, defined by the following formula:

[0052]

[0053]

[0054]

[0055]

[0056] Where a is the imaginary factor

[0057] Compared with the prior art, the present invention has the following beneficial effects:

[0058] The present invention can calculate the injection amount of the tracer in the injection well in the field tracer experiment under the premise of considering the diffusion and convection of the tracer, and simultaneously determine the arrangement of the monitoring well.

[0059] The present invention can calculate the migration concentration profile and release flux of the tracer according to the different moisture contents of the landfill cover layer, garbage body and bottom liner. Therefore, the migration law of the tracer can be obtained under different weather conditions. By comparing with the detection limit of the tracer, the layout of the monitoring well can be determined, and the injection boundary of the tracer of the injection well can be determined at the same time. By comparing with the field measured data, the approximate location of the leakage point can be preliminarily obtained.

[0060] By comparing with numerical simulation theoretical calculations, the method of the present invention has good reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 The present invention is a flow chart for calculating the migration of landfill leakage detection tracers.

[0062] Figure 2 It is the calculation model of the present invention.

[0063] Figure 3 It is a profile cloud diagram of the tracer concentration calculated by the present invention.

[0064] Figure 4 It is the curve of tracer release flux changing with time under different gas-containing porosity conditions of the calculation model of the present invention. Specific implementation plan

[0065] The present invention will be further described below in conjunction with the accompanying drawings and embodiments, but the scope of implementation of the present invention is not limited thereto.

[0066] The invention mainly determines the scale of the landfill according to the landfill design drawings, mainly including the depth of the landfill, the thickness of the bottom liner l3, the thickness of the garbage body l1 and the thickness of the cover layer l2. The moisture content of the landfill garbage body, the moisture content of the upper liner, the type of the cover layer and the bottom liner material are obtained through on-site detection, and the diffusion coefficient and convection rate of the tracer in the landfill system are determined. The injection boundary conditions of the tracer in the injection well and the arrangement of the monitoring well are established.

[0067] The present invention is further explained below by taking the calculation of the migration law of SF6 tracer in a simplified landfill model as an example:

[0068] like Figure 1 As shown, the present invention is divided into four steps. Step 1 is to construct a physical model of tracer migration in a landfill (physical model of gas phase tracer injection into a landfill), which mainly determines the types and parameters of various parts of the landfill, tracer migration parameters, and injection well boundary conditions; Step 2 is to establish a control equation, solve the equation to obtain the tracer migration concentration profile and surface release flux expression; Step 3 is to draw the tracer migration concentration profile and release flux curve, and at the same time determine the position of the monitoring point and its distance from the injection well (i.e., calculate the concentration profile and release flux of the tracer migration in the landfill system, and draw its curve of change with time and space; determine the position coordinates of the monitoring point according to the drawn concentration profile and the detection limit of the tracer). Step 4 is to determine the concentration and release flux of the tracer at the monitoring point and compare them with the detection limit. If the tracer concentration or release flux at the monitoring point is greater than the detection limit, the injection concentration boundary of the injection well in step 1 is determined to be the actual injection concentration; if the tracer concentration or release flux at the monitoring point is less than the detection limit, the tracer cannot be detected at the monitoring point, the injection boundary concentration or flux of the injection well is increased, and steps 3 and 4 are repeated.

[0069] The present invention can obtain the injection amount of the tracer and the arrangement method of the monitoring well. In the subsequent monitoring process, the actual concentration of the tracer obtained in the monitoring well is compared with the calculated result, and the approximate position of the leakage point can be preliminarily obtained.

[0070] The present invention is further described below in conjunction with the examples. Step 1) Construction of a physical model of tracer migration in a landfill

[0071] like Figure 2 As shown, the physical model of the migration of the tracer in the landfill of the present invention mainly consists of the following parts: cover layer ①, garbage body ②, bottom liner ③, injection well ④. It is assumed that the thickness of the cover layer is 1m, the thickness of the garbage body is 2m, and the thickness of the bottom liner is 1m. According to the materials used for the cover layer and bottom liner of typical domestic landfills, clay is selected as the main cover material, and the porosity n of the soil body is selected as 0.46. Therefore, the specific working conditions and parameter values ​​of the cover layer and the bottom liner are as follows in Table 1:

[0072] Table 1 Tracer migration parameters in soil

[0073]

[0074] The migration parameters of the tracer in the garbage body are shown in Table 2:

[0075] Table 2. Migration parameters of tracers in garbage bodies

[0076]

[0077] Assuming that the injection boundary of the injection well is a constant concentration injection, that is, the tracer concentration in the injection well remains constant, due to the concentration difference and pressure difference, the tracer enters the landfill system in the form of diffusion and convection. The injection boundary is as follows:

[0078]

[0079] In order to facilitate on-site operation and calculation, this example selects the initial concentration of the tracer in the injection well as C0 = 10 g / m 3 .

[0080] Step 2) Model establishment and solution

[0081] The concentration profiles of the tracer in the landfill cover layer, garbage body and bottom liner in the Laplace domain, as well as the release flux expression of the tracer on the cover layer surface, are obtained by formulas (1)-(10). Then, numerical inversion is performed by formulas (13)-(17).

[0082] Step 3.1) Plotting tracer migration concentration profile and release flux curve

[0083] like Figure 3 The figure shows the concentration profile cloud diagram of the migration of the tracer in the landfill system. The working conditions are that the air porosity of the cover layer, the garbage body and the bottom liner are 0.1, 0.2 and 0.1 respectively. The diffusion coefficient and convection rate of the tracer in the cover layer, the garbage body and the bottom liner are obtained by looking up Table 1 and Table 2 respectively. Assuming that the detection limit of the SF6 detector used in the present invention is 1ppm, it can be based on Figure 3The concentration profile cloud map can be used to obtain a curve with a concentration of 1 ppm, which can determine the radial coordinates of the monitoring points at different depths when t = 0.5 years. The monitoring points can be arranged at a position 0.5 m below the surface layer of the overburden, that is, z = 1.5 m, and at a radial distance of r = 5 m from the injection well.

[0084] Step 4) Determination of injection volume of injection well and analysis of parameter impact

[0085] Assuming that the detection limit of the SF6 detector used in the present invention is 1ppm, according to Figure 3 From the concentration profile drawn, it can be obtained that when t=0.5 years, the tracer concentration at the monitoring point is less than 1ppm. This shows that in order to detect the tracer at the monitoring point at t=0.5 years, it is necessary to increase the tracer concentration in the injection well. Therefore, the tracer concentration in the injection well is increased by 10 times, and substituted into steps 2 and 3 for calculation. This is repeated until the tracer concentration obtained at the monitoring point is greater than the detection limit. The final tracer injection concentration value of the injection well is the concentration of the tracer injected in the field test. The final tracer injection concentration ensures that at t=0.5 years, the tracer can be detected at the monitoring point (the concentration is greater than the detection limit). In the subsequent monitoring process, for example at 0.5 years, by detecting the tracer concentration data of each monitoring point, the approximate location of the leakage point can be preliminarily obtained. Figure 4 The figure shows the calculated tracer release flux curve of the cover layer under different air porosity (water content) conditions. It can be seen from the figure that the air porosity of the cover layer has the greatest influence on the tracer release flux, while the air porosity of the bottom liner has almost no effect on the tracer release flux. Figure 4 It can be seen that when θ=0.2,θ l =0.1, t = 100day, when the air-containing porosity of the overburden increases from 0.1 to 0.3, the tracer release flux at r = 6m increases by 5 orders of magnitude. u =0.3,θ l = 0.1, 100 days, when the gas porosity of the garbage body increases from 0.2 to 0.4, the release flux of the tracer at r = 6m increases by 2000 times. Therefore, it is necessary to use the method of the present invention to arrange the monitoring wells and determine the tracer injection boundary according to the specific conditions of each landfill.

[0086] The above embodiments are intended to illustrate the present invention, not to limit the present invention. Any simple transformation of the present invention falls within the protection scope of the present invention.

Claims

1. A method for arranging monitoring wells and determining tracer injection boundaries for landfill leakage detection, characterized in that: The steps include: Step 1): Based on the construction scale of the closed landfill, determine the thickness of the garbage body as l1, the thickness of the cover layer as l2, and the thickness of the bottom liner as l3; measure the moisture content of the landfill cover layer, garbage body and bottom liner on site, and obtain the diffusion coefficient and convection rate of the gas phase tracer in the landfill cover layer, garbage body and bottom liner according to the diffusion coefficient calculation model (Formula 1) and Darcy's law (Formula 2). D=D0(θ a 10 / 3 / n 2 ) (1) Where D and D0 are the diffusion coefficients of the tracer in soil and air, respectively, θ a is the air-containing porosity of the soil, n is the porosity of the soil, v is the convection velocity of the tracer in the soil, k a is the permeability coefficient of the tracer in the soil, dP / dz is the pressure gradient at both ends of the soil; Step 2.1): Determine the governing equations and boundary conditions A two-dimensional radial axisymmetric coordinate system is established with the middle point of the garbage body as the origin, and the two-dimensional control equation of tracer migration is established. The tracer injection condition is a constant concentration C0 injection, and the boundary conditions used in the model are determined; Step 2.2): Obtain the migration law of the tracer in the landfill, specifically including: Step 2.2.1): Calculate the tracer migration concentration profile in the Laplace domain; Step 2.2.2): Calculate the tracer release flux on the overburden surface in the Laplace domain and calculate the tracer concentration on the bottom of the liner in the Laplace domain; Step 2.2.3): Use Talbot numerical inversion to obtain the tracer concentration distribution profile and release flux; Step 3.1): Calculate the concentration profile and release flux of the tracer in the landfill system and plot its variation curve over time and space; Step 3.2): Determine the position coordinates of the monitoring point based on the concentration profile drawn in step 3.1) and the detection limit of the tracer; Step 4): Determine the release flux and concentration of the tracer at the monitoring point, and compare them with the detection limit. If the tracer concentration or release flux at the monitoring point is greater than the detection limit, the injection concentration boundary of the injection well in step 1) is determined to be the actual injection concentration; if the tracer concentration or release flux at the monitoring point is less than the detection limit, the tracer cannot be detected at the monitoring point, increase the injection boundary concentration or flux of the injection well, and repeat steps 3.1)-step 4).

2. A method for arranging monitoring wells and determining tracer injection boundaries for landfill leakage detection according to claim 1, characterized in that: The step 2.2.1) is specifically: The injection well is injected at the fixed concentration boundary, and the tracer migration concentration in the landfill is Where C0 is the concentration of the tracer in the injection well, p is the time factor in the Laplace domain, μ n and ω n is the characteristic root, r is the radial coordinate, r w is the radius of the injection well, z is the vertical coordinate, and m2 is the migration parameter of the tracer in the garbage body, which can be calculated using the following formula: Where v is the convection velocity of the tracer in the garbage body; θ is the air-containing porosity of the garbage body; D z D is the diffusion coefficient of the tracer in the garbage along the axial direction; r is the radial diffusion coefficient of the tracer in the garbage body; The concentration of the tracer in the overburden is Where l1 and l2 are the thickness of the garbage body and the thickness of the cover layer, and m3 and m4 are the migration parameters of the tracer in the cover layer, which can be calculated by the following formulas: Where v u is the convection rate of the tracer in the overburden; D u is the diffusion coefficient of the tracer in the overburden along the axial direction; The tracer concentration profile in the bottom liner is Where m5 and m6 are the migration parameters of the tracer in the bottom liner, which are solved by the following equations: Where v l is the convection rate of the tracer in the bottom pad; D l is the diffusion coefficient of the tracer in the bottom liner along the axial direction.

3. A method for arranging monitoring wells and determining tracer injection boundaries for landfill leakage detection according to claim 2, characterized in that: The step 2.2.2) is specifically: Calculate the tracer release flux on the overburden surface in the Laplace domain: Calculate the tracer concentration at the bottom of the pad on the Laplace domain: Where l3 is the bottom pad thickness.

4. A method for arranging monitoring wells and determining tracer injection boundaries for landfill leakage detection according to claim 3, characterized in that: The step 2.2.3) is specifically: The concentration distribution profile and release flux of the tracer are obtained by Talbot numerical inversion. The calculation formula is as follows: Where M is the superposition constant, Re is the real part of the complex number, γ i , δ i is the Laplace inversion constant, defined by the following formula: Where a is an imaginary factor.

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