A landfill leakage channel detection method based on tracer migration

Abstract

The present invention discloses a landfill leakage channel detection method based on tracer migration, which belongs to the technical field of urban domestic waste landfill. The method mainly includes on-site tracer injection in the landfill, downstream monitoring well arrangement, obtaining its output curve through tracer concentration sampling monitoring in the injection well and the monitoring well, establishing a dual-medium mathematical model of tracer migration in the landfill garbage body, and inverting the tracer concentration output curve through the model to obtain the crack-to-void ratio w f Parameters such as crack-to-void ratio w f Determining on-site leakage channels is of great significance for the accurate detection and repair of landfill bottom leakage.

Description

Technical Field

[0001] The invention belongs to the field of fluid migration simulation experimental methods in urban domestic garbage landfills, and in particular relates to a landfill leakage channel detection method based on tracer migration. Technical Background

[0002] The garbage disposal process produces leachate with complex composition and high concentration. The horizontal liner of the landfill is easily damaged by force during the landfill process. In addition, the rapid clogging of the landfill drainage layer causes the water head to be too high, making the problem of leachate leakage very prominent. The buried depth of the leakage point is often more than 10 meters. There is an urgent need to detect deep leakage channels for subsequent repairs.

[0003] In order to quickly and effectively carry out repair and treatment, it is first necessary to find out the location of the leakage. However, the thickness of the landfill in operation ranges from a few meters to tens of meters, and it is impossible to directly find out the location of the leakage. The detection depth of the dual-electrode method commonly used at home and abroad does not exceed 3 meters. It is mainly used to detect defects during the construction period of the horizontal liner and is difficult to use for leakage detection in operating or sealed landfills. The detection accuracy of the existing high-density electrical method and geological radar decreases significantly with the increase of burial depth, and cannot meet the requirements of leakage channel detection. Summary of the invention

[0004] In view of the situation that the bottom liner of the landfill in domestic landfills is damaged and the leachate flows to the outside of the landfill through the seepage channel to pollute the groundwater, the present invention provides a landfill leakage channel detection method based on tracer migration, which is used to detect the leakage channel when the landfill leaks, and is of great significance for the remediation of pre-landfills.

[0005] The present invention is achieved through the following technical solutions:

[0006] A landfill leakage channel detection method based on tracer migration comprises the following steps:

[0007] Step 1: Organize background information and prepare for the experiment;

[0008] Step 2: On-site injection and monitoring to obtain the output curve;

[0009] Step 3: Calculate the ratio of cracks to voids w at each point based on the inversion of the output curve f , according to the crack-to-void ratio w f Get a leak channel.

[0010] As a preferred embodiment of the present invention, the step 1 is specifically as follows:

[0011] Step 1.1: Summarize and organize on-site data

[0012] Obtain hydrogeological data related to the landfill, including geological maps, profiles, and parameters of existing water wells; based on the hydrogeological data, preliminarily determine the groundwater flow field information around the landfill, obtain the maximum groundwater depth data and water flow line direction data, and provide a reference for the depth of injection wells and monitoring wells.

[0013] Step 1.2: Laying out injection wells and monitoring wells

[0014] The location and depth of the injection well and the monitoring well are determined according to the site conditions of the landfill. The injection well is located in the middle of the landfill, and the monitoring well is located downstream of the landfill, perpendicular to the direction of the water flow line, and arranged in a "one" shape; the depth of the injection well and the monitoring well is 1m below the known maximum groundwater depth;

[0015] Insert a well pipe with an inner diameter of not less than 75 mm into the injection well and the monitoring well, wherein the injection well pipe is only opened at the last 1 m, and the monitoring well pipe is fully opened, and the openings are wrapped with a water filter section;

[0016] Step 1.3, On-site background check

[0017] Conduct water level surveys and water quality surveys on all injection wells and monitoring wells, and take samples to test tracer concentrations to determine the background value of tracer concentration.

[0018] As a preferred embodiment of the present invention, the step 2 is specifically as follows:

[0019] Step 2.1, determination of tracer dosage

[0020] First, establish the tracer input model:

[0021] M=μ·MDL·πR 2 h·φ

[0022] Where M is the amount of effective tracer material added; μ is the security factor, and the recommended value for general landfills is 1×10 -4 -5×10 -3 ; MDL is the minimum detection limit of the tracer equipment; R is the average distance between the injection well and the monitoring well; h is the average effective aquifer thickness of the garbage; is the porosity;

[0023] Step 2.2, tracer injection

[0024] The tracer is injected at a constant rate and pressure.

[0025] Step 2.3, On-site monitoring

[0026] After the tracer is injected, sampling and monitoring of the injection well and monitoring well shall be started immediately. The monitoring frequency of the monitoring wells in the contaminated area shall be no less than twice a day, and the monitoring frequency of the monitoring wells outside the contaminated area shall be no less than once a day. The monitoring time shall be no less than 10 days, and the water samples shall be tested. If no tracer is detected in the sample, it means that the leakage in the test area of ​​the landfill is not obvious. If the tracer is detected, the tracer output curve of each point is obtained according to the tracer concentration at each point at each time.

[0027] As a preferred embodiment of the present invention, the step three is specifically as follows:

[0028] Step 3.1, build the model

[0029] According to the size of the voids, the voids in the saturated garbage body, rock and soil are divided into pores and cracks, and a simplified model of the injection well and the monitoring well is established, wherein the simplified model includes a water flow control equation and a solute transport control equation;

[0030] The governing equation for water flow in the garbage as the tracer migrates between the garbage body and the well is:

[0031]

[0032]

[0033] h w =w f ×h wf +(1-w f )×h wm (7)

[0034] In the formula, K wfx is the lateral permeability coefficient in the garbage cracks, K wfz is the vertical permeability coefficient of garbage cracks, u wfx is the lateral velocity of the liquid in the garbage crack, u wfz is the vertical velocity of the liquid in the garbage cracks, K wmx is the lateral permeability coefficient in the garbage pores, K wmz is the vertical permeability coefficient of garbage pores, u wmx is the lateral velocity of the liquid in the pores of the garbage, u wmz is the vertical velocity of the liquid in the pores of the garbage, h wf is the water head of the garbage fissure, h wm is the water head of garbage pores, h w is the leachate head in the landfill, x is the horizontal direction, z is the vertical direction, β is the convection exchange coefficient, K is the permeability coefficient at the pore fracture exchange, and w f is the crack-to-void ratio;

[0035] The governing equation for solute transport in garbage with tracer migration in the profile is:

[0036]

[0037] Then the control equation for solute transport in the garbage on this path is:

[0038]

[0039] The final tracer concentration of the monitoring well is expressed as:

[0040]

[0041]

[0042] c w =w f ×c wf +(1-w f )×c wm (18)

[0043] In the formula, c wf is the concentration of the tracer in the garbage cracks, D wfx is the lateral hydrodynamic diffusion coefficient of the tracer in the garbage cracks, D wfz is the vertical hydrodynamic diffusion coefficient of the tracer in the garbage cracks, c wm is the concentration of the tracer in the pores of the garbage, D wmx is the lateral diffusion coefficient of the tracer in the pores of garbage, D wmz is the vertical diffusion coefficient of the tracer in the pores of garbage, c w is the tracer concentration in the landfill.

[0044] Similarly, the governing equation for water flow in soil is:

[0045]

[0046] h s =w f ×h sf +(1-w f )×h sm (25)

[0047] In the formula, K sfx is the lateral permeability coefficient in soil cracks, K sfz is the vertical permeability coefficient of soil cracks, u sfx is the lateral velocity of the liquid in the soil cracks, u sfz is the vertical velocity of the liquid in the soil cracks, K smx is the lateral permeability coefficient in soil pores, K smz is the vertical permeability coefficient of soil pores, u smxis the lateral velocity of the liquid in the soil pores, u smz is the vertical velocity of the liquid in the soil pores, h sf is the hydraulic head of soil fissure, h sm is the water head of soil pores, h s is the water head in the soil;

[0048] The governing equation for solute transport in soil is:

[0049]

[0050]

[0051] c s =w f ×c sf +(1-w f )×c sm (30)

[0052] In the formula, c sfψ is the concentration of the tracer in soil cracks at angle ψ, c smψ is the concentration of the tracer in the soil pores at angle ψ, c sf is the concentration of the tracer in soil cracks, D sfx is the lateral hydrodynamic diffusion coefficient of the tracer in soil cracks, D sfz is the vertical hydrodynamic diffusion coefficient of the tracer in soil cracks, c sm is the concentration of the tracer in the soil pores, D smx is the lateral diffusion coefficient of the tracer in the soil pores, D smz is the vertical diffusion coefficient of the tracer in the soil pores, c s is the tracer concentration in the soil.

[0053] Similarly, the governing equation for water flow in rock is:

[0054]

[0055] h r =w f ×h rf +(1-w f )×h rm (37)

[0056] In the formula, K rfx is the lateral permeability coefficient in rock fractures, K rfz is the vertical permeability coefficient of rock fracture, u rfx is the lateral velocity of rock in soil cracks, u rfz is the vertical velocity of the liquid in the rock fracture, K rmx is the lateral permeability coefficient in rock pores, Krmz is the vertical permeability coefficient of rock pores, u rmx is the lateral velocity of the liquid in the rock pores, u rmz is the vertical velocity of the liquid in the rock pores, h rf is the hydraulic head of the rock fissure, h rm is the hydraulic head of rock pores, h r is the water head in the rock;

[0057] The governing equation for solute transport in rocks is:

[0058]

[0059] c r =w f ×c rf +(1-w f )×c rm (42)

[0060] In the formula, c rfψ is the concentration of the tracer in the rock fracture at angle ψ, c rmψ is the concentration of the tracer in the rock pores at angle ψ, c rf is the concentration of the tracer in the rock fractures, D rfx is the lateral hydrodynamic diffusion coefficient of the tracer in rock fractures, D rfz is the vertical hydrodynamic diffusion coefficient of the tracer in rock fractures, c rm is the concentration of the tracer in the rock pores, D rmx is the lateral diffusion coefficient of the tracer in the rock pores, D rmz is the vertical diffusion coefficient of the tracer in the rock pores, c r is the tracer concentration in the rock.

[0061] Step 3.2, calculate the profile groundwater flow field

[0062] According to the results of the on-site hydrogeological data survey, determine the actual conditions of the injection point and the monitoring point, and determine the scope, depth, and soil depth of the landfill. Determine its profile according to its actual conditions. According to the on-site conditions, determine the water level in the monitoring well as the soil water level h s or rock water level h r According to the specific conditions on site, the injection boundary and other boundary conditions and initial conditions are set, the profile flow field is calculated according to the water flow control equation in soil, rock and garbage, and the pore permeability coefficient and fracture permeability coefficient of soil, rock and garbage are adjusted until the water level h of the calculated monitoring well is s or r The water level difference from the actual water level measured in the water level survey shall not exceed 10%.

[0063] Step 3.3, calculate the crack-to-void ratio w f , obtain leakage channel

[0064] The output curves of pores and fractures in the monitoring wells are calculated based on the solute transport control equations in soil, rock and garbage. The fracture-void ratio w of each profile is obtained by combining the actual output curves of the monitoring wells. f The ratio of cracks to voids is w f The larger the value, the more cracks there are in the flow direction, and the greater the possibility of leakage channels. It is generally believed that the crack-to-void ratio w f There is a leakage channel in the section where the largest point is located.

[0065] The present invention has the following beneficial effects:

[0066] The present invention takes into account the strong heterogeneity of garbage and divides the voids in garbage, rock and soil into pores and fissures. The fissure has a large volume, the liquid flows fast in the fissure, and more liquid flows through the fissure. The opposite is true for pores. The pores and fissures exchange mass at the interface, and the convection-diffusion equation of the tracer on the profile is established. The present invention can calculate the permeability coefficient, diffusion dilution and other parameters of garbage, soil and rock based on the control equation and experimental results, and can also obtain the ratio of fissure to void w f , according to w f The spatial distribution of the leakage channels in the study area was inferred.

[0067] The tracer of the invention adopts fluorobenzoic acid tracer, and combines its migration equation to infer the leakage channel in the landfill, which is low in price, economical and practical, and has little impact on the environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0068] Figure 1 It is a flow chart of the landfill leakage channel detection method based on tracer migration proposed by the present invention.

[0069] Figure 2 It is a scene layout diagram of a computing example shown in an embodiment of the present invention.

[0070] Figure 3 It is a diagram of partial point monitoring results and numerical fitting results shown in an embodiment of the present invention.

[0071] Figure 4 It is a schematic diagram of a computing example model shown in an embodiment of the present invention. DETAILED DESCRIPTION

[0072] 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.

[0073] The present invention mainly selects injection wells and monitoring wells according to the actual situation of the landfill, injects tracers into the injection wells, monitors the tracers in the monitoring wells to obtain output curves, and obtains the leakage channels according to model inversion.

[0074] like Figure 1 As shown, the landfill leakage channel detection method based on tracer migration proposed by the present invention mainly includes the following steps:

[0075] Step 1: Organize background information and prepare for the experiment;

[0076] Step 1.1): Summarize and organize on-site data

[0077] Before laying out injection wells and monitoring wells, relevant hydrogeological data of the landfill should be collected, mainly including: geological maps, profiles, relevant parameters of existing water wells (well location, drilling date, well depth, well completion method, pumping test data, water quality data, etc.). Based on the existing hydrogeological data, the groundwater flow field information around the landfill can be roughly determined, and the maximum groundwater depth data and water flow line direction data can be obtained to provide a reference for the depth of injection wells and monitoring wells.

[0078] Step 1.2): Laying out injection wells and monitoring wells

[0079] Injection wells and monitoring wells are determined according to the on-site conditions of the landfill. The injection well should be located in the middle of the landfill, that is, in the garbage; monitoring wells are arranged outside the landfill. The monitoring wells should be located downstream of the landfill, perpendicular to the direction of the water flow line, in the shape of a "one", and the distance between two adjacent off-site monitoring wells is 10-100m. In this embodiment, the arrangement of off-site monitoring wells is suitable for valley-type landfills.

[0080] According to the site conditions, a suitable drilling method is selected to drill at the injection well and monitoring well locations to obtain the injection well and monitoring well. The well depth should be determined according to the local groundwater and leachate burial depth. In this embodiment, the well depth is 1m below the known maximum groundwater burial depth. The inner diameter of the injection well and monitoring well pipe is greater than 0.1m. The injection well and monitoring well pipe are made of strong, corrosion-resistant materials that do not pollute the groundwater quality, such as PVC pipes.

[0081] Insert the well pipe into the injection well and the monitoring well. The injection well pipe is only open at the last 1m, and the monitoring well pipe is fully open. The filter section must be wrapped around the open section, and the filter section must have good water permeability. The inner diameter of the well pipe should not be less than 75mm. The off-site monitoring well should be pumped and washed.

[0082] Step 1.3): On-site background check

[0083] Water quality surveys are conducted on all injection wells and monitoring wells in the study area, generally 10 days after the wells are completed. For example, sampling is done using a Beyler tube, with one Beyler tube used for each well. When sampling, the water sample must fill the container, leaving no gaps on the top. After collection, it is fixed on site, capped with a bottle stopper and sealed with water, and sent to the testing unit for testing. The test items should include chemical oxygen demand, five-day chemical oxygen demand, ammonia nitrogen, chloride ions, sulfate ions, etc. The test results are compared with Class III water in the groundwater quality standard (GB / T 148448-2017) to determine the scope of pollution.

[0084] Water level surveys were conducted for all injection wells and monitoring wells in the study area, and water samples were surveyed and recorded at least every 24 hours during the experiment.

[0085] All wells in the study area were sampled and tested for tracer concentration to determine the background value.

[0086] Step 2: On-site injection and monitoring to obtain the output curve;

[0087] Step 2.1): Determination of tracer dosage

[0088] First, establish the tracer input model:

[0089] M=μ·MDL·πR 2 h·φ

[0090] Where M is the amount of effective tracer material added; μ is the security factor, and the recommended value for general landfills is 1×10 -4 -5×10 -3 ; MDL is the minimum detection limit of the tracer equipment; R is the average distance between the injection well and the monitoring well; h is the average effective aquifer thickness of the garbage; is the porosity;

[0091] Step 2.2): Tracer injection

[0092] The total amount of injected tracer is determined according to the M value calculated by the total tracer dilution model. In this embodiment, a fluorobenzoic acid tracer is used, which has a slight toxicity, almost no pollution to the environment, and a low detection limit of only 1ug / L. The tracer is configured into a 500-1000mg / L solution, stirred evenly, and injected into the injection well at a constant speed and normal pressure.

[0093] Step 2.3): On-site monitoring

[0094] After the tracer is injected, sampling and monitoring of the injection well and monitoring well shall be started immediately. The monitoring frequency of the monitoring wells in the contaminated area shall be no less than twice a day, and the monitoring frequency of the monitoring wells outside the contaminated area shall be no less than once a day. The monitoring time shall be no less than 10 days, and the water samples shall be tested. If no tracer is detected in the sample, it means that the leakage in the test area of ​​the landfill is not obvious. If the tracer is detected, the tracer output curve of each point is obtained according to the tracer concentration at each point at each time.

[0095] Step 3: Calculate the ratio of cracks to voids w at each point based on the inversion of the output curve f , according to the crack-to-void ratio w f Get a leak channel.

[0096] Step 3.1): Build the model

[0097] According to the on-site geological conditions, a simplified model of the injection well and a monitoring well is established. Considering that the garbage body has strong heterogeneity, the size of the voids in the garbage body at different locations is quite different, and the permeability coefficients vary greatly. Therefore, according to the size of the voids, the voids in the saturated garbage body, rock and soil are divided into two types of media: pores and fissures. Among them, the pore volume is small, the liquid in it diffuses slowly, and less liquid migrates through the pores; the fissure volume is large, the liquid in it diffuses quickly, and more liquid migrates through the fissures. In general, fissures are the main channels and storage spaces for the migration of leachate and groundwater. The more fissures there are between the injection well and the monitoring well, the more channels there are for the migration of leachate and groundwater in this direction, and the possibility of leakage channels in this direction is high.

[0098] A two-dimensional model of the migration of tracers between garbage bodies and wells was established. The two-dimensional model includes a water flow control equation and a solute transport control equation. The equation assumptions are as follows:

[0099] 1. The migration direction of the liquid is parallel to the horizontal axis (r axis) and the vertical axis (y axis);

[0100] 2. The migration behavior of tracers in leachate and groundwater only considers convection-diffusion operations, ignoring the effects of dispersion, adsorption / desorption, etc.;

[0101] 3. The flow of liquid obeys Darcy's law;

[0102] 4. Liquid flows from the cracks and pores to the downstream together. The liquid in the cracks and pores undergoes mass exchange at the interface between the cracks and pores, and the anisotropy values ​​of the two regions are the same;

[0103] The governing equation for water flow in the garbage as the tracer migrates between the garbage body and the well is:

[0104]

[0105] hw =w f ×h wf +(1-w f )×h wm (7)

[0106] In the formula, K wfx is the lateral permeability coefficient in the garbage cracks, K wfz is the vertical permeability coefficient of garbage cracks, u wfx is the lateral velocity of the liquid in the garbage crack, u wfz is the vertical velocity of the liquid in the garbage cracks, K wmx is the lateral permeability coefficient in the garbage pores, K wmz is the vertical permeability coefficient of garbage pores, u wmx is the lateral velocity of the liquid in the pores of the garbage, u wmz is the vertical velocity of the liquid in the pores of the garbage, h wf is the water head of the garbage fissure, h wm is the water head of garbage pores, h w is the leachate head in the landfill, x is the horizontal direction, z is the vertical direction, β is the convection exchange coefficient, K is the permeability coefficient at the pore fracture exchange, and w f is the crack-to-void ratio;

[0107] The governing equation for solute transport in garbage with tracer migration in the profile is:

[0108]

[0109] All flow paths between wells are assumed to be arcs passing through the wells. For any angle, there is a given flow path, and the length of the flow path is expressed as:

[0110] L ψ =2Lψcscψ(10)

[0111] Arrival time:

[0112] t ψ =3(1-ψcotψ+cot 2 ψ-ψcot 3 ψ)t0(11)

[0113] Then the average flow velocity on the path section is:

[0114]

[0115] Simplifying further:

[0116]

[0117] Where ψ is the angle of the flow path, 2L is the distance between the two wells, and L ψ is the length of the flow path, t0 is the migration time of the tracer under the straight path, t ψ is the migration time of the tracer under the flow path, v0 is the average migration velocity of the tracer under the straight path, and v ψ is the average migration velocity of the tracer in this flow path.

[0118] Then the control equation for solute transport in the garbage on this path is:

[0119]

[0120] The final tracer concentration of the monitoring well is expressed as:

[0121]

[0122] c w =w f ×c wf +(1-w f )×c wm (18)

[0123] In the formula, c wf is the concentration of the tracer in the garbage cracks, D wfx is the lateral hydrodynamic diffusion coefficient of the tracer in the garbage cracks, D wfz is the vertical hydrodynamic diffusion coefficient of the tracer in the garbage cracks, c wm is the concentration of the tracer in the pores of the garbage, D wmx is the lateral diffusion coefficient of the tracer in the pores of garbage, D wmz is the vertical diffusion coefficient of the tracer in the pores of garbage, c w is the tracer concentration in the landfill.

[0124] Similarly, the governing equation for water flow in soil is:

[0125]

[0126] h s =w f ×h sf +(1-w f )×h sm (25)

[0127] In the formula, K sfx is the lateral permeability coefficient in soil cracks, K sfz is the vertical permeability coefficient of soil cracks, u sfx is the lateral velocity of the liquid in the soil cracks, u sfz is the vertical velocity of the liquid in the soil cracks, Ksmx is the lateral permeability coefficient in soil pores, K smz is the vertical permeability coefficient of soil pores, u smx is the lateral velocity of the liquid in the soil pores, u smz is the vertical velocity of the liquid in the soil pores, h sf is the hydraulic head of soil fissure, h sm is the water head of soil pores, h s is the water head in the soil;

[0128] The governing equation for solute transport in soil is:

[0129]

[0130]

[0131] c s =w f ×c sf +(1-w f )×c sm (30)

[0132] In the formula, c sfψ is the concentration of the tracer in soil cracks at angle ψ, c smψ is the concentration of the tracer in the soil pores at angle ψ, c sf is the concentration of the tracer in soil cracks, D sfx is the lateral hydrodynamic diffusion coefficient of the tracer in soil cracks, D sfz is the vertical hydrodynamic diffusion coefficient of the tracer in soil cracks, c sm is the concentration of the tracer in the soil pores, D smx is the lateral diffusion coefficient of the tracer in the soil pores, D smz is the vertical diffusion coefficient of the tracer in the soil pores, c s is the tracer concentration in the soil.

[0133] Similarly, the governing equation for water flow in rock is:

[0134]

[0135] h r =w f ×h rf +(1-w f )×h rm (37)

[0136] In the formula, K rfx is the lateral permeability coefficient in rock fractures, K rfz is the vertical permeability coefficient of rock fracture, u rfxis the lateral velocity of rock in soil cracks, u rfz is the vertical velocity of the liquid in the rock fracture, K rmx is the lateral permeability coefficient in rock pores, K rmz is the vertical permeability coefficient of rock pores, u rmx is the lateral velocity of the liquid in the rock pores, u rmz is the vertical velocity of the liquid in the rock pores, h rf is the hydraulic head of the rock fissure, h rm is the hydraulic head of rock pores, h r is the water head in the rock;

[0137] The governing equation for solute transport in rocks is:

[0138]

[0139] c r =w f ×c rf +(1-w f )×c rm (42)

[0140] In the formula, c rfψ is the concentration of the tracer in the rock fracture at angle ψ, c rmψ is the concentration of the tracer in the rock pores at angle ψ, c rf is the concentration of the tracer in the rock fractures, D rfx is the lateral hydrodynamic diffusion coefficient of the tracer in rock fractures, D rfz is the vertical hydrodynamic diffusion coefficient of the tracer in rock fractures, c rm is the concentration of the tracer in the rock pores, D rmx is the lateral diffusion coefficient of the tracer in the rock pores, D rmz is the vertical diffusion coefficient of the tracer in the rock pores, c r is the tracer concentration in the rock.

[0141] Step 3.2): Calculate the groundwater flow field of the profile

[0142] According to the results of the on-site hydrogeological data survey, determine the actual conditions of the injection point and the monitoring point, and determine the scope, depth, and soil depth of the landfill. Determine its profile according to its actual conditions. According to the on-site conditions, determine the water level in the monitoring well as the soil water level h s or rock water level h r According to the specific conditions on site, the injection boundary and other boundary conditions and initial conditions are set, the profile flow field is calculated according to the water flow control equation in soil, rock and garbage, and the pore permeability coefficient and fracture permeability coefficient of soil, rock and garbage are adjusted until the water level h of the calculated monitoring well iss or r The water level difference from the actual water level measured in the water level survey shall not exceed 10%.

[0143] Step 3.3): Calculate the crack-to-void ratio w f , obtain leakage channel

[0144] The output curves of pores and fractures in the monitoring wells are calculated based on the solute transport control equations in soil, rock and garbage. The fracture-void ratio w of each profile is obtained by combining the actual output curves of the monitoring wells. f The ratio of cracks to voids is w f The larger the value, the more cracks there are in the flow direction, and the greater the possibility of leakage channels. It is generally believed that the crack-to-void ratio w f There is a leakage channel in the section where the largest point is located.

[0145] Taking a landfill in Xianju County as an example, the implementation process of the present invention in this example is further explained:

[0146] Step 1: Organize background information and prepare for the experiment.

[0147] Step 1.1: On-site data collection

[0148] First, we collected hydrogeological data related to the landfill, mainly including: geological maps, cross-sections, and relevant parameters of existing wells (well location, drilling date, well depth, well construction method, pumping test data, water quality data, etc.). The data showed that the landfill was a typical valley-type landfill, surrounded by mountains in the northwest and intercepted by a mortar-laid block stone cement garbage dam on the east side; the landfill foundation was composed of 1.3-6.7m of clayey gravel, 0.8-3.8m of strongly weathered tuff, and medium-weathered tuff from top to bottom, with no bottom exposed; the landfill was a typical valley-type informal simple landfill, with a pile covering an area of ​​50 mu, a maximum thickness of 16m, an average thickness of 10m, and a total pile of 300,000m 3 There are no anti-seepage measures under the pile; there are currently no monitoring water wells in the landfill.

[0149] Step 1.2, lay out injection wells and monitoring wells.

[0150] Injection wells and monitoring wells are determined according to the on-site conditions of the landfill. Injection wells are arranged in the middle of the landfill, and off-site monitoring wells are arranged in a "one" shape in the depression downstream of the landfill. The off-site monitoring wells are about 10m apart, 15m deep, and wet drilled.

[0151] In this embodiment, the various points on site are arranged as follows: Figure 2 shown.

[0152] Step 1.3, On-site background check

[0153] The exploration showed that the landfill had serious leakage, the ammonia nitrogen in the groundwater downstream of the landfill seriously exceeded the standard, and the heavy metal ions in the soil samples downstream seriously exceeded the standard. Among them, the iron in the downstream groundwater exceeded the standard by 30 times, the ammonia nitrogen in the surface water exceeded the standard by 7 times, and the copper in the soil exceeded the standard by 1.5-2.5 times. Sampling of all wells in the study area to test their tracer concentrations showed that there was no tracer in the landfill. Water level surveys were conducted in all wells in the study area, and water samples were surveyed and recorded every 24 hours during the experiment.

[0154] Step 2: Inject on site and monitor at different points to obtain the output curve.

[0155] Step 2.1, determination of tracer dosage

[0156] According to the total dilution model of the tracer, the amount of tracer used in the landfill is determined. In this embodiment, the maximum distance R between the on-site injection well and the monitoring well is about 100m, the average effective aquifer thickness h of the garbage is about 8m, the porosity φ is 0.75, and the minimum detection limit of the tracer equipment is 1mg / L, that is, the MDL value is 1g / m 3 , the security coefficient μ is taken as 0.0025. Substituting it into the total dilution model of the tracer, it can be obtained that 471g of tracer is injected into the landfill. For convenience, 500g of tracer is injected into the landfill.

[0157] Step 2.2, Injection of tracer

[0158] According to step 2.1, prepare 500 L of 1000 mg / L 2,3,4,5-tetrafluorobenzoic acid solution, stir evenly, randomly take 5 samples to verify the tracer injection concentration, and inject into the injection well at a uniform speed of 1.2 L / min and normal pressure.

[0159] Step 2.3, On-site monitoring

[0160] After the tracer is injected, sampling and monitoring of the monitoring wells begins immediately. The monitoring frequency is 2-3 times a day and the monitoring time is 200 hours. The water samples are tested with a liquid chromatograph to obtain the output curve of each point.

[0161] In this embodiment, the output curves of some points of the landfill (taking points d1 and d4 as examples) are as follows: Figure 3 shown.

[0162] Step three, obtain the leakage channel based on the inversion of the production curve.

[0163] Step 3.1, build the model

[0164] According to the on-site geological conditions, a simplified model of the injection well and a monitoring well was established. The voids in the saturated garbage body, rock and soil were divided into pores and cracks according to the size of the voids.

[0165] A tracer migration model is established, which includes a water flow control equation and a solute transport control equation. The boundary conditions are determined according to the actual situation on site. In this embodiment, the boundary conditions from the injection hole m1 to the monitoring hole d2 of the landfill are as follows: Figure 4 .

[0166] The boundary conditions are as follows:

[0167] Injection Boundary:

[0168] h wf =h m1 ,h wm =h m1

[0169] c wf =c m1 ,c wm =c m1

[0170] The rest are set as zero flux boundary conditions;

[0171] In the formula, h m1 is the injection hole head, c m1 is the injection concentration of the injection hole;

[0172] Step 3.2, calculate the profile groundwater flow field.

[0173] Taking point d2 as an example, according to the on-site hydrogeological data survey results, the landfill depth is set to 8m and the soil depth is 0.5m. According to its actual situation, its profile is determined as follows Figure 3 According to the on-site conditions, the water level in the monitoring well is determined to be the rock water level h r The measured water level of m1 is input into the model as the boundary condition, and the configured solution concentration is input into the model as the injection concentration of the injection hole as the boundary condition. The remaining boundary conditions are set according to the specific conditions on site. The profile flow field is calculated according to the water flow control equation in soil, rock and garbage, and the pore permeability coefficient and fracture permeability coefficient of soil, rock and garbage are adjusted until the water level h of the monitoring well d2 is calculated. r The difference from the actual measurement is no more than 10%.

[0174] Step 3.3, calculate the crack-to-void ratio w f , to obtain a leakage channel.

[0175] The output curves of pores and fractures in the monitoring well are calculated based on the solute transport control equations in soil, rock and garbage. The fracture-void ratio w of d2 is obtained by combining the actual output curves of the monitoring well. f , the crack-to-void ratio w of d2 can be calculated f is 0.21. Repeat the above steps to calculate the fracture-void ratio w of all monitoring wells fBy comparing the crack-to-void ratio w on each section f In this embodiment, the fitting curve of the downstream d1-d4 monitoring holes and the crack-to-void ratio w f The results are as follows Figure 3 As shown. Figure 3 It can be seen that the crack-void ratio of d2 hole is w f The largest is 2-5 times that of other monitoring wells. Therefore, the leakage channels in the study area are as follows Figure 2 shown.

Claims

1. A landfill leakage channel detection method based on tracer migration, characterized in that: The following steps are involved: Step 1: Arrange injection wells and monitoring wells in the landfill, and take samples to test the tracer concentration in each well to determine the background value; The step 1 is specifically as follows: Step 1.1: Laying out injection wells and monitoring wells The location and depth of the injection well and monitoring well are determined according to the site conditions of the landfill. The injection well is located in the middle of the landfill, and the monitoring well is located downstream of the landfill, perpendicular to the direction of the water flow line, and arranged in a "one" shape; the depth of the injection well and monitoring well is 1m below the known maximum groundwater depth; Step 1.2, insert well tube Insert a well pipe with an inner diameter of not less than 75 mm into the injection well and the monitoring well, wherein the injection well pipe is only opened at the last 1 m, and the monitoring well pipe is fully opened, and the openings are wrapped with a water filter section; Step 1.3, water level and tracer concentration background value detection Conduct water level surveys and water quality surveys on all injection wells and monitoring wells, and take samples to test tracer concentrations to determine the background value of tracer concentrations; Step 2: adding tracers into the injection wells to obtain the production curves of different monitoring wells; The step 2 is specifically as follows: Step 2.1, determine the amount of tracer First, the total tracer dilution model is established: M=μ·MDL·πR 2 h·φ Where M is the amount of tracer active substance added, in g; μ is the security factor, and the recommended value for landfills is 1×10 -4 -5×10 -3 ; MDL is the minimum detection limit of the tracer equipment, unit: g / m 3 ; R is the average distance between the injection well and the monitoring well, unit is m; h is the average effective aquifer thickness of the garbage, unit is m; is the porosity; The M value is calculated based on the total tracer dilution model to determine the total amount of tracer injected; Step 2.2, Injection of tracer Inject the tracer into the injection well at a constant speed and pressure; Step 2.3, On-site monitoring After the tracer is injected, sampling and monitoring of the monitoring wells shall be started immediately. The monitoring frequency of the monitoring wells in the contaminated area shall be no less than twice a day, and the monitoring frequency of the monitoring wells outside the contaminated area shall be no less than once a day. The monitoring time shall be no less than 10 days, and the water samples shall be tested. If no tracer is detected in the sample, it means that the leakage in the test area of ​​the landfill is not obvious. If the tracer is detected, the tracer output curve of each point shall be obtained according to the tracer concentration at each point at each time. Step 3: According to the output curve of the monitoring well, the groundwater flow field information of the path profile from the injection well to each monitoring well is inverted, the void ratio of the fracture in each path profile is calculated, and the leakage channel is obtained according to the void ratio of the fracture; The step three is specifically as follows: Step 3.1, according to the size of the voids, the voids in the saturated garbage body, rock and soil are divided into pores and fractures, and a simplified model of the injection well and the monitoring well is established, wherein the simplified model includes a water flow control equation and a solute transport control equation; Step 3.2, according to the results of the on-site hydrogeological data survey, determine the actual situation of the injection point and the monitoring point in the profile, determine the scope, depth, soil depth of the landfill, and determine the profile; and according to the on-site conditions, determine the water level in the monitoring well as the soil water level h s or rock water level h r , set the injection boundary conditions and initial conditions, calculate the profile flow field according to the water flow control equations in soil, rock and garbage, adjust the pore permeability coefficient and fracture permeability coefficient of soil, rock and garbage until the water level h of the calculated monitoring well is s or r The difference between the water level measured in the water level survey and that measured in the water level survey shall not exceed 10%; Step 3.3, according to the on-site conditions, determine the concentration in the monitoring well as the soil concentration c s or rock concentration c r According to the solute migration control equation in saturated garbage bodies, rocks and soils, the concentration output curves in each monitoring well under different fracture-void ratios are calculated; combined with the actual output curves of the monitoring wells, the fracture-void ratio of the path profile from the injection well to each monitoring well is obtained. The larger the fracture-void ratio, the more fractures there are in this flow direction, and the path with the largest fracture-void ratio is taken as the leakage channel.

2. A landfill leakage channel detection method based on tracer migration according to claim 1, characterized in that: Before the step 1, the method further includes: Obtain hydrogeological data related to the landfill, including geological maps, profiles, and parameters of existing water wells; based on the hydrogeological data, preliminarily determine the groundwater flow field information around the landfill, obtain the maximum groundwater depth data and water flow line direction data, and provide a reference for the depth of injection wells and monitoring wells.

3. The landfill leakage channel detection method based on tracer migration according to claim 1 is characterized in that: The simplified model is specifically: The governing equation for the flow of tracer in saturated garbage is: h w =w f ×h wf +(1-w f )×h wm In the formula, K wfx is the lateral permeability coefficient in the garbage cracks, K wfz is the vertical permeability coefficient of garbage cracks, u wfx is the lateral velocity of the liquid in the garbage crack, u wfz is the vertical velocity of the liquid in the garbage cracks, K wmx is the lateral permeability coefficient in the garbage pores, K wmz is the vertical permeability coefficient of garbage pores, u wmx is the lateral velocity of the liquid in the pores of the garbage, u wmz is the vertical velocity of the liquid in the pores of the garbage, h wf is the water head of the garbage fissure, h wm is the water head of garbage pores, h w is the leachate head in the landfill, x is the horizontal direction, z is the vertical direction, β is the convection exchange coefficient, K is the permeability coefficient at the pore fracture exchange, and w f is the crack-to-void ratio; The control equation for solute transport of tracer in saturated garbage is: The tracer concentration in the monitoring well is expressed as: Where ψ is the angle of the flow path between the two wells, c wf is the concentration of the tracer in the garbage cracks, D wfx is the lateral hydrodynamic diffusion coefficient of the tracer in the garbage cracks, D wfz is the vertical hydrodynamic diffusion coefficient of the tracer in the garbage cracks, c wm is the concentration of the tracer in the pores of the garbage, D wmx is the lateral diffusion coefficient of the tracer in the pores of garbage, D wmz is the vertical diffusion coefficient of the tracer in the pores of garbage, c w is the tracer concentration in the landfill; The governing equation for water flow of the tracer in soil is: h s =w f ×h sf +(1-w f )×h sm In the formula, K sfx is the lateral permeability coefficient in soil cracks, K sfz is the vertical permeability coefficient of soil cracks, u sfx is the lateral velocity of the liquid in the soil cracks, u sfz is the vertical velocity of the liquid in the soil cracks, K smx is the lateral permeability coefficient in soil pores, K smz is the vertical permeability coefficient of soil pores, u smx is the lateral velocity of the liquid in the soil pores, u smz is the vertical velocity of the liquid in the soil pores, h sf is the hydraulic head of soil fissure, h sm is the water head of soil pores, h s is the water head in the soil; The governing equation for solute transport of tracers in soil is: c s =w f ×c sf +(1-w f )×c sm (30) In the formula, c sfψ is the concentration of the tracer in soil cracks at angle ψ, c smψ is the concentration of the tracer in the soil pores at angle ψ, c sf is the concentration of the tracer in soil cracks, D sfx is the lateral hydrodynamic diffusion coefficient of the tracer in soil cracks, D sfz is the vertical hydrodynamic diffusion coefficient of the tracer in soil cracks, c sm is the concentration of the tracer in the soil pores, D smx is the lateral diffusion coefficient of the tracer in the soil pores, D smz is the vertical diffusion coefficient of the tracer in the soil pores, c s is the tracer concentration in soil; The governing equation for the flow of tracer in rock is: h r =w f ×h rf +(1-w f )×h rm In the formula, K rfx is the lateral permeability coefficient in rock fractures, K rfz is the vertical permeability coefficient of rock fracture, u rfx is the lateral velocity of rock in soil cracks, u rfz is the vertical velocity of the liquid in the rock fracture, K rmx is the lateral permeability coefficient in rock pores, K rmz is the vertical permeability coefficient of rock pores, u rmx is the lateral velocity of the liquid in the rock pores, u rmz is the vertical velocity of the liquid in the rock pores, h rf is the hydraulic head of the rock fissure, h rm is the hydraulic head of rock pores, h r is the water head in the rock; The control equation for solute transport of tracer in rock is: c r =w f ×c rf +(1-w f )×c rm In the formula, c rfψ is the concentration of the tracer in the rock fracture at angle ψ, c rmψ is the concentration of the tracer in the rock pores at angle ψ, c rf is the concentration of the tracer in the rock fractures, D rfx is the lateral hydrodynamic diffusion coefficient of the tracer in rock fractures, D rfz is the vertical hydrodynamic diffusion coefficient of the tracer in rock fractures, c rm is the concentration of the tracer in the rock pores, D rmx is the lateral diffusion coefficient of the tracer in the rock pores, D rmz is the vertical diffusion coefficient of the tracer in the rock pores, c r is the tracer concentration in the rock.

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