A cyclodextrin-enhanced prediction model for the migration distance of a pollution plume in the natural attenuation process of polycyclic aromatic hydrocarbons in a groundwater aquifer

Abstract

The application belongs to the technical field of groundwater aquifer organic pollution remediation, and discloses a cyclodextrin reinforced prediction model for migration distance of a pollution plume in a natural attenuation process of polycyclic aromatic hydrocarbons in a groundwater aquifer, which covers parameters influencing the natural attenuation process of polycyclic aromatic hydrocarbons, such as hydrogeological characteristics of the groundwater aquifer, pollution degree of a site, biodegradation rate, and cyclodextrin reinforcement technology, and quantitatively characterizes the influence law of changes of the above parameters on changes of the migration distance of the polycyclic aromatic hydrocarbon pollution plume. The application can be applied to optimization of a monitoring natural attenuation remediation technology for a polycyclic aromatic hydrocarbon contaminated site, and provides technical guidance for efficient remediation of the contaminated site.

Description

Technical Field

[0001] This invention belongs to the field of organic pollution remediation technology for underground aquifers, specifically involving a predictive model for the change in plume migration distance during the natural decay process of polycyclic aromatic hydrocarbons in underground aquifers enhanced by cyclodextrin. Background Technology

[0002] Oil-contaminated sites, coking sites, and natural gas manufacturing plants contain large amounts of polycyclic aromatic hydrocarbons (PAHs). PAHs are a diverse group of hydrophobic organic pollutants. Most PAHs in soil are adsorbed in the solid phase, with a small amount existing in the soil solution. Microorganisms cannot absorb or transform these adsorbed compounds, and the PAHs slowly dissolve in the soil, entering the atmosphere and water bodies through evaporation and seepage. Ultimately, they affect human health through respiration and skin contact. Remediation is necessary to reduce the health risks posed by PAH-contaminated sites.

[0003] Monitoring natural degradation (MNA) has been used for the remediation of sites contaminated with chlorinated solvents, petroleum hydrocarbons, BTEX, polycyclic aromatic hydrocarbons, perfluorinated compounds, and inorganic heavy metals. It offers advantages such as simple engineering facilities, easy maintenance, low operating costs, and no secondary environmental pollution, making it suitable for sites with low levels of contamination or for risk management of residual pollutants after source removal. Microbial degradation is the only natural process that can reduce the mass of pollutants, thus it is a key factor in the feasibility of MNA technology. Microbial degradation, along with other processes, influences the transport of pollutant plumes in the subsurface environment, specifically manifested as changes in the plume's migration distance. The variation in plume migration distance during transport is related to the risk posed to downstream receptors and is a crucial aspect that must be assessed when applying MNA technology.

[0004] To enhance the natural degradation efficiency of PAHs, improving the bioavailability of adsorbed PAHs in soil is crucial, and the basic principle is to increase the solubility of adsorbed PAHs. CDs are naturally non-toxic and harmless, and can selectively encapsulate organic pollutants through interactions, forming water-soluble inclusion compounds, thus increasing the apparent solubility and bioavailability of the compounds. Using CDs to desorb PAHs from soil allows them to be degraded by microorganisms, and can also enhance natural degradation through other natural processes when the microbial degradation rate is slow.

[0005] Currently, there are few actual engineering cases of natural attenuation of polycyclic aromatic hydrocarbon (PAH) contaminated sites. Different PAH sites vary greatly in terms of contamination levels, hydrogeological factors, and other characteristics, and on-site attenuation experiments are lengthy. Therefore, it is difficult to predict the changes in the extent of the contamination plume in the actual underground environment over time under the influence of multiple factors. In engineering, [the following is commonly used]... 13 C / 12Experimental tools such as compound-specific isotope analysis (CSIA), characteristic metabolite analysis (SMA), and redox sensitive band analysis (RST) were used to investigate and evaluate small-scale biochemical processes during natural attenuation. The literature (Hang Lv., et al. "Effectiveness and mechanism of natural attenuation at a petroleum-hydrocarbon contaminated site." Chemosphere 206(2018): 293-301.) reported the effectiveness and mechanism of the natural attenuation process of petroleum hydrocarbons (TPH) leaked from an abandoned oil well in Jilin, China, through... 13 C 34 S isotope analysis, microbial community sample analysis, and radioactivity 14 C-analysis identified and characterized the impact of in-situ biodegradation mechanisms on TPH removal and plume reduction. However, the aforementioned methods for assessing plume range changes require the collection of numerous geochemical indicators from underground monitoring wells for laboratory analysis, making the process complex. Furthermore, they place high demands on the setup of the in-situ monitoring network and monitoring frequency, resulting in long cycles and high costs. Numerical modeling methods offer the advantages of flexibility and low cost, and can be used to study solute transport in complex underground environments. The literature (Katherina Seiter., et al. "Nutzung von Natural Attenuation zur Reduktion der PAK-Belastung in heterogenen Porengrundwasserleitern." Grundwasser-Zeitschrift der Fachsektion Hydrogeologie 24(2019): 129–144.) reports that an abandoned industrial park in Bremen, Germany, demonstrated that MNA technology is a safe and effective strategy for the remediation of PAH-contaminated sites. The potential of MNA was demonstrated from two aspects. First, the microbial degradation process was validated by detecting characteristic metabolites. Second, a numerical transport model (MT3DMS) based on the MODFLOW flow model was used to simulate and predict the future behavior of plumes in heterogeneous aquifers, demonstrating the effectiveness of the natural attenuation technique for PAHs in heterogeneous aquifers. However, the aforementioned numerical model is tailored to the specific aquifer conditions of this region, and the density of aquifer hydrogeological data is low, making it insufficient as a model for predicting changes in the migration distance of PAH plumes using MNA technology. Currently, there is no rapid predictive model that comprehensively and quantitatively influences the temporal variation of the pollution plume range during the natural attenuation process of PAHs in the subsurface environment.

[0006] Therefore, this invention addresses the difficulty in predicting the migration distance of pollutant plumes during the natural decay of PAHs. It employs numerical simulation tools to provide a comprehensive, wide-range, multi-factor prediction model for PAH pollutant plume migration distance under the bioavailability enhancement effect of cyclodextrin. First, a numerical simulation study was conducted to continuously inject cyclodextrin to increase the solubility of PAHs in the soil, utilizing natural processes such as convection, dispersion, dilution, adsorption, and microbial degradation to accelerate PAH decay in underground aquifers. Then, data on the maximum migration distance of PAH pollutant plumes under underground aquifer hydrogeological parameters, microbial degradation rates, site contamination levels, and cyclodextrin-enhanced technology parameters were obtained. By characterizing the effects of cyclodextrin desorption / desorption, convection, dispersion, and microbial degradation on the natural decay process of PAHs, a prediction model for the migration distance of pollutant plumes during the natural decay process of PAHs in underground aquifers enhanced by cyclodextrin was finally constructed, providing theoretical support for the application and optimization of cyclodextrin-enhanced PAH natural decay technology. Summary of the Invention

[0007] This invention addresses the difficulty in determining the migration distance of PAHs during the natural decay process in cyclodextrin-enhanced aquifers. It constructs a rapid prediction model for PAHs migration distance, enabling the direct use of this model to obtain the time-varying patterns of PAHs migration distance under different aquifer hydrogeological characteristics, site pollution levels, microbial degradation rates, and cyclodextrin enhancement technical parameters.

[0008] The technical solution of the present invention:

[0009] A predictive model for the migration distance of a pollution plume during the natural decay of polycyclic aromatic hydrocarbons (PAHs) in an underground aquifer enhanced by cyclodextrin is presented, comprising the following steps:

[0010] (1) Numerical simulation of natural decay of PAHs in soil-groundwater based on cyclodextrin enhancement

[0011] The soil-groundwater saturation transport was simulated based on the Richards equation. Considering the physicochemical properties of cyclodextrin inclusion complexes, the adsorption loss in the groundwater aquifer was ignored. A convection-dispersion equation with a specific desorption term was established to simulate the enhanced natural decay process of PAHs in soil-groundwater, as shown in equation (1). The concentration change of PAHs in the groundwater aquifer includes the concentration change caused by the desorption of polycyclic aromatic hydrocarbons on soil particles by cyclodextrin (the fourth term on the right side of the equation), the concentration change caused by the combined effects of convection in soil-groundwater (the second term on the right side of the equation), dispersion (the first term on the right side of the equation), and microbial degradation (the third term on the right side of the equation).

[0012]

[0013] Where: C is the mass concentration of PAHs, mg / L; t is the natural decay time, d; x i D is the distance, in meters; ij Let m be the dispersion coefficient tensor. 2 / d;v i Groundwater, or soil water flow velocity, is expressed in m / d; n e Γ1 represents the effective porosity; Γ2 represents the consumption of PAHs due to biodegradation; Γ3 represents the concentration change of PAHs due to cyclodextrin injection and desorption; R d The hindering factor for pollutants is 1;

[0014] Among them, the consumption term Γ1 of PAHs due to biodegradation in the control equation is Equation (3), and the concentration change of PAHs in the plume due to biodegradation is described by the first-order kinetic equation, as shown in Equation (2).

[0015]

[0016] Γ1=-k C PAHs (3)

[0017] In the formula: k is the first-order biodegradation rate, d -1 ;

[0018] The concentration change Γ2 of PAHs caused by cyclodextrin injection desorption in the governing equation is given by equation (5); In soil, the cyclodextrin desorption process of PAHs follows a first-order two-chamber kinetic model, as shown in equation (4); Based on the collected soil from PAHs-contaminated sites, different concentrations of HPCD were used to extract PAHs from the soil, and the influence relationship between HPCD concentration and the fast and slow desorption constants and rates of PAHs was obtained, which is the concentration change of PAHs caused by cyclodextrin injection desorption in the governing equation.

[0019]

[0020] Γ2=-F rap k rap C PAHs -F slow k slow C PAHs (5)

[0021] Where: S0 is the initial concentration of PAHs on soil particles, mg / Kg; S t The concentration of PAHs on soil particles at a given time, in mg / Kg; F rap F slow It is the fraction of PAHs desorption at fast and slow rates, %; k rap k slow It refers to the fast and slow desorption rates of PAHs;

[0022] Considering the influence of water level fluctuation, the distance from the center of the soil pollution source area to the maximum distance of the contour line of the exceedance concentration, 2m below the water level along the groundwater flow direction, is defined as the pollution plume migration distance. The exceedance concentration of the plume is defined as the Class IV groundwater concentration value of four-ring PAHs in the national groundwater quality standard GB / T 14848-2017. That is, the essence of the change in the pollution plume migration distance is the change in the vertical position when the concentration of PAHs entering the groundwater is 0.48 mg / L. The change of PAH pollution plume during the natural decay process is jointly controlled by the process of continuous plume formation and the decay process of entering the groundwater. Therefore, the influence of the plume formation stage and the groundwater decay stage on the change of PAH pollution plume migration distance is studied separately.

[0023] ① Changes in the migration distance of PAHs pollution plumes during the plume formation stage

[0024] The plume formation phase is relatively short, and the impact of microbial degradation is negligible. The variation in the PAH plume migration distance during this process is influenced by the spatiotemporal interaction of cyclodextrin injection and desorption, vertical transport and mixing of dissolved PAHs in soil water, and dilution over the water table. In other words, the plume migration variation is mainly caused by the convection, dispersion, and desorption terms on the right-hand side of the governing equation. The change in the PAH plume migration distance during this phase is denoted as L. inj ;

[0025] ② Changes in the migration distance of PAHs pollution plumes during the groundwater depletion stage

[0026] Dissolved PAHs flow across the water table and form a plume in the groundwater. The process of the plume migrating along the groundwater flow direction and its concentration continuously decreasing is defined as the groundwater attenuation process. Groundwater convection only causes the entire PAH plume to shift along the groundwater flow direction and does not affect the attenuation process of PAHs within the plume. Both groundwater diffusion and microbial degradation affect the PAH concentration distribution within the plume. Therefore, the combined effects of microbial degradation and diffusion need to be considered. That is, the change in plume migration distance at this stage is mainly caused by convection, diffusion, biodegradation, and their interactions on the right-hand side of the governing equation.

[0027] (2) Construction of PAHs pollution plume migration distance prediction model

[0028] Determine L respectively inj Research parameters related to L1, L2, L3, and L4;

[0029] 2.1) Variation in PAH pollution plume migration distance L caused by the combined effects of the plume formation stage inj

[0030] The technical parameters for cyclodextrin enhancement are mainly related to the desorption and desorption process of PAHs caused by cyclodextrin injection, as shown in equations (2) and (3), including the injection concentration C. inj Injected traffic Q inj Injection time t inj The initial concentration of PAHs (Cs) in the soil; the velocity of dissolved PAHs flowing vertically from the soil water to the groundwater surface is affected by convection and dispersion mixing, and has a longitudinal dispersion α. L The permeability coefficient K, which characterizes the permeability of soil. s The desorbed PAHs are instantly diluted in concentration as they flow across the groundwater surface at depth Z. w These are the main factors influencing this process;

[0031] L inj With cyclodextrin injection concentration C inj Injected traffic Q inj Injection time t inj Initial concentration of PAHs in soil C s Longitudinal diffusion α L Permeability coefficient K s Groundwater depth Z w related;

[0032] 2.2) Changes in PAH pollution plume migration distance caused by groundwater convection during the groundwater depletion stage L1

[0033] The value of L1 is the distance that the polycyclic aromatic hydrocarbon plume is shifted along the groundwater flow direction due to groundwater convection during the groundwater attenuation stage. L1 is related to the groundwater flow velocity v and the attenuation time t in the groundwater. L1 is expressed by equation (6):

[0034] L1 = vt(6)

[0035] In the formula: v is the fluid velocity in the porous medium, here referring to the groundwater velocity, in m / d; K s The permeability coefficient of porous media is expressed in m / d; here, it refers to the permeability coefficient of groundwater; I is the hydraulic gradient, dimensionless; n e Effective porosity;

[0036] 2.3) Changes in PAH pollution plume migration distance caused by groundwater dispersion during the groundwater depletion stage L2

[0037] According to equation (1), the change in PAH concentration caused by groundwater diffusion during the groundwater depletion stage can be expressed by equation (7):

[0038]

[0039] In the formula: D L The longitudinal dispersion coefficient, m 2 / d,D L =α L v;D T m is the lateral dispersion coefficient. 2 / d; Migration distance mainly examines the longitudinal changes of the pollution plume along the groundwater flow direction;

[0040] L2 and longitudinal dispersion α L Permeability coefficient K s It is related to the hydraulic gradient I and the decay time t in groundwater;

[0041] 2.4) Changes in PAH pollution plume migration distance caused by biodegradation during the groundwater depletion stage L3

[0042] Under the action of microorganisms, the mass of PAHs in the plume decreases and the plume range shrinks; the magnitude of the first-order degradation rate coefficient k affects the strength of biodegradation.

[0043] L3 is related to the primary biodegradation rate k and the decay time t in groundwater;

[0044] 2.5) Changes in PAH pollution plume migration distance caused by the combined effects of biodegradation and groundwater dispersion during the groundwater depletion stage L4

[0045] The combined effect of biodegradation and groundwater diffusion helps reduce PAH concentration through biodegradation and decreases the increase in migration distance due to groundwater diffusion.

[0046] L4 and primary biodegradation rate k, longitudinal dispersion α L Permeability coefficient K s It is related to the hydraulic gradient I and the decay time t in groundwater;

[0047] Using the power function empirical rule and a multivariate nonlinear regression method, the variation L of the polycyclic aromatic hydrocarbon plume migration distance caused by the five parts of influence shown in Equation (8) is constructed. inj L1, L2, L3, L4; A j X is the fitting constant term. i Representative and L j The relevant parameters are i = 1, 2, 3...; a i For X i The power exponent; n j To be with L j The number of relevant parameters;

[0048]

[0049] In summary, the change in the migration distance of polycyclic aromatic hydrocarbon (PAH) plumes during natural decay is affected by five factors, and the PAH plume migration distance prediction model is constructed as Equation (9):

[0050] L = L inj +L1+L2+L3+L4 (9)

[0051] Where: L is the migration distance of the PAHs plume during natural decay, in meters; L inj L1 represents the change in plume migration distance caused by the plume formation stage, in meters; L2 represents the change in plume migration distance caused by groundwater convection, in meters; L3 represents the change in plume migration distance caused by groundwater dispersion, in meters; L4 represents the change in plume migration distance caused by biodegradation, in meters; and L5 represents the change in plume migration distance caused by the combined effects of biodegradation and groundwater dispersion, in meters.

[0052] (3) External verification of the PAHs transport convection-diffusion equation in groundwater

[0053] Based on field monitoring data of natural attenuation, the accuracy of the numerical simulation of PAHs migration in groundwater based on the convection-diffusion equation was verified. The accuracy was further validated by comparing the field monitoring data and the model results, using the adjusted coefficient of determination (R²) of the equation. 2 R is used to evaluate the accuracy of numerical models. 2 The closer the value is to 1, the higher the accuracy of the numerical model.

[0054] The beneficial effects of this invention are as follows: The predictive model for the migration distance variation of polycyclic aromatic hydrocarbon (PAH) plumes during the natural decay process of PAHs in underground aquifers, constructed in this invention, encompasses parameters affecting the natural decay process of PAHs, such as the hydrogeological characteristics of the underground aquifer, the degree of site contamination, the biodegradation rate, and the cyclodextrin enhancement technology. It also quantitatively characterizes the influence of changes in these parameters on the migration distance of PAH plumes. This invention can be applied to the optimization of monitoring and natural decay remediation technologies for PAH-contaminated sites, providing technical guidance for achieving efficient remediation of contaminated sites. Attached Figure Description

[0055] Figure 1 A conceptual model diagram for enhancing the natural decay of polycyclic aromatic hydrocarbons in underground aquifers using cyclodextrin.

[0056] Figure 2 The effects of various factors on the migration distance of polycyclic aromatic hydrocarbon (PAH) plumes are shown in the following figures: (a) the effects of cyclodextrin injection flow rate and concentration on the migration distance of PAH plumes during the plume formation stage; (b) the effects of groundwater depth and site contamination level on the migration distance of PAH plumes during the plume formation stage; (c) the effects of longitudinal dispersion and permeability coefficient on the migration distance of PAH plumes during the plume formation stage; and (d) the effects of the first-order biodegradation rate coefficient on the migration distance of PAH plumes during the plume formation stage.

[0057] Figure 3 The effects of various factors on the migration distance of polycyclic aromatic hydrocarbon (PAH) plumes are shown, where (a) is the effect of vertical dispersion during the groundwater decay stage on the migration distance of PAH plumes; and (b) is the effect of permeability coefficient and hydraulic gradient during the groundwater decay stage on the migration distance of PAH plumes.

[0058] Figure 4 The figures show a comparison of the raw and fitted data of the influence of various factors on the migration distance of polycyclic aromatic hydrocarbon (PAH) plumes. (a) shows the raw data of the influence of the first-order biodegradation coefficient during the groundwater depletion stage on the migration distance of PAH plumes; (b) shows the fitted data of the influence of the first-order biodegradation coefficient during the groundwater depletion stage on the migration distance of PAH plumes.

[0059] Figure 5 This is a comparison chart of simulated values ​​from an external validation model and measured values ​​from field experiments. Figure 6 This is a diagram showing the actual measurement results at the contaminated production site. Detailed Implementation

[0060] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and technical solutions.

[0061] Example 1: Numerical simulation of the natural decay of polycyclic aromatic hydrocarbons in soil-groundwater based on cyclodextrin enhancement

[0062] Based on the premise that continuous injection of cyclodextrin increases the bioavailability of adsorbed polycyclic aromatic hydrocarbons (PAHs), a scenario is established where dissolved PAHs rapidly flow into the saturation zone and decay through natural processes. It is assumed that the aquifer is homogeneous and isotropic, and that the groundwater flow velocity and direction are constant. The conceptual model study area is 200m × 10m (…). Figure 1 The study area consisted of an upper unsaturated zone 3m thick and a saturated zone 7m thick, with a 2m × 1m pollution source area at the center of the unsaturated zone. The left and right sides of the study area were designated as constant head boundaries, and the lower boundary was designated as an impermeable boundary to maintain a stable groundwater flow field within the study area. Based on the initial groundwater flow field, cyclodextrin at a constant concentration was continuously injected into the aquifer at a constant flow rate. After a certain injection time, the injection was stopped, allowing the polycyclic aromatic hydrocarbons to continue decaying within the original stable groundwater flow field.

[0063] The study selected nine parameters, considering the effects of cyclodextrin enhancement technology (injection concentration, injection flow rate, injection time, and initial site contamination level), aquifer media characteristics (hydraulic gradient, permeability coefficient, dispersion coefficient, and groundwater depth), and polycyclic aromatic hydrocarbon (PAH) biodegradation rate, and set eight factor levels (arithmetic progression). An orthogonal design table with nine parameters and eight factor levels was constructed.

[0064] Numerical simulations of the natural decay of polycyclic aromatic hydrocarbons in soil-groundwater were conducted using FEFLOW 7.2 (Finite Element subsurface FLOW system) software.

[0065] Table 1. Parameters and value ranges for the study of the migration distance of the natural decay plume of cyclodextrin-enhanced polycyclic aromatic hydrocarbons.

[0066]

[0067] Example 2: Construction of a Prediction Model for the Maximum Migration Distance of Polycyclic Aromatic Hydrocarbon Plumes

[0068] ① Changes in the migration distance of polycyclic aromatic hydrocarbon (PAH) pollution plumes caused by the combined effects of the plume formation stage (L) inj

[0069] With L inj The relevant single factor is C inj Q inj t inj C s α L K s Z w Through single-factor experiments ( Figure 2 The baseline model parameters were randomly selected as C. inj =75310mg / L, t inj =300d, Q inj =0.1m 3 / d、Z w =1m, C s =800mg / kg, α L =1m, K s =3.68m / d, I=0, k=0, and L is found to be... inj -t inj The evolution trend follows a power function model (Equation 10) under different parameter conditions, where the goodness of fit R 2 The value of A is 0.986-0.996, the power is 0.54-0.62, the proportionality constant A is 0.39-0.66, and the value of A is related to C. inj Q inj C s α L K s Z w This relates to the strength of the influence of various factors during the plume formation stage on the migration distance of polycyclic aromatic hydrocarbon (PAH) plumes. Considering that six factors simultaneously affect the changes in PAH migration distance during the plume formation stage and that the underlying patterns are unclear, this study uses the common power function rule to describe the relationship between the scaling factor A and the six factors (see Equation 11), where x... m(m = 1, 2, 3, ...) are undetermined coefficients.

[0070]

[0071]

[0072] ② Changes in the maximum migration distance of polycyclic aromatic hydrocarbons (PAHs) caused by groundwater convection during the attenuation phase (L1)

[0073] The value of L1 is the distance by which the polycyclic aromatic hydrocarbon plume is shifted along the groundwater flow direction due to groundwater convection during the decay stage. The single factors related to L1 are groundwater flow velocity v (Equation 12) and decay time t, where v is calculated based on Darcy's law (Equation 13).

[0074] L1 = vt(12)

[0075]

[0076] Where: K s denoted as the groundwater permeability coefficient (m / d); I is the hydraulic gradient of groundwater (dimension 1); and n is the porosity of the aquifer (dimension 1).

[0077] ③ Changes in the maximum migration distance of polycyclic aromatic hydrocarbon (PAH) plumes caused by groundwater dispersion during the attenuation phase (L2)

[0078] The research parameters related to L2 include α L K s I and t were determined through a single-factor experiment. Figure 3 ), select benchmark model C inj =75310mg / l, Q inj =0.1m / d, t inj =300m / d Z w =1m, C s =800mg / kg, α L =1m, K s =3.68m / d, I=0.005, k=0, it was found that the evolution trend of L2-t follows a linear function model (Equation 14) under different parameter conditions. The goodness of fit R0 2 ≥0.990, the proportionality constant (A2) is 0.00414-0.00666, and its value is related to α. L K s The parameters A2 and I are related to the strength of the influence of groundwater dispersion during the decay stage on the migration distance of polycyclic aromatic hydrocarbon (PAH) pollution plumes. Similarly, considering that the above parameters simultaneously affect the dispersion of groundwater during the decay stage and the influence law is unclear, this study uses the power function rule to describe the relationship between the scaling factor A2 and the three parameters (Equation 15).

[0079] L2＝ A2t(14)

[0080]

[0081] ④ Changes in the maximum migration distance of polycyclic aromatic hydrocarbon (PAH) plumes caused by biodegradation during the decay phase (L3)

[0082] Microbial degradation during the downward gradient transport of polycyclic aromatic hydrocarbon plumes from the source region was simulated using first-order reaction coefficients. The parameters related to L3 were k and t, determined through single-factor experiments. Figure 4 Select the baseline model parameter C. inj =75310mg / l, Q inj =0.1m 3 / d、t inj =300d, Z w =1m, C s =800mg / kg, α L =1m, K s =3.68 m / d, I = 0.005, k = 0.0018 / d. Through fitting analysis of L3 and groundwater decay time t, it was found that L3-t exhibits a negative correlation, and the evolution trend follows a power function model (Equation 15). The goodness of fit of the power function model is R0. 2 The value of A3 is 0.985-0.997, the power is 1.33-1.87, and the proportionality coefficient A3 ranges from 5×10. -6 -7×10 -5 Its value is mainly related to k, reflecting the strength of the influence of biodegradation during the decay stage on the maximum migration distance of polycyclic aromatic hydrocarbon pollution plumes. Similarly, this study uses the power function rule to describe the relationship between the scaling factor A3 and k (Equation 16).

[0083] L3=A3t 1.33-1.87 (15)

[0084]

[0085] ⑤ Changes in the maximum migration distance of polycyclic aromatic hydrocarbons caused by the combined effects of biodegradation and groundwater dispersion during the decay phase (L4)

[0086] The combined effects of biodegradation and groundwater dispersion accelerate the decrease in the concentration gradient of polycyclic aromatic hydrocarbons (PAHs) within the plume. The resulting L4 change depends on α. L K s The parameters I, k, and t reflect groundwater dispersion and microbial degradation. Similarly, the power function rule is used to describe the relationship between L4 and the above five parameters (Equation 17).

[0087]

[0088] Based on equations (10)(12)(14)(15)(17), a multivariate nonlinear fitting analysis was conducted on the results of the study on the migration distance of polycyclic aromatic hydrocarbon (PAH) pollution plumes during the plume formation stage and the groundwater decay stage obtained from the FEFLOW numerical simulation. Equation (18) can be obtained. This result can be used as a comprehensive model (n=1285) to predict the changes in the migration distance of PAH plumes in groundwater aquifers with solute transport parameters, degradation rate and decay time.

[0089]

[0090] Example 3: External Validation of the Convection-Dispersion Equation for Polycyclic Aromatic Hydrocarbon Transport in Groundwater

[0091] Based on the hydrogeological characteristics of the aquifer (Table 2) from the literature (Kao C M., et al. "Assessing of Natural Attenuation and Intrinsic Bioremediation Rates at a Petroleum-Hydrocarbon Spill Site: Laboratory and Field Studies." Journal of Environmental Engineering 136(2010):54-67.), a simulation verification of the transport of dissolved polycyclic aromatic hydrocarbons (PAHs) in groundwater using FEFLOW software was conducted. The governing equation model for the transport of PAHs in groundwater was used to simulate the natural decay rate of dissolved components benzene and toluene at a gasoline spill contaminated site over 18 months. The simulation results were found to be basically the same as the measured results (Table 3), showing a highly significant correlation. The goodness of fit was 0.998, the slope was 1, and P < 0.001. Figure 5 The validation results show that the polycyclic aromatic hydrocarbon (PAH) plume migration distance prediction model constructed using FEFLOW is accurate and reliable.

[0092] Table 2. Hydrogeological characteristics of underground aquifers in the external validation model.

[0093]

[0094] Table 3 Comparison of the natural decay rates of benzene and toluene in the literature with those in numerical simulations.

[0095]

Claims

1. A predictive model for the migration distance of a pollution plume during the natural decay of polycyclic aromatic hydrocarbons (PAHs) in an underground aquifer enhanced by cyclodextrin, characterized in that, The steps are as follows: (1) Numerical simulation of natural decay of PAHs in soil-groundwater based on cyclodextrin enhancement The saturated transport of soil-groundwater was simulated based on the Richards equation. Combining the physicochemical properties of cyclodextrin inclusion complexes, and neglecting the adsorption loss in the aquifer, a convection-diffusion equation with a specific desorption term was established to simulate the enhanced natural decay process of PAHs in soil-groundwater, as shown in equation (1). The concentration change of PAHs in the aquifer includes the concentration change caused by the desorption of polycyclic aromatic hydrocarbons on soil particles by cyclodextrin, and the concentration change caused by the combined effects of convection, diffusion and microbial degradation in soil-groundwater. where C is the mass concentration of PAHs, mg / L; t is the time of natural attenuation, d; x i is the distance, m; D ij is the dispersion coefficient tensor, m 2 / d; v i is the groundwater, i.e. soil water flow rate, m / d; n e is the effective porosity; Γ1 is the depletion term of PAHs due to biodegradation; Γ2 is the concentration change of PAHs due to cyclodextrin injection desorption; R d is the retardation factor of the pollutant, is 1; Among them, the consumption term Γ1 of PAHs due to biodegradation in the control equation is Equation (3), and the concentration change of PAHs in the plume due to biodegradation is described by the first-order kinetic equation, as shown in Equation (2). Γ1=-kC(3) wherein: k is the first order biodegradation rate, d -1 ; The concentration change Γ2 of PAHs caused by cyclodextrin injection desorption in the governing equation is given by equation (5); In soil, the cyclodextrin desorption process of PAHs follows a first-order two-chamber kinetic model, as shown in equation (4); Based on the collected soil from PAHs-contaminated sites, different concentrations of HPCD were used to extract PAHs from the soil, and the influence relationship between HPCD concentration and the fast and slow desorption constants and rates of PAHs was obtained, which is the concentration change of PAHs caused by cyclodextrin injection desorption in the governing equation. Γ2=-F rap k rap C PAHs -F slow k slow C PAHs (5) Where: S0 is the initial concentration of PAHs on soil particles, mg / Kg; S t The concentration of PAHs on soil particles at a given time, in mg / Kg; F rap F slow It is the fraction of PAHs desorption at fast and slow rates, %; k rap k slow It refers to the fast and slow desorption rates of PAHs; Considering the influence of water level fluctuation, the distance from the center of the soil pollution source area to the maximum distance of the contour line of the exceedance concentration, 2m below the water level along the groundwater flow direction, is defined as the pollution plume migration distance. The exceedance concentration of the plume is defined as the Class IV groundwater concentration value of four-ring PAHs in the national groundwater quality standard GB / T 14848-2017. That is, the essence of the change in the pollution plume migration distance is the change in the vertical position when the concentration of PAHs entering the groundwater is 0.48 mg / L. The change of PAH pollution plume during the natural decay process is jointly controlled by the process of continuous plume formation and the decay process of entering the groundwater. Therefore, the influence of the plume formation stage and the groundwater decay stage on the change of PAH pollution plume migration distance is studied separately. ① Changes in the migration distance of PAHs pollution plumes during the plume formation stage The plume formation phase is relatively short, and the impact of microbial degradation is negligible. The variation in the PAH plume migration distance during this process is influenced by the spatiotemporal interaction of cyclodextrin injection and desorption, vertical transport and mixing of dissolved PAHs in soil water, and dilution over the water table. In other words, the plume migration variation is caused by the convection, dispersion, and desorption terms on the right-hand side of the governing equation. The change in the PAH plume migration distance during this phase is denoted as L. inj ; ② Changes in the migration distance of PAHs pollution plumes during the groundwater depletion stage Dissolved PAHs flow across the water table and form a plume in the groundwater. The process of the plume migrating along the groundwater flow direction and its concentration continuously decreasing is defined as the groundwater attenuation process. Groundwater convection only causes the entire PAH plume to shift along the groundwater flow direction and does not affect the attenuation process of PAHs within the plume. Both groundwater diffusion and microbial degradation affect the PAH concentration distribution within the plume. Therefore, the combined effects of microbial degradation and diffusion need to be considered. That is, the change in plume migration distance at this stage is caused by convection, diffusion, biodegradation, and their interactions on the right-hand side of the governing equation. (2) Construction of PAHs pollution plume migration distance prediction model Determine L respectively inj Research parameters related to L1, L2, L3, and L4; 2.1) Variation in PAH pollution plume migration distance L caused by the combined effects of the plume formation stage inj The technical parameters for cyclodextrin enhancement are related to the desorption and desorption process of PAHs induced by cyclodextrin injection, as shown in equations (2) and (3), including the injection concentration C. inj Injected traffic Q inj Injection time t inj The initial concentration of PAHs (Cs) in the soil; the velocity of dissolved PAHs flowing vertically from the soil water to the groundwater surface is affected by convection and dispersion mixing, and has a longitudinal dispersion α. L The permeability coefficient K, which characterizes the permeability of soil. s The desorbed PAHs are instantly diluted in concentration as they flow across the groundwater surface at depth Z. w These are the main factors influencing this process; L inj With cyclodextrin injection concentration C inj Injected traffic Q inj Injection time t inj Initial concentration of PAHs in soil C s Longitudinal diffusion α L Permeability coefficient K s Groundwater depth Z w related; 2.2) Changes in PAH pollution plume migration distance caused by groundwater convection during the groundwater depletion stage L1 The value of L1 is the distance that the polycyclic aromatic hydrocarbon plume is shifted along the groundwater flow direction due to groundwater convection during the groundwater attenuation stage. L1 is related to the groundwater flow velocity v and the attenuation time t in the groundwater. L1 is expressed by equation (7): L1 = vt (7) In the formula: v is the fluid velocity in the porous medium, here referring to the groundwater velocity, in m / d; K s ρ is the permeability coefficient of the porous medium, m / d; I is the hydraulic gradient, dimensionless; n e Effective porosity; 2.3) Changes in PAH pollution plume migration distance caused by groundwater dispersion during the groundwater depletion stage L2 According to equation (1), the change in PAH concentration caused by groundwater diffusion during the groundwater depletion stage can be expressed by equation (8): In the formula: D L The longitudinal dispersion coefficient, m 2 / d,D L =α L v;D T m is the lateral dispersion coefficient. 2 / d; Migration distance examines the longitudinal variation of the pollution plume along the groundwater flow direction; L2 and longitudinal dispersion α L Permeability coefficient K s It is related to the hydraulic gradient I and the decay time t in groundwater; 2.4) Changes in PAH pollution plume migration distance caused by biodegradation during the groundwater depletion stage L3 Under the action of microorganisms, the mass of PAHs in the plume decreases and the plume range shrinks; the magnitude of the first-order degradation rate coefficient k affects the strength of biodegradation. L3 is related to the primary biodegradation rate k and the decay time t in groundwater; 2.5) Changes in PAH pollution plume migration distance caused by the combined effects of biodegradation and groundwater dispersion during the groundwater depletion stage L4 The combined effect of biodegradation and groundwater diffusion helps reduce PAH concentration through biodegradation and decreases the increase in migration distance due to groundwater diffusion. L4 and primary biodegradation rate k, longitudinal dispersion α L Permeability coefficient K s It is related to the hydraulic gradient I and the decay time t in groundwater; Using the power function empirical rule and a multivariate nonlinear regression method, the variation L of the polycyclic aromatic hydrocarbon plume migration distance caused by the five components shown in Equation (9) is constructed. inj L1, L2, L3, L4; A j X is the fitting constant term. i Representative and L j The relevant parameters are i = 1, 2, 3...; a i For X i The power exponent; n j To be with L j The number of relevant parameters; In summary, the change in the migration distance of polycyclic aromatic hydrocarbon (PAH) plumes during natural decay is affected by five factors, and the PAH plume migration distance prediction model is constructed as Equation (10): L=L inj +L1+L2+L3+L4 (10) Where: L is the migration distance of the PAHs plume during natural decay, in meters; L inj L1 represents the change in plume migration distance caused by the plume formation stage, in meters; L2 represents the change in plume migration distance caused by groundwater convection, in meters; L3 represents the change in plume migration distance caused by groundwater dispersion, in meters; L4 represents the change in plume migration distance caused by biodegradation, in meters; and L5 represents the change in plume migration distance caused by the combined effects of biodegradation and groundwater dispersion, in meters. (3) External verification of the PAHs transport convection-diffusion equation in groundwater Based on field monitoring data of natural decay, the accuracy of numerical simulation of PAHs migration in groundwater based on the convection-diffusion equation was verified.

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