Boron isotope-based method for identifying underground water polluted by flowback fluid

Abstract

The invention discloses a boron isotope-based method for identifying underground water polluted by flowback fluid, and relates to the technical field of environmental protection. The method comprises the following steps: depicting a boron isotope equilibrium fractionation state between a water-containing medium and underground water before pollution; after pollution occurs, when boron ions and boron isotopes in flowback fluid and underground water are only conservatively mixed but not subjected to new equilibrium fractionation, the ratio of the boron isotopes in the mixed liquid is conservative under different pollution intensities; when boric acid in the polluted water body reaches new hydrolysis equilibrium and boron isotope equilibrium fractionation is initiated, the boron concentration and the boron isotope ratio are adsorbed to the water-containing medium through new equilibrium fractionation; according to the isotope conservative binary mixed model, the boron isotope ratio and the boron concentration of the polluted underground water are obtained, a response curve of the boron isotope and the boron concentration of the polluted underground water is constructed, and early pollution is recognized. By establishing the boron isotope quantitative model of the underground water polluted by the flow-back fluid, the accuracy of a pollution result reflecting real field conditions is improved.

Description

Technical Field

[0001] This application relates to the field of environmental protection technology, and in particular to a method for identifying groundwater contaminated by backflow fluid based on boron isotopes. Background Technology

[0002] While shale gas has garnered widespread attention due to its high energy efficiency and clean environmental benefits, the potential impact on groundwater has also attracted considerable attention from government departments and experts. The core of shale gas development is hydraulic fracturing, but this process generates a large amount of flowback fluid, typically accounting for 20-30% of the fracturing fluid volume. Flowback fluid mainly consists of fracturing fluid, formation water from the shale layer, and shale particles formed during fracturing. Artificially prepared fracturing fluids contain various organic compounds, and formation water is usually brine with high total dissolved solids (TDS). Furthermore, clay minerals in shale preferentially adsorb heavy metals and uranium from the formation, resulting in flowback fluids typically characterized by high TDS, high radioactivity, and excessive levels of toxic and harmful components. Statistics from the New York State Environmental Protection Department indicate that fracturing flowback fluids within its jurisdiction contain an average of 59 dissolved heavy metals, 27 organochlorine compounds, and 177 volatile and semi-volatile organic compounds, including large amounts of heavy metals and organic compounds that are toxic and harmful to humans. Toxic and hazardous flowback fluids returned to the surface are typically stored in wastewater pools at shale gas well sites. After the flowback process is completed, they are transported to wastewater treatment plants for treatment and discharge. However, leaks and spills that may occur during the fracturing, storage, transportation, and treatment processes make the flowback fluids a potential source of shallow groundwater pollution. Therefore, how to efficiently identify flowback fluids in shallow aquifers has become a new hot topic in the field of environmental impact assessment for shale gas development.

[0003] With the increasing global shale gas extraction, major shale gas producing countries are paying increasing attention to the potential groundwater pollution from shale gas development, making the identification techniques for hydraulic fracturing flowback fluid in shallow groundwater a current research hotspot. Currently, there are three main methods for identifying groundwater contamination by flowback fluid: one is the characteristic components in the fracturing fluid that distinguish it from shallow groundwater; another is the artificial tracer added to the fracturing fluid; and the third is the geochemical characteristics of the flowback fluid itself. However, the first two identification methods are limited in application due to the following factors: (1) the fracturing fluid formulation is protected by patents, making it impossible to obtain detailed chemical composition and concentration; (2) the chemical reactions, complexation effects, and adsorption that may occur between the artificial tracer and the flowback fluid during fracturing operations reduce the effectiveness of the artificial tracer. Therefore, current research mainly focuses on the geochemical characteristics of the flowback fluid itself.

[0004] In response to the geochemical characteristics of flowback fluids, the United States, with its highest level of shale gas extraction, pioneered a series of studies to identify and address groundwater contamination caused by flowback fluids. Taking the Marcellus gas field, the largest marine shale gas field in the United States, as an example, the Cl concentration in the flowback fluid is typically greater than 10 g / L, its hydrochemical type is primarily Cl-Na type water, and the Br / Cl ratio (>0.1×10−2) is greater than the corresponding value in seawater; the flowback fluid contains… (25~31‰) and 87 Sr / 86 The distribution range of Sr (0.7101~0.7121) is also similar to the characteristic values ​​of seawater. The Sichuan Basin contains my country's largest marine shale gas reservoir to date, namely the Silurian Longmaxi Formation shale. The TDS of the hydraulic fracturing flowback fluid from the Longmaxi Formation is greater than 10,000 mg / L, and the Br / Cl ratio is greater than... , The concentration ranges from 20 to 30‰. Meanwhile, the formation water in the Longmaxi Formation shale is Cl-Na type water, with TDS ranging from 57.6 to 65.3 g / L. The contents of Na, Cl, and Br are twice that of seawater, while the Br / Cl and Na / Cl ratios are close to those of seawater. Therefore, it can be concluded that the identification indicators and characteristic values ​​of fracturing flowback fluids from marine shale are similar to those of seawater in terms of their distribution range. Both the Marcellus Shale in the United States and the Longmaxi Formation shale in Sichuan were deposited in a marine environment, and the formation water contained within them is paleoseawater that has been concentrated through evaporation. This indicates that during fracturing and flowback, marine shale formation water, through mixing with fracturing fluid, dominates the hydrochemical and isotopic characteristics of the flowback fluid. Therefore, the current identification indicators for groundwater contamination by marine shale fracturing flowback fluid are mainly hydrogeochemical indicators that differ from those of groundwater, including salinity, bromine-chlorine concentration ratio (Br / Cl), and boron isotope values ​​(…). The pollution monitoring threshold and identification process are as follows: (1) Monitor whether the salinity of groundwater exceeds the background value; (2) Monitor whether the bromine-chlorine concentration ratio (Br / Cl) in groundwater exceeds the corresponding value in seawater; (3) Monitor the boron isotope value in groundwater ( (3) Whether it is less than the corresponding value of seawater, especially 30‰; (4) If all of the above conditions are met, it indicates that the fracturing flowback fluid of marine shale is polluting groundwater.

[0005] However, existing methods for identifying groundwater contamination by flowback fluids in my country's widely distributed marine-continental transitional shale gas reservoirs have the following shortcomings when applied to fracturing flowback fluids from marine-continental transitional shale gas reservoirs: (1) Regarding pollution identification indicators, salinity is the ratio of the mass of dissolved substances in water to the mass of water, and it is a comprehensive indicator that includes multiple ionic components. In addition to pollution from fracturing flowback fluid, the salinity of groundwater is also abnormally elevated due to the input of chloride ions, potassium ions, nitrate ions, sulfate ions, etc. from domestic, agricultural, and industrial pollution sources, making salinity a highly uncertain indicator for identifying flowback fluid pollution. The Br / Cl ratio is also subject to similar limitations as salinity due to the multi-source nature of chloride ions in groundwater. Therefore, compared with ions and their related identification indicators, isotopes with pollution source fingerprints are a good indicator that exclusively points to flowback fluid pollution of groundwater.

[0006] (2) Regarding the pollution monitoring threshold, the hydrogeochemical end-member characteristic value of the shale fracturing flowback fluid is the monitoring threshold. However, since the sedimentary environment of marine-continental transitional shale is quite different from that of marine shale, the source of geochemical components in its formation water is also different from that of marine shale formation water. As a result, the formation water imparts significantly different geochemical characteristic values ​​to the flowback fluid through mixing during the fracturing process. Therefore, the monitoring threshold for groundwater pollution by marine shale fracturing flowback fluid is not applicable to marine-continental transitional shale.

[0007] (3) Regarding pollution identification methods, when fracturing flowback fluid enters the aquifer, it alters the original hydrochemical balance of shallow groundwater, triggering water-rock interactions such as dissolution / precipitation, adsorption / desorption, and cation exchange. These geochemical processes may reduce the significance of flowback fluid characteristic indicators compared to shallow groundwater. However, pollution identification methods based on indicator monitoring thresholds do not consider the complex groundwater chemical processes involved in these pollution events, thus affecting their effectiveness as an identifier for flowback fluid pollution of shallow groundwater. Furthermore, this pollution identification method is only applicable to scenarios where groundwater is severely polluted and cannot effectively identify pollution from small-flux flowback fluids in the early stages of pollution.

[0008] Therefore, there is an urgent need for an isotope-based method to identify groundwater contamination by flowback fluid, reducing the interference caused by dynamic fluctuations in groundwater on the identification of contamination by flowback fluid from fracturing shale gas in marine-continental transitional facies. Summary of the Invention

[0009] Therefore, it is necessary to provide a method for identifying groundwater contaminated by backflow fluid based on boron isotopes to address the aforementioned technical problems.

[0010] The following technical solution is adopted in this specification: This specification provides a method for identifying groundwater contaminated by backflow fluid based on boron isotopes, including: Based on the boron distribution coefficient and boron isotope fractionation coefficient in the groundwater aquifer, the boron isotope equilibrium fractionation state between the aquifer medium and the uncontaminated groundwater is determined, and the initial concentration and isotope ratio of boron ions in the aquifer medium are obtained. Based on the boron concentration and boron isotope ratio in the backflow solution and uncontaminated groundwater, the boron concentration and boron isotope ratio in the conservative mixture under different pollution intensities were obtained; and the pH value of the conservative mixture and the boron partition coefficient and boron isotope fractionation coefficient at the pH value were determined. Based on the initial concentration and boron isotope ratio of boron ions in the aqueous medium, the concentration and boron isotope ratio of boron ions in the conservative mixed solution, and the partition coefficient and fractionation coefficient of boron at the pH value, the process by which boron reaches hydrolysis equilibrium at the pH value and triggers boron isotope equilibrium fractionation in polluted water is determined. The boron concentration removed from groundwater based on selective adsorption and the boron isotope ratio adsorbed from groundwater to the aqueous medium during the equilibrium fractionation process are obtained. Furthermore, through a conservative binary mixing model of ion and isotope mass, the boron concentration and boron isotope ratio in polluted groundwater are obtained, and a response curve of the boron concentration and boron isotope ratio in polluted groundwater is constructed. The monitoring data of boron isotopes and boron concentration in the target groundwater are plotted onto the response curve to determine whether the groundwater is contaminated by backflow fluid and to determine the pollution intensity of the target groundwater.

[0011] Furthermore, the boron concentration and boron isotope ratio in the backflow fluid and uncontaminated groundwater include: The boron concentration and boron isotope ratio in the uncontaminated groundwater aquifer and the flowback fluid were obtained by testing and analyzing them separately.

[0012] Furthermore, the conservative mixture is obtained by conservatively mixing the backflow fluid and the uncontaminated groundwater without any other water-rock interactions.

[0013] Furthermore, the pH value of the conservative mixture is obtained by simulating and analyzing the conservative mixture using PHREEQC software.

[0014] Furthermore, the determination of the boron isotope equilibrium fractionation state between the aquifer medium and the uncontaminated groundwater based on the boron distribution coefficient and boron isotope fractionation coefficient in the groundwater aquifer includes: By testing and analyzing uncontaminated aquifers, the boron concentration and boron isotope ratio in the uncontaminated groundwater and the weight percentage of clay particles in the aquifer medium were obtained. Based on the initial concentration of boron ions in the aquifer and groundwater and the weight ratio of clay particles in the aquifer, the distribution coefficient of boron ions in the uncontaminated aquifer is obtained. The allocation coefficient is calculated using the following formula: ; in, The partition coefficient of boron ions in an uncontaminated aquifer; and The initial concentrations of boron ions in the aqueous medium and groundwater are respectively. c This represents the weight percentage of clay particles in the aqueous medium. Based on the boron concentration and boron isotope ratio in uncontaminated groundwater, the fractionation coefficients of boron isotopes in the aqueous medium and groundwater are obtained; The fractionation coefficient is calculated using the following formula: ; in ，α I The fractionation coefficients of boron isotopes in the aqueous medium and groundwater under initial conditions; and These represent the boron isotope ratios in uncontaminated aquatic media and groundwater, respectively. Based on the distribution coefficient and the fractionation coefficient, the boron isotope equilibrium fractionation state between the aqueous medium and the uncontaminated groundwater is characterized.

[0015] Furthermore, the calculation process for the boron concentration and boron isotope ratio in the conservative mixed solution under different pollution intensities is as follows: The boron concentration in the conservative mixed solution is obtained based on the boron concentration in the backflow solution, the volume percentage of the backflow solution in the contaminated groundwater, and the initial concentration of boron ions in the groundwater. The formula for calculating the boron concentration in the contaminated groundwater is as follows: ; in, The boron concentration in the conservative mixture; The boron concentration in the effluent; x Iw This represents the volume percentage of the backflow fluid in the contaminated groundwater. This represents the initial concentration of boron ions in the groundwater. The boron isotope ratio in the polluted groundwater was obtained based on the boron concentration in the polluted groundwater and the boron isotope ratio in the unpolluted groundwater. The formula for calculating the boron isotope ratio in the contaminated groundwater is as follows: ; in, The boron isotope ratio in the conservative mixture; The boron isotope ratio in uncontaminated groundwater; This represents the boron isotope ratio in the effluent.

[0016] Furthermore, the process by which boron reaches hydrolysis equilibrium at the stated pH value and initiates boron isotope equilibrium fractionation in polluted water includes: Based on the boron partition coefficient in the polluted aquifer, the pH value of the conservative mixed solution, the boron concentration entering the aquifer through selective adsorption, and the fractionation coefficient between the aquifer and the polluted groundwater after equilibrium fractionation, the polluted water body that reaches hydrolysis equilibrium at the pH value and triggers boron isotope equilibrium fractionation is characterized. The equation is expressed as follows: ; ; ; in, The partition coefficient of boron in the contaminated aquifer; and The boron concentrations in the aqueous medium and polluted groundwater after selective adsorption of boron are shown, respectively. and These represent the boron concentrations that enter the aqueous medium through selective adsorption and the boron concentrations that are removed from groundwater, respectively. u The water content is the weight of the aqueous medium. To balance the fractionation coefficient between the aqueous medium and the contaminated groundwater after fractionation; and These represent the boron isotope ratios in the aqueous medium and polluted groundwater after equilibrium fractionation, respectively. To balance the boron isotope ratios adsorbed from the solid phase to the liquid phase during the fractionation process.

[0017] The above-mentioned technical solutions adopted in this specification can achieve the following beneficial effects: In the boron isotope-based method for identifying groundwater contamination by backflow fluid provided in this specification, boron isotopes in the aquifer-groundwater system are selected as indicators for identifying backflow fluid contamination. The contamination status of groundwater is identified and determined by cross-monitoring of boron isotopes and boron concentration. The selective adsorption / desorption equilibrium fractionation of boron isotopes, which is only affected by pH-controlled hydrolysis reactions, improves their tracer performance.

[0018] Furthermore, by establishing a boron isotope quantitative model for groundwater contaminated by backflow fluid, and by introducing the weight ratio of clay particles in the aqueous medium into the boron isotope quantitative model, the previous model overcame the assumption that the aqueous medium was entirely composed of clay particles, and the simulation results reflected the pollution results under real field conditions. Attached Figure Description

[0019] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments of this application and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0020] Figure 1 This document presents a flowchart illustrating a method for identifying groundwater contaminated by backflow fluid based on boron isotopes. Figure 2 A schematic diagram of the response curves of boron concentration and boron isotope ratio in groundwater to pollution under Scenario 1 provided in this specification; Figure 3 This is a schematic diagram of the response curves of boron concentration and boron isotope ratio in groundwater to pollution under Scenario 2 provided in this specification. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this specification clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments in this specification without creative effort are within the scope of protection of this application.

[0022] When backflow fluid contaminates shallow groundwater, changes in the groundwater pH level trigger the hydrolysis of boric acid. This leads to preferential adsorption by clay particles in the aquifer. 10 The boron isotope equilibrium fractionation effect formed by B Since aquifers typically contain clay minerals, when backflow fluid continuously enters the aquifer, causing changes in the pH of the groundwater, these clay minerals will trigger a significant boron isotope equilibrium fractionation.

[0023] To this end, the present invention provides a method for identifying groundwater contaminated by backflow fluid based on boron isotopes, and establishes a quantitative model of boron isotopes for groundwater contaminated by backflow fluid considering boron isotope equilibrium fractionation.

[0024] The technical solutions provided by the various embodiments of this application are described in detail below with reference to the accompanying drawings.

[0025] like Figure 1 A flowchart illustrating a method for identifying groundwater contaminated by backflow fluid based on boron isotopes is provided, specifically including the following steps: S101: Describes the boron isotope equilibrium fractionation state between the aquatic medium and shallow groundwater before pollution occurs.

[0026] This can be expressed as: (1) (2) in, The partition coefficient of boron ions in an uncontaminated aquifer; and The initial concentrations of boron ions in the aqueous medium and groundwater are respectively ( ). mM ); c The percentage of clay particles in the aqueous medium by weight (%). The fractionation coefficients of boron isotopes in the aqueous medium and groundwater under initial conditions; and These represent the boron isotope ratios (‰) between uncontaminated aquifers and groundwater. and α Previous research findings can be consulted.

[0027] Substituting the known parameters, the test and analysis results of boron concentration and boron isotope ratio in uncontaminated groundwater, and the particle analysis results of the aquatic medium into equations (1) and (2), we obtain... and .

[0028] The second step describes how, after pollution occurs, boron ions and boron isotopes in the backflow solution and groundwater only undergo conservative mixing and have not yet fractionated, which can be expressed as: (3) (4) In the formula: and The values ​​are boron concentration (mM) and boron isotope ratio (‰) in the conservative mixture, respectively. and These represent the boron concentration (mM) and boron isotope ratio (‰) in the effluent, respectively. The percentage (%) is the volume percentage of the backflow fluid in the contaminated groundwater.

[0029] Substituting the test and analysis results of the backflow fluid and uncontaminated groundwater into equations (3) and (4), the conservative concentrations of the mixed liquid under different pollution intensities were calculated. and 。

[0030] The third step describes how, after pollution occurs, boric acid in the polluted water reaches a new hydrolysis equilibrium at the new pH value, triggering equilibrium fractionation of boron isotopes, which can be expressed as: (5) (6) (7) In the formula: The partition coefficient of boron in a polluted aquifer; and The boron concentrations (mM) in the aqueous medium and polluted groundwater after selective adsorption of boron are shown, respectively. and The boron concentrations (mM) entering the aqueous medium through selective adsorption and the boron concentrations (mM) removed from groundwater are respectively. u The water content (%) is the weight of the aqueous medium. To balance the fractionation coefficient between the aqueous medium and the contaminated groundwater after fractionation; and The boron isotope ratios (‰) in the aqueous medium and polluted groundwater after equilibrium fractionation are respectively. The boron isotope ratio (‰) from the solid phase to the liquid phase (or from the solid phase to the liquid phase) during the equilibrium fractionation process.

[0031] Based on equation (6), equations (5) and (7) are transformed and eliminated. Next, the data obtained in steps one and two will be used to... 、 、 and Substituting the transformed equations (5) and (7), we obtain the result. and .

[0032] Fourth, based on the following isotopic conservative binary mixing model, calculate the boron isotope ratios in the contaminated groundwater: (8) (9) The meanings of the symbols in the formula are explained above.

[0033] The third step obtained and Substituting into equations (8) and (9), we obtain and .

[0034] To verify the effectiveness and sensitivity of this invention, an example was used. This example selected a typical marine-continental transitional shale gas reservoir in my country, namely the Taiyuan Formation of the Permian system in the North China Plain, as the object of study. The fracturing flowback fluid from the Taiyuan Formation shale gas reservoir was used as the groundwater pollution source, and typical shallow groundwater in the North China Plain was used as the groundwater pollution receptor. Based on the hydrochemical and isotopic analysis results of the flowback fluid and shallow groundwater, the response of boron concentration and boron isotope ratio in groundwater under different pollution intensities (represented by the volume ratio of flowback fluid to polluted groundwater) was simulated. Since the proportion of clay minerals in the aquifers of this area is less than 5%, and the weight water content is 15%~25%, simulations were conducted for aquifers with different characteristic values. For example... Figure 2 Scenario 1 illustrates the response curves of boron concentration and boron isotope ratio in polluted groundwater under a constant weight water content of 20% and clay mineral content of 1%, 2%, 3%, 4%, and 5%, respectively, as pollution intensity changes. (Example:) Figure 3 Scenario 2 shows the response curves of boron concentration and boron isotope ratio in polluted groundwater as the pollution intensity changes, under the condition that the clay mineral content of the aquifer is 3% and the weight water content is 15%, 20%, and 25%.

[0035] The results of the examples show that, under the conditions of an aquifer weight water content of 20% and clay mineral content of 1%, 2%, 3%, 4%, and 5%, when the backflow fluid reaches 0.1% of the total volume of contaminated groundwater, although the boron concentration in the groundwater shows a slight change, the boron isotope ratio shows a detectable change (the uncertainty in boron concentration testing is 0.001 mg / L, and the uncertainty in boron isotope ratio testing is 0.000015). When the backflow fluid reaches 1% of the total volume of contaminated groundwater, both the boron concentration and the boron isotope ratio in the groundwater show significant changes. Table 1 shows the parameters required for step 1 and the results obtained; Table 2 shows the results obtained for step 2 under different pollution intensities; Table 3 shows the data obtained for step 3 under different aquifer parameters and pollution intensities; Table 4 shows the data obtained for step 4 under different aquifer parameters and pollution intensities.

[0036] Table 1. Boron ion and boron isotope parameters in aquifers under different conditions, measured boron concentration and boron isotope ratio in groundwater, and calculated boron concentration and boron isotope ratio in aquifers. Table 2. Boron concentration and boron isotope ratios in the conservative mixture of backflow fluid and uncontaminated groundwater under different pollution intensities. Table 3. Adsorption capacity of boron ions in polluted groundwater induced by new boron isotope equilibrium fractionation and the isotope ratio of adsorbed boron under different aqueous media conditions and pollution intensities. unit: For mM, for‰ Table 4. Final boron concentration and boron isotope ratio in polluted groundwater under different aquifer conditions and pollution intensities. unit: It is mg / L. for‰ The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A method for identifying groundwater contaminated by backflow fluid based on boron isotopes, characterized in that, include: Based on the boron distribution coefficient and boron isotope fractionation coefficient in the groundwater aquifer, the boron isotope equilibrium fractionation state between the aquifer medium and the uncontaminated groundwater is determined, and the initial concentration and isotope ratio of boron ions in the aquifer medium are obtained. Based on the boron concentration and boron isotope ratio in the backflow solution and uncontaminated groundwater, the boron concentration and boron isotope ratio in the conservative mixture under different pollution intensities were obtained; and the pH value of the conservative mixture and the boron partition coefficient and boron isotope fractionation coefficient at the pH value were determined. Based on the initial concentration and boron isotope ratio of boron ions in the aqueous medium, the concentration and boron isotope ratio of boron ions in the conservative mixed solution, and the partition coefficient and fractionation coefficient of boron at the pH value, the process by which boron reaches hydrolysis equilibrium at the pH value and triggers boron isotope equilibrium fractionation in polluted water is determined. The boron concentration removed from groundwater based on selective adsorption and the boron isotope ratio adsorbed from groundwater to the aqueous medium during the equilibrium fractionation process are obtained. Furthermore, through a conservative binary mixing model of ion and isotope mass, the boron concentration and boron isotope ratio in polluted groundwater are obtained, and a response curve of the boron concentration and boron isotope ratio in polluted groundwater is constructed. The monitoring data of boron isotopes and boron concentration in the target groundwater are plotted onto the response curve to determine whether the groundwater is contaminated by backflow fluid and to determine the pollution intensity of the target groundwater.

2. The method for identifying groundwater contaminated by backflow fluid based on boron isotopes as described in claim 1, characterized in that, The boron concentration and boron isotope ratios in the backflow fluid and uncontaminated groundwater include: The boron concentration and boron isotope ratio in the uncontaminated groundwater aquifer and the flowback fluid were obtained by testing and analyzing them separately.

3. The method for identifying groundwater contaminated by backflow fluid based on boron isotopes as described in claim 1, characterized in that, The conservative mixture is obtained by conservatively mixing the backflow fluid and the uncontaminated groundwater without any other water-rock interactions.

4. The method for identifying groundwater contaminated by backflow fluid based on boron isotopes as described in claim 1, characterized in that, The pH value of the conservative mixture was obtained by simulating and analyzing the conservative mixture using PHREEQC software.

5. The method for identifying groundwater contaminated by backflow fluid based on boron isotopes as described in claim 1, characterized in that, The determination of the boron isotope equilibrium fractionation state between the aquifer medium and uncontaminated groundwater based on the boron distribution coefficient and boron isotope fractionation coefficient in the groundwater aquifer includes: By testing and analyzing uncontaminated aquifers, the boron concentration and boron isotope ratio in the uncontaminated groundwater and the weight percentage of clay particles in the aquifer medium were obtained. Based on the initial concentration of boron ions in the aquifer and groundwater and the weight ratio of clay particles in the aquifer, the distribution coefficient of boron ions in the uncontaminated aquifer is obtained. The allocation coefficient is calculated using the following formula: ； in, The partition coefficient of boron ions in an uncontaminated aquifer; and The initial concentrations of boron ions in the aqueous medium and groundwater are respectively. c This represents the weight percentage of clay particles in the aqueous medium. Based on the boron concentration and boron isotope ratio in uncontaminated groundwater, the fractionation coefficients of boron isotopes in the aqueous medium and groundwater are obtained; The fractionation coefficient is calculated using the following formula: ； in ，α I The fractionation coefficients of boron isotopes in the aqueous medium and groundwater under initial conditions; and These represent the boron isotope ratios in uncontaminated aquatic media and groundwater, respectively. Based on the distribution coefficient and the fractionation coefficient, the boron isotope equilibrium fractionation state between the aqueous medium and the uncontaminated groundwater is characterized.

6. The method for identifying groundwater contaminated by backflow fluid based on boron isotopes as described in claim 1, characterized in that, The calculation process for the boron concentration and boron isotope ratio in the conservative mixed solution under different pollution intensities is as follows: The boron concentration in the conservative mixed solution is obtained based on the boron concentration in the backflow solution, the volume percentage of the backflow solution in the contaminated groundwater, and the initial concentration of boron ions in the groundwater. The formula for calculating the boron concentration in the contaminated groundwater is as follows: ； in, The boron concentration in the conservative mixture; The boron concentration in the effluent; x Iw This represents the volume percentage of the backflow fluid in the contaminated groundwater. This represents the initial concentration of boron ions in the groundwater. The boron isotope ratio in the polluted groundwater was obtained based on the boron concentration in the polluted groundwater and the boron isotope ratio in the unpolluted groundwater. The formula for calculating the boron isotope ratio in the contaminated groundwater is as follows: ； in, The boron isotope ratio in the conservative mixture; The boron isotope ratio in uncontaminated groundwater; This represents the boron isotope ratio in the effluent.

7. The method for identifying groundwater contaminated by backflow fluid based on boron isotopes as described in claim 1, characterized in that, The process by which boron reaches hydrolysis equilibrium at the stated pH value and initiates boron isotope equilibrium fractionation in polluted water includes: Based on the boron partition coefficient in the polluted aquifer, the pH value of the conservative mixed solution, the boron concentration entering the aquifer through selective adsorption, and the fractionation coefficient between the aquifer and the polluted groundwater after equilibrium fractionation, the polluted water body that reaches hydrolysis equilibrium at the pH value and triggers boron isotope equilibrium fractionation is characterized. The equation is expressed as follows: ； ； ； in, The partition coefficient of boron in the contaminated aquifer; and The boron concentrations in the aqueous medium and polluted groundwater after selective adsorption of boron are shown, respectively. and These represent the boron concentrations that enter the aqueous medium through selective adsorption and the boron concentrations that are removed from groundwater, respectively. u The water content is the weight of the aqueous medium. To balance the fractionation coefficient between the aqueous medium and the contaminated groundwater after fractionation; and These represent the boron isotope ratios in the aqueous medium and polluted groundwater after equilibrium fractionation, respectively. To balance the boron isotope ratios adsorbed from the solid phase to the liquid phase during the fractionation process.

Images