Method for identifying underground water polluted by flowback fluid based on strontium isotope

Abstract

The invention discloses a method for identifying underground water polluted by flowback fluid based on strontium isotope, and relates to the technical field of environmental protection. The method comprises the following steps: obtaining activity coefficients of ions in the flowback fluid and underground water through PHREEQC; depicting the cation exchange equilibrium state of the water-containing medium and the shallow groundwater before the pollution, conservative mixed liquid with different pollution intensities after the flowback liquid and the groundwater are mixed after the pollution, and the state when the polluted groundwater and the water-containing medium pass through new cation exchange equilibrium under different pollution intensities; according to the isotope mass conservative binary mixed model, the strontium isotope ratio and the strontium concentration in the underground water in a new cation exchange equilibrium state after pollution are obtained, and a response curve of the strontium concentration and the strontium isotope ratio in the polluted underground water is constructed, so that early pollution is identified and determined. The strontium isotope and strontium concentration cross monitoring mode effectively reduces the interference of fluctuation of unpolluted strontium isotope caused by dynamic fluctuation of underground water on pollution identification of the fracturing flow-back fluid.

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 strontium 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 fluid pollution in shallow groundwater 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 indicators of flowback fluid contamination of groundwater: one is the characteristic components in the fracturing fluid that differ 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 indicators 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, and its hydrochemical type is primarily Cl-Na type water, with a Br / Cl ratio > 0.1 × 10⁻⁶. −2 The δ value in the backflow fluid is greater than the corresponding value in seawater. 11 B (25~31‰) and 87 Sr / 86 The distribution range of Sr (0.7101~0.7121) is also similar to that 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 10000 mg / L, and the Br / Cl ratio is greater than 0.2 × 10⁻⁶. −2 δ 11 B is distributed at 20-30‰. 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, and the Br / Cl and Na / Cl ratios are close to those of seawater. Therefore, the identification indicators and characteristic values ​​of fracturing flowback fluids from marine shale are similar to those of seawater and their distribution range. Both the Marcellus Shale in the United States and the Longmaxi Formation shale in Sichuan were deposited in marine environments, 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 value (δ¹⁴). 11 B). 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 (δ) in groundwater. 11 B) 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 from marine shale is polluting groundwater.

[0005] However, in my country's widely distributed marine-continental transitional shale gas reservoirs, existing methods for identifying groundwater contamination from flowback fluids are ineffective when applied to fracturing flowback fluids from these reservoirs. When flowback fluids enter aquifers, they alter the original hydrochemical balance of shallow groundwater, initiating 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. Furthermore, contamination identification methods based on monitoring thresholds for these indicators do not consider the complex groundwater chemistry involved in these contamination processes, thus affecting their effectiveness as identifiers of shallow groundwater contamination from flowback fluids. Moreover, these methods are only applicable to scenarios with severe groundwater contamination and cannot effectively identify contamination in the early stages of contamination, particularly with small-flux flowback fluids. Summary of the Invention

[0006] Therefore, it is necessary to provide a method for identifying groundwater contaminated by backflow fluid based on strontium isotopes to address the aforementioned technical problems.

[0007] The following technical solution is adopted in this specification: This specification provides a method for identifying groundwater contaminated by backflow fluid based on strontium isotopes, including: Characterizing groundwater contamination by backflow fluid, determining the sodium ion concentration and ion content in the aquifer and shallow groundwater under cation exchange equilibrium before the backflow fluid contaminates the groundwater. I i+ Milligram equivalents of solid-phase ion exchange; After the backflow fluid contaminates groundwater, strontium ions in the backflow fluid and shallow groundwater undergo conservative mixing and do not undergo new cation exchange with the aqueous medium. The strontium concentration and strontium isotope ratio in the conservative mixture with different pollution intensities are determined and denoted as the first ratio. The new cation exchange equilibrium between the backflow liquid-contaminated groundwater and the aquifer is characterized. Based on the solid-phase exchange milliequivalent and the first ratio, the strontium concentration and strontium isotope ratio of the groundwater entering the groundwater from the solid phase through the new cation exchange equilibrium between the contaminated groundwater and the aquifer under different pollution intensities are obtained and denoted as the second ratio. Based on the first ratio and the second ratio, the strontium isotope ratios and strontium concentrations in groundwater with different intensities under the new cation exchange equilibrium state after groundwater pollution were obtained through a conservative binary mixing model of isotope mass, so as to construct response curves of strontium concentration and strontium isotope ratios in polluted groundwater. The monitoring data of strontium isotopes and strontium 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.

[0008] Furthermore, the cations include: sodium, potassium, calcium, magnesium, strontium, chloride, sulfate, and bicarbonate.

[0009] Furthermore, the determination of the activity coefficients of various ions in the backflow fluid and groundwater before the backflow fluid contaminates the groundwater includes: By acquiring and analyzing backflow fluid and uncontaminated groundwater, the concentrations of various ionic components and the strontium isotope ratios in the backflow fluid and uncontaminated groundwater were obtained. Based on the concentration of each ion component and the strontium isotope ratio, the activity coefficients of each ion in the backflow solution and groundwater were obtained using PHREEQC software.

[0010] Furthermore, the sodium ions in the aqueous medium and shallow groundwater under cation exchange equilibrium state and I i+ The calculation process for milliequivalents of ions in solid-phase exchange includes: Based on sodium ions in uncontaminated groundwater and The activity of ions, sodium ions in uncontaminated aqueous media and The equilibrium coefficient of ions is used to obtain the sodium ion and... I The exchange coefficient of ions; The calculation formula is: ; ; in, Sodium ions and I The exchange coefficient of ions; and Sodium ions in uncontaminated groundwater and Ion activity; and Sodium ions in uncontaminated aqueous media and The equilibrium coefficient of ions; Uncontaminated water-containing media The equilibrium coefficient of ions is calculated using the following formula: ; in, and Sodium ions and sodium ions under equilibrium conditions, respectively. Milligram equivalents of solid-phase ion exchange; CEC This represents the cation exchange capacity of the aqueous medium. The cation exchange capacity of the aqueous medium is obtained based on an empirical formula; The calculation formula is: ; in, c The percentage of clay in the water-bearing medium; Based on the cation exchange capacity of the aqueous medium CEC,Uncontaminated groundwater The activity coefficients of ions and sodium ions and uncontaminated groundwater The molar concentrations of sodium ions and ions are used to convert ion activity into activity coefficients and ion concentrations, thus obtaining the sodium ion concentrations under equilibrium conditions. Milligram equivalents of solid-phase ion exchange; The calculation formula is: ; ; in, and In uncontaminated groundwater Activity coefficients of sodium ions and sodium ions; and In uncontaminated groundwater The molar concentrations of sodium ions and sodium ions; and Sodium ions under equilibrium conditions and Milligram equivalents of solid-phase ion exchange.

[0011] Furthermore, the calculation process for the strontium concentration and strontium isotope ratio in the conservative mixtures with different pollution intensities includes: By mixing backflow fluid and groundwater, conservative mixtures with different levels of contamination were constructed. Based on the strontium concentrations in the backflow fluid and background groundwater, the strontium concentration in the conservatively mixed contaminated groundwater was obtained using a conservative binary mixing model based on isotopic mass. The calculation formula is: ; in, To conserve the concentration of strontium in mixed contaminated groundwater; and The strontium concentrations are shown in the flowback fluid and background groundwater, respectively. Based on the strontium concentration in the backflow solution, the strontium concentration in the background groundwater, the strontium concentration in the conservatively mixed contaminated groundwater, and the strontium isotope ratio between the backflow solution and the uncontaminated groundwater, the strontium isotope ratio in the conservatively mixed contaminated groundwater was obtained. The calculation formula is: ; in, To preserve the strontium isotope ratios in mixed-contaminated groundwater; and The values ​​are the strontium isotope ratios of the backflow fluid and the uncontaminated groundwater, respectively. x This represents the volume percentage of the backflow fluid in contaminated groundwater.

[0012] Furthermore, the exchange coefficient between sodium ions and strontium ions at the cation exchange equilibrium is calculated using the following formula: ; ; in, This refers to the cation exchange capacity. and for and Exchange quantity; u This refers to the weight water content of the groundwater-bearing medium. CEC This represents the cation exchange capacity of the aqueous medium. The exchange coefficient between sodium ions and strontium ions; This represents the milliequivalent of solid-phase exchange of sodium ions under the original equilibrium conditions. This represents the milliequivalent of solid-phase exchange of strontium ions under the original equilibrium conditions. To conserve the concentration of strontium in mixed contaminated groundwater; To conserve sodium concentration in mixed contaminated groundwater; To conserve the activity coefficient of strontium ions in mixed polluted groundwater; To conserve the sodium ion activity coefficient in mixed-contaminated groundwater.

[0013] Furthermore, the calculation process for the strontium isotope ratios and strontium concentrations in groundwater of different intensities under the new cation exchange equilibrium state after groundwater pollution includes: Based on the conservative mixed contamination of groundwater and the emergence of new cation exchange mechanisms The exchange rate is used to determine the strontium concentration in the polluted groundwater. The strontium concentration in the contaminated groundwater is calculated using the following formula: ; in, The concentration of strontium in the polluted groundwater; To conserve the concentration of strontium in mixed contaminated groundwater; for Exchange quantity; Strontium isotope ratios in conserved mixed-contaminated groundwater, strontium isotope ratios in uncontaminated groundwater, and strontium concentrations in conserved mixed-contaminated groundwater are all influenced by new cation exchange-induced factors. The strontium isotope ratios of the polluted groundwater were obtained. The strontium isotope ratio of the contaminated groundwater is calculated using the following formula: ; in, Strontium isotope ratios for polluted groundwater; To preserve the strontium isotope ratios in mixed-contaminated groundwater; The strontium isotope ratio of uncontaminated groundwater.

[0014] The above-mentioned technical solutions adopted in this specification can achieve the following beneficial effects: In the method for identifying groundwater contaminated by backflow fluid based on strontium isotopes provided in this specification, the strontium isotope ratio in the aquifer-groundwater system is selected ( 87 Sr / 86 Sr Strontium isotopes are used as an indicator for identifying flowback fluid contamination. By cross-monitoring strontium isotopes and strontium concentration, the contamination status of groundwater can be identified and determined. Strontium isotopes do not undergo isotopic fractionation due to phase transfer and chemical transformation, thus improving their tracer performance. The cross-monitoring of strontium isotopes and strontium concentration can reduce the interference of fluctuations in uncontaminated strontium isotopes caused by dynamic fluctuations in groundwater on the identification of fracturing flowback fluid contamination.

[0015] Furthermore, by introducing PHREEQC software to accurately obtain ion activity, the error generated by empirical formula calculations is reduced. Attached Figure Description

[0016] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0017] Figure 1 A schematic flowchart illustrating a method for identifying groundwater contaminated by strontium isotope-based backflow fluid, as provided in this specification. Figure 2 The response curves of strontium concentration and strontium isotope ratio in groundwater to pollution under Scenario 1 provided in this specification; Figure 3 The response curves of strontium concentration and strontium isotope ratio in groundwater to pollution under Scenario 2 provided in this specification are shown. Detailed Implementation

[0018] 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.

[0019] Existing methods for identifying groundwater contamination from flowback fluid, when applied to shale gas fracturing flowback fluid from marine-continental transitional facies, suffer from the following problems in addition to their limitations in contamination identification: (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.

[0020] (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.

[0021] And groundwater 87 Sr and 86 Sr does not undergo isotopic fractionation due to phase transitions, chemical reactions, microbial activity, or evaporation and concentration; only the incorporation of exogenous strontium alters its isotope composition. 87 Sr / 86 Therefore, in simulating the scenario of flowback fluid contaminating shallow groundwater, in addition to the pollutant (flowback fluid) and the pollutant receptor (groundwater) end-members, it is also necessary to consider the impact of exogenous strontium entering the groundwater through the following two water-rock interactions on groundwater contamination. 87 Sr / 86 The effect of Sr:

[0022] (6) Dissolution: Silicates are the main minerals containing strontium, primarily found in Quaternary loose rocks, which are the main medium for shallow groundwater storage. The dissolution of silicate minerals typically occurs under acidic conditions (pH < 7), and the dissolution rate is negatively correlated with pH. Previous indoor dissolution tests have shown that... The half-lives of quartz, biotite, and potassium feldspar crystals with a radius of 1 mm in water at pH 5 are 3.4 × 10⁷, 2.7 × 10⁶, and 5.2 × 10⁵ years, respectively. Therefore, the dissolved strontium source can be ignored in the actual scenario of groundwater contamination by backflow fluid.

[0023] (7) Cation exchange: After the backflow liquid pollutes the groundwater, it will disrupt the original water chemical balance, causing the cations adsorbed on the surface of clay minerals to become more concentrated. Sr 2+ It enters groundwater through cation exchange, thereby affecting and polluting groundwater.87 Sr / 86 Sr. clay minerals play a dominant role in cation exchange and pollute groundwater with Na. + The content of Na+ is 1-2 orders of magnitude higher than that of other cations, therefore, liquid-phase Na+ mainly occurs in the aquifer. + It exchanges with other cations in the solid phase.

[0024] Based on this, the present invention establishes a quantitative model of strontium isotopes for groundwater contaminated by backflow fluid, taking into account cation exchange.

[0025] The technical solutions provided by the various embodiments of this application are described in detail below with reference to the accompanying drawings.

[0026] like Figure 1 A flowchart illustrating a method for identifying groundwater contaminated by strontium isotope-based flowback fluid is provided, which includes the following steps: S101: Describes the cation exchange equilibrium state between the aquatic medium and shallow groundwater before pollution occurs.

[0027] The equilibrium state can be represented as: (1) (2) (3) in, Sodium ions and I Ion exchange coefficients (refer to Table 1); and Sodium ions in uncontaminated groundwater and Ion activity; and Sodium ions in uncontaminated aqueous media and The equilibrium coefficient of ions; and For sodium ions and under equilibrium conditions Milligram equivalents (meq / kg) of solid-phase exchange of ions. CEC The cation exchange capacity (meq / kg) is the capacity of the aqueous medium.

[0028] CEC It can be obtained through the following empirical formula: (4) in, c The percentage of clay in the water-bearing medium; Substitute equation (3) into equation (2) and convert the ion activity into the activity coefficient ( γ Expressed in terms of ion concentration, we can obtain: (5) in, and In uncontaminated groundwater Activity coefficients of sodium ions and sodium ions; and In uncontaminated groundwater The molar concentrations of ions and sodium ions.

[0029] Then, the following equations can be used to calculate... and : (6) The test and analysis results of ionic components in the backflow fluid and uncontaminated groundwater were imported into the SOLUTION module of PHREEQC (Version 3, USGS, Reston, USA) software to calculate the activity coefficient of each ionic component.

[0030] Based on the known parameters, the test and analysis results of the backflow fluid and uncontaminated groundwater, and the calculated activity coefficient solution formula (6), the following is obtained: and .

[0031] S102: Characterizes the conservative mixing of strontium ions in the backflow liquid and shallow groundwater after the pollution occurred, before new cation exchange with the aqueous medium occurs, and calculates the strontium isotope ratio using the isotope mass conservative mixing model. The calculation formula is: (7) (8) in, To preserve the strontium isotope ratio in mixed-contaminated groundwater and The values ​​represent the strontium isotope ratios of the backflow fluid and the uncontaminated groundwater, respectively. To conserve the concentration of strontium in mixed contaminated groundwater; and The strontium concentrations are shown in the flowback fluid and background groundwater, respectively. x This represents the volume percentage of the backflow fluid in contaminated groundwater.

[0032] Based on the test and analysis results of the backflow fluid and uncontaminated groundwater (see Table 2), the strontium concentration and strontium isotope ratio in the conservative mixed liquid under different pollution intensities were obtained.

[0033] S103: A new cation exchange equilibrium is formed between contaminated groundwater and the aquifer, and the groundwater level is calculated after the new equilibrium is reached. This can be expressed as: (9) (10) in, Cation exchange capacity (meq / L); and for and Exchange volume (mM); u This refers to the weight water content of the groundwater-bearing medium. To conserve the activity coefficient of strontium ions in mixed polluted groundwater; To conserve the sodium ion activity coefficient in mixed-contaminated groundwater.

[0034] Based on the SOLUTION module superimposed with the MIXING module in step one, the concentration and activity coefficient of ionic components in the conservative mixed solution under different pollution intensities are calculated.

[0035] Based on the obtained data (see Table 3), substituting Equation 10 into Equation 8 yields the solution. .

[0036] S104: Based on the following isotopic conservative binary mixing model, calculate the strontium isotope ratios in the polluted groundwater (see Table 4).

[0037] The calculation formula is: (11) (12) in, Strontium isotope ratios for polluted groundwater; The strontium isotope ratio of the conservative mixture of backflow fluid and groundwater; Strontium isotope ratios for uncontaminated groundwater; The concentration of strontium in the polluted groundwater.

[0038] To verify the effectiveness and sensitivity of the present invention, examples were used for verification.

[0039] This example selects the Taiyuan Formation of the Permian system in the North China Plain, a typical marine-continental transitional shale gas reservoir in my country, as the research object. The fracturing flowback fluid from the Taiyuan Formation shale gas reservoir is used as the groundwater pollution source, and typical shallow groundwater in the North China Plain is used as the groundwater pollution receptor. Based on the hydrochemical and isotopic analysis results of the flowback fluid and shallow groundwater, the response of strontium concentration and strontium isotope ratio in groundwater under different pollution intensities (represented by the volume ratio of flowback fluid to polluted groundwater) is 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 are conducted for aquifers with different characteristic values. For example… Figure 2Scenario 1 illustrates the response curves of strontium concentration and strontium isotope ratios in polluted groundwater under a constant aquifer weight moisture content of 20% and clay mineral content of 1%, 2%, 3%, 4%, and 5%, respectively, as pollution intensity changes. Figure 3 Scenario 2 shows the response curves of strontium concentration and strontium isotope ratio in polluted groundwater under the condition that the clay mineral content of the aquifer is 3% and the weight water content is 15%, 20%, and 25%, respectively, as the pollution intensity changes.

[0040] The results of the examples show that, under the conditions of an aquifer weight water content of 20% and a clay mineral content of 1%, 2%, 3%, 4%, and 5%, when the backflow fluid reaches 1% of the total volume of contaminated groundwater, the strontium concentration in the groundwater changes significantly, and the strontium isotope ratio shows a detectable change (the uncertainty of the strontium concentration test is 0.001 mg / L, and the uncertainty of the strontium isotope ratio test is 0.000015); when the backflow fluid reaches 5% of the total volume of contaminated groundwater, both the strontium concentration and the strontium isotope ratio in the groundwater change significantly.

[0041] Table 1 Sodium ions and I The ion exchange coefficient, the molar concentration and activity coefficient of cations in the groundwater before pollution, and the calculated milliequivalent of cation solid-phase exchange. Table 2. Calculation results of strontium isotope ratios under different pollution intensities. Table 3. Ion calculation data under different aquifer parameters and pollution intensities Table 4. Ion calculation data under different aquifer parameters and pollution intensities 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 strontium isotope-based backflow fluid, characterized in that, include: Characterizing groundwater contamination by backflow fluid, determining the sodium ion concentration and ion content in the aquifer and shallow groundwater under cation exchange equilibrium before the backflow fluid contaminates the groundwater. I i+ Milligram equivalents of solid-phase ion exchange; After the backflow fluid contaminates groundwater, strontium ions in the backflow fluid and shallow groundwater undergo conservative mixing and do not undergo new cation exchange with the aqueous medium. The strontium concentration and strontium isotope ratio in the conservative mixture with different pollution intensities are determined and denoted as the first ratio. The new cation exchange equilibrium between the backflow liquid-contaminated groundwater and the aquifer is characterized. Based on the solid-phase exchange milliequivalent and the first ratio, the strontium concentration and strontium isotope ratio of the groundwater entering the groundwater from the solid phase through the new cation exchange equilibrium between the contaminated groundwater and the aquifer under different pollution intensities are obtained and denoted as the second ratio. Based on the first ratio and the second ratio, the strontium isotope ratios and strontium concentrations in groundwater with different intensities under the new cation exchange equilibrium state after groundwater pollution were obtained through a conservative binary mixing model of isotope mass, so as to construct response curves of strontium concentration and strontium isotope ratios in polluted groundwater. The monitoring data of strontium isotopes and strontium 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 strontium isotopes as described in claim 1, characterized in that, The cations include: sodium, potassium, calcium, magnesium, strontium, chloride, sulfate, and bicarbonate.

3. The method for identifying groundwater contaminated by backflow fluid based on strontium isotopes as described in claim 1, characterized in that, The determination of the activity coefficients of various ions in the backflow fluid and groundwater before the occurrence of groundwater contamination by the backflow fluid includes: By acquiring and analyzing backflow fluid and uncontaminated groundwater, the concentrations of various ionic components and the strontium isotope ratios in the backflow fluid and uncontaminated groundwater were obtained. Based on the concentration of each ion component and the strontium isotope ratio, the activity coefficients of each ion in the backflow solution and groundwater were obtained using PHREEQC software.

4. The method for identifying groundwater contaminated by backflow fluid based on strontium isotopes as described in claim 1, characterized in that, Sodium ions and shallow groundwater in the aqueous medium under cation exchange equilibrium state I i+ The calculation process for milliequivalents of ions in solid-phase exchange includes: Based on sodium ions in uncontaminated groundwater and The activity of ions, sodium ions in uncontaminated aqueous media and The equilibrium coefficient of ions is used to obtain the sodium ion and... I The exchange coefficient of ions; The calculation formula is: ； ； in, Sodium ions and I The exchange coefficient of ions; and Sodium ions in uncontaminated groundwater and Ion activity; and Sodium ions in uncontaminated aqueous media and The equilibrium coefficient of ions; Uncontaminated water-containing media The equilibrium coefficient of ions is calculated using the following formula: ； in, and Sodium ions and sodium ions under equilibrium conditions, respectively. Milligram equivalents of solid-phase ion exchange; CEC This represents the cation exchange capacity of the aqueous medium. The cation exchange capacity of the aqueous medium is obtained based on an empirical formula; The calculation formula is: ； in, c The percentage of clay in the water-bearing medium; Based on the cation exchange capacity of the aqueous medium CEC, Uncontaminated groundwater The activity coefficients of ions and sodium ions and uncontaminated groundwater The molar concentrations of sodium ions and ions are used to convert ion activity into activity coefficients and ion concentrations, thus obtaining the sodium ion concentrations under equilibrium conditions. Milligram equivalents of solid-phase ion exchange; The calculation formula is: ； ； in, and In uncontaminated groundwater Activity coefficients of ions and sodium ions; and In uncontaminated groundwater The molar concentrations of sodium ions and sodium ions; and Sodium ions under equilibrium conditions and Solid-phase exchange of ions in milliequivalents.

5. The method for identifying groundwater contaminated by backflow fluid based on strontium isotopes as described in claim 1, characterized in that, The calculation process for the strontium concentration and strontium isotope ratio in the conservative mixtures with different pollution intensities includes: By mixing backflow fluid and groundwater, conservative mixtures with different levels of contamination were constructed. Based on the strontium concentrations in the backflow fluid and background groundwater, the strontium concentration in the conservatively mixed contaminated groundwater was obtained using a conservative binary mixing model based on isotopic mass. The calculation formula is: ； in, To conserve the concentration of strontium in mixed contaminated groundwater; and The strontium concentrations are shown in the flowback fluid and background groundwater, respectively. Based on the strontium concentration in the backflow solution, the strontium concentration in the background groundwater, the strontium concentration in the conservatively mixed contaminated groundwater, and the strontium isotope ratio between the backflow solution and the uncontaminated groundwater, the strontium isotope ratio in the conservatively mixed contaminated groundwater was obtained. The calculation formula is: ； in, To preserve the strontium isotope ratios in mixed-contaminated groundwater; and The values ​​are the strontium isotope ratios of the backflow fluid and the uncontaminated groundwater, respectively. x This refers to the volume percentage of backflow fluid in contaminated groundwater.

6. The method for identifying groundwater contaminated by backflow fluid based on strontium isotopes as described in claim 1, characterized in that, The exchange coefficient between sodium ions and strontium ions at the cation exchange equilibrium is calculated using the following formula: ； ； in, This refers to the cation exchange capacity. and for and Exchange quantity; u This refers to the weight water content of the groundwater-bearing medium. CEC This represents the cation exchange capacity of the aqueous medium. The exchange coefficient between sodium ions and strontium ions; This represents the milliequivalent of solid-phase exchange of sodium ions under the original equilibrium conditions. This represents the milliequivalent of solid-phase exchange of strontium ions under the original equilibrium conditions. To conserve the concentration of strontium in mixed contaminated groundwater; To conserve sodium concentration in mixed contaminated groundwater; To conserve the activity coefficient of strontium ions in mixed polluted groundwater; To conserve the sodium ion activity coefficient in mixed-contaminated groundwater.

7. The method for identifying groundwater contaminated by backflow fluid based on strontium isotopes as described in claim 1, characterized in that, The calculation process for strontium isotope ratios and strontium concentrations in groundwater of different intensities under the new cation exchange equilibrium state after groundwater pollution includes: Based on the conservative mixed contamination of groundwater and the emergence of new cation exchange mechanisms The exchange rate is used to determine the strontium concentration in the polluted groundwater. The strontium concentration in the contaminated groundwater is calculated using the following formula: ； in, The concentration of strontium in the polluted groundwater; To conserve the concentration of strontium in mixed contaminated groundwater; for Exchange quantity; Strontium isotope ratios in conserved mixed-contaminated groundwater, strontium isotope ratios in uncontaminated groundwater, and strontium concentrations in conserved mixed-contaminated groundwater are all influenced by new cation exchange-induced factors. The strontium isotope ratios of the polluted groundwater were obtained. The strontium isotope ratio of the contaminated groundwater is calculated using the following formula: ； in, Strontium isotope ratios for polluted groundwater; To preserve the strontium isotope ratios in mixed-contaminated groundwater; The strontium isotope ratio of uncontaminated groundwater.

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