A method for inversion of NAPLs pollutant concentration based on resistivity method

Abstract

The present invention discloses a NAPLs pollutant concentration inversion method based on the resistivity method, including: selecting a NAPLs polluted survey area and an unpolluted survey area of ​​equal area in the survey site, obtaining corresponding resistivity profiles in the two survey areas, and measuring relevant parameters at different depths in the two survey areas; based on the Archie extended model, introducing the pollutant influence coefficient b and correcting the soil porosity to obtain the NAPLs polluted soil resistivity model; using the relevant parameters in the two survey areas to correct the NAPLs polluted soil resistivity model; using the corrected NAPLs polluted soil resistivity model to invert and analyze the resistivity profile diagram. The present invention combines the indoor test results with the drilling sampling results of the survey site, based on the Archie extended model, considering that NAPLs pollutants and air are both high-resistance materials, and correcting the soil porosity to water porosity, which is closer to the actual current conduction path.

Description

Technical Field

[0001] The invention relates to a resistivity method exploration technology for contaminated sites, and in particular to a NAPLs pollutant concentration inversion method based on the resistivity method. Background Art

[0002] Non-aqueous Phase Liquids (NAPLs) are common pollutants in oil-contaminated sites. They migrate in the strata as the groundwater level changes, posing a great threat to the groundwater environment and human health. At present, the monitoring technology for the migration of NAPLs pollutants in the underground environment includes geophysical exploration methods such as resistivity, ground penetrating radar, transient electromagnetic method and seismic wave method, and combines sampling and analysis technology to comprehensively analyze the distribution of pollutants. Among them, the resistivity method is considered to be one of the most effective methods for detecting and monitoring contaminated sites. This method mainly establishes a relationship model between soil resistivity and porosity, pore fluid resistivity, pore fluid saturation, temperature and other influencing factors to quantitatively analyze the degree of contamination of the site. Therefore, whether the resistivity model can correctly describe the relationship between resistivity and pollutant concentration will directly affect the accuracy of the evaluation results.

[0003] In 1942, Archie first proposed a resistivity model for saturated cohesionless soil, which was later extended to unsaturated soil. This model has been continuously improved and revised as the basis for soil resistivity research. Liu Guohua et al. applied the Archie model to clay and obtained a regional clay resistivity model by modifying the parameters; Ma Yuanyuan et al. established a resistivity model for leachate-contaminated sand by modifying the pore fluid resistivity and introducing a conductivity reduction factor based on the Archie model; Sun Yakun et al. modified the pore fluid resistivity and replaced the saturation with volumetric water content to obtain a resistivity model for chromium-contaminated sand.

[0004] At present, the improvement methods of the resistivity model of contaminated soil are mostly aimed at sandy soil and water-soluble pollutants, and there is a lack of research on the pollution of clay soil and non-aqueous liquid. In addition, the resistivity model is mostly obtained based on the summary of indoor test results and lacks adaptability to the geological conditions of the survey site, resulting in the inability to accurately assess the degree of pollution at NAPLs contaminated sites. Summary of the invention

[0005] Purpose of the invention: In view of the above problems, the purpose of the present invention is to provide a NAPLs pollutant concentration inversion method based on the resistivity method, analyze the resistivity characteristics of NAPLs contaminated soil through indoor resistivity tests, and establish a corresponding resistivity model in combination with the formation conditions at the survey site, so as to more accurately invert the NAPLs pollutant concentration based on the resistivity profile.

[0006] Technical solution: A NAPLs pollutant concentration inversion method based on resistivity method of the present invention comprises the following steps:

[0007] Step 1: select a NAPLs contaminated survey area and an uncontaminated survey area of ​​equal area at the survey site, obtain corresponding resistivity profiles in the two survey areas, and measure relevant parameters at different depths in the two survey areas; the relevant parameters include resistivity, porosity, water content, pollutant content and pore fluid resistivity;

[0008] Step 2: Take an appropriate amount of soil from the survey site as indoor test soil, measure the resistivity of the sample, and based on the Archie extended model, introduce the pollutant influence coefficient b and correct the soil porosity to obtain the NAPLs contaminated soil resistivity model;

[0009] Step 3, using the relevant parameters in the two measurement areas in step 1 to correct the NAPLs contaminated soil resistivity model in step 2;

[0010] Step 4: Use the modified NAPLs contaminated soil resistivity model to invert and analyze the resistivity profiles of the two measurement areas in step 1 to quantitatively explain the distribution of pollutant concentrations.

[0011] Furthermore, the resistivity model expression of NAPLs contaminated soil in step 2 is:

[0012] ρ=aρ w n w m R w -p (1+bR O )

[0013] Where a is the pore distribution inhomogeneity coefficient, ρ w is the pore fluid resistivity; n w is the soil water porosity; m is the cementation index; R w is the volumetric water content; p is the saturation index; R o is the volume contamination rate, which is the ratio of the volume of NAPLs pollutants in the soil to the volume of soil particles.

[0014] Furthermore, the four parameters of the NAPLs contaminated soil resistivity model in step 2, namely, the cementation index m, the saturation index p, the pollutant influence coefficient b, and the pore distribution heterogeneity coefficient a, were measured and calculated respectively:

[0015] Calculate the cementation index m: measure the water-saturated soil resistivity of the indoor test soil at different porosities, and plot the measurement results into a relationship curve between the formation factor and the porosity in double logarithmic coordinates. The slope of the relationship curve is the cementation index.

[0016] Calculate the saturation index p: The saturation index p is a function of the porosity n and the volume moisture content β, and the expression is:

[0017] p=f(n)·f(β)

[0018] In the formula, the volume moisture content β is the ratio of the sum of the volume of water and NAPLs pollutants in the pores to the volume of soil particles;

[0019] Calculate the pollutant impact coefficient b: Use the control variable method, when the volume contamination rate R o When the resistivity is constant, the volume water content R w It decreases in the form of a power function; when the volume water content R w When the resistivity is constant, the volume contamination rate R o It increases linearly, and the expression of the resistivity model is:

[0020] ρ=cR w -p (1+bR O )

[0021] In the formula, b and c are fitting parameters, and the value of parameter b is obtained by fitting;

[0022] Calculation of pore distribution inhomogeneity coefficient a: The expression of pore distribution inhomogeneity coefficient a is calculated by the resistivity model expression of NAPLs contaminated soil:

[0023]

[0024] Further, the step 3 comprises:

[0025] Step 31, when the pollutant content of the indoor test soil is measured to be zero, the resistivity theoretical value under the same conditions is corrected for the first time according to the resistivity of the uncontaminated test area at different depths in step 1, and the ratio of the measured resistivity value of the uncontaminated sample to the theoretical value is defined as the correction coefficient, and the average of the correction coefficients corresponding to different depths is taken as the a1 value; the same conditions refer to the same porosity, water content, and pore fluid resistivity conditions of the indoor test soil and the uncontaminated test area;

[0026] Step 32, when the pollutant content of the indoor test soil is not zero, the resistivity theoretical value under the same conditions is corrected for the second time based on step 31 according to the resistivity at different depths of the NAPLs contaminated measurement area, and the ratio of the measured resistivity value to the theoretical value of the contaminated sample is defined as the correction coefficient, and the average of the correction coefficients corresponding to different depths is taken as the a2 value to obtain the final NAPLs contaminated soil resistivity model; the same conditions refer to the indoor test soil and the NAPLs contaminated measurement area having the same porosity, water content, pollutant content, and pore fluid resistivity conditions;

[0027] The final NAPLs contaminated soil resistivity model expression is:

[0028] ρ=aa1a2ρ w n w m R w -p (1+bR O ).

[0029] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0030] 1. The present invention uses the Archie extended model as the basis for the indoor resistivity model research process, which better reflects the variation of soil resistivity with porosity and water content. On this basis, NAPLs pollutants and air are both high-resistance substances, and the porosity of the soil is corrected to water porosity, which weakens the contribution of NAPLs pollutants to current conduction and is closer to the actual current conduction path.

[0031] 2. Sampling was conducted simultaneously in both the uncontaminated and contaminated areas of the NAPLs contaminated site, taking into account the effects of formation factors such as formation cracks, plant roots, and gravel on soil resistivity, as well as the effects of the microscopic distribution of NAPLs pollutants on soil resistivity;

[0032] 3. Since soil resistivity is affected by many factors, the resistivity model established through indoor tests is difficult to be universal. The present invention combines the indoor test results with the drilling sampling results of the survey site, which can provide some new ideas for the establishment of the resistivity model and has important guiding significance for the quantitative evaluation of NAPLs contaminated sites. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a flow chart of the present invention;

[0034] Figure 2 is the relationship curve between formation factor and porosity;

[0035] Figure 3 is the test result of parameter f(β);

[0036] Figure 4 is the test result of parameter f(n);

[0037] Figure 5 is the relationship curve between soil resistivity and volume water content;

[0038] Figure 6 It is the relationship curve between soil resistivity and volume contamination rate;

[0039] Figure 7is the calculation result of the pore micro-distribution heterogeneity coefficient a. DETAILED DESCRIPTION

[0040] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0041] In this embodiment, the survey site is a sample site in Fengxian District, Shanghai. The soil type is low-permeability clay, and soil is taken from the site as indoor test soil. The NAPLs pollutant concentration inversion method based on resistivity method described in this embodiment includes the following steps, as shown in the flow chart: Figure 1 As shown:

[0042] Step 1: Select a NAPLs contaminated survey area and an uncontaminated survey area of ​​equal area at the survey site, obtain corresponding resistivity profiles in the two survey areas, and measure relevant parameters at different depths in the two survey areas; the relevant parameters include resistivity, porosity, water content, pollutant content and pore fluid resistivity.

[0043] Select NAPLs contaminated sites, and select uncontaminated survey areas and NAPLs contaminated survey areas in the survey site based on the historical data of the site, design the length and direction of the survey line, and use the high-density resistivity method to lay electrodes along the survey line to obtain the corresponding resistivity profile. At the same time, samples were drilled at three different depths in each of the two survey areas, and the resistivity, porosity, water content, pollutant content and pore fluid resistivity parameters of the samples at different depths were measured.

[0044] Step 2: Take an appropriate amount of soil from the survey site as indoor test soil, measure the resistivity of the sample, and based on the Archie extended model, introduce the pollutant influence coefficient b and correct the soil porosity to obtain the NAPLs contaminated soil resistivity model;

[0045] In this embodiment, the indoor test uses the dipole method to measure the resistivity of the sample, and the measuring device is a Miller Soil Box, whose internal dimensions are 12cm×3cm×2.5cm. Methyl tert-butyl ether (MTBE) is used as the NAPLs pollutant, and distilled water is used as the pore water. The test sample preparation steps are: air-dry the test soil and grind it through a 2mm sieve, add water and pollutants according to the design value for curing, and put it into the Miller soil box for layered compaction after curing. After preparation, measure the resistance, and conduct three groups of parallel tests, and take the average value as the resistivity measurement result. Finally, the NAPLs contaminated soil resistivity model is established based on the indoor resistivity measurement results and the on-site sampling and analysis results.

[0046] Based on the Archie extended model, the expression is:

[0047] ρ=aρ w n-m S r -p

[0048] In the formula, ρ is the soil resistivity; a is the test parameter; ρ w is the resistivity of soil pore fluid; n is the porosity; m is the cementation coefficient; S r is the saturation; P is the saturation index.

[0049] By introducing the pollutant influence coefficient b and correcting the soil porosity, the resistivity model expression of NAPLs contaminated soil obtained using indoor test soil is:

[0050] ρ=aρ w n w m R w -p (1+bR O )

[0051] Where a is the pore distribution inhomogeneity coefficient, ρ w is the pore fluid resistivity; n w is the water porosity of the soil. Considering that NAPLs pollutants are high-resistance substances like air, filling in the pores is equivalent to reducing the pores of the soil. Therefore, the soil porosity n is corrected to the water porosity n w ; m is the cementation index; R w is the volumetric water content, which is the ratio of the volume of water in the soil to the volume of soil particles; p is the saturation index; R o is the volume contamination rate, which is the ratio of the volume of NAPLs pollutants in the soil to the volume of soil particles; b is defined as the pollutant influence coefficient, which reflects the influence of the presence of pollutants on the resistivity of the soil.

[0052] The four parameters of the NAPLs contaminated soil resistivity model in step 2, namely, the cementation index m, the saturation index p, the pollutant influence coefficient b, and the pore distribution heterogeneity coefficient a, were measured and calculated respectively:

[0053] (1) Calculation of cementation index m: The cementation index m is obtained by measuring the resistivity of water-saturated soil with different porosities. The measurement results are plotted as a relationship curve between formation factor and porosity in double logarithmic coordinates. The slope of the relationship curve is the cementation index. The measurement results are shown in Figure 2 shown.

[0054] (2) Calculation of saturation index p: According to Archie's formula, the saturation index p is related to the soil porosity n and water saturation S w In this embodiment, the saturation index p is a function of the porosity n and the volume moisture content β, and the expression is:

[0055] p=f(n)·f(β)

[0056] In the formula, the volume moisture content β is the ratio of the sum of the volume of water and NAPLs pollutants in the pores to the volume of soil particles. When the soil porosity n is constant, the relationship curve between the saturation index p and the volume moisture content β is as follows: Figure 3 As shown, the results show that f(β) satisfies the lower open parabola form. The relationship curve with n is as follows Figure 4 As shown, the results show that f(n) satisfies the linear form.

[0057] (3) Calculate the pollutant impact coefficient b: Figure 5-6 As shown in the figure, using the control variable method, when the volume contamination rate R o When the resistivity is constant, the volume water content R w It decreases in the form of a power function; when the volume water content R w When the resistivity is constant, the volume contamination rate R o It increases linearly, and the expression of the resistivity model is:

[0058] ρ=cR w -p (1+bR O )

[0059] In the formula, b and c are fitting parameters, and the value of parameter b is obtained by fitting. Figure 5 and Figure 6 The curve in is expressed in the form of the above resistivity model. The parameter b is taken as the average value to obtain the value of the pollutant influence coefficient, which reflects the influence of the presence of NAPLs pollutants on the soil resistivity.

[0060] (4) Calculation of pore distribution inhomogeneity coefficient a: The expression of pore distribution inhomogeneity coefficient a is calculated by the resistivity model expression of NAPLs contaminated soil:

[0061]

[0062] The calculation result of a is as follows Figure 7 As shown in the figure, the results show that the change of a with the increase of volumetric water content can be divided into three stages: steep drop stage (OA), flattening stage (AB), and slow rise stage (BC).

[0063] Step 3, using the relevant parameters in the two measurement areas in step 1 to correct the NAPLs contaminated soil resistivity model in step 2, specifically includes the following steps:

[0064] Step 31, when the pollutant content of the indoor test soil is zero, the resistivity theoretical value under the same conditions is corrected for the first time according to the resistivity of the uncontaminated test area at three samples in step 1, and the ratio of the measured resistivity value of the uncontaminated sample to the theoretical value is defined as the correction coefficient. The correction coefficient corresponding to different depths is averaged as the a1 value. The correction coefficient reflects the influence of factors such as cracks, plant roots, soil heterogeneity, and microscopic distribution state differences of pore water in the surveyed stratum on the resistivity measurement results. The same conditions refer to the same porosity, water content, and pore fluid resistivity conditions of the indoor test soil and the uncontaminated test area. The theoretical value is calculated by the established resistivity model.

[0065] Step 32, when the pollutant content of the indoor test soil is not zero, the resistivity theoretical value under the same conditions is corrected for the second time based on step 31 according to the resistivity of the NAPLs contaminated measurement area at the three samples, and the ratio of the measured resistivity value of the contaminated sample to the theoretical value is defined as the correction coefficient, and the correction coefficient corresponding to different depths is averaged as the a2 value. The correction coefficient reflects the influence of the difference in the microscopic distribution state of NAPLs pollutants on the resistivity measurement results, and the final NAPLs contaminated soil resistivity model is obtained. The same conditions refer to the indoor test soil and the NAPLs contaminated measurement area having the same porosity, water content, pollutant content, and pore fluid resistivity conditions.

[0066] The final NAPLs contaminated soil resistivity model expression is:

[0067] ρ=aa1a2ρ w n w m R w -p (1+bR O ).

[0068] Step 4: Use the modified NAPLs contaminated soil resistivity model to perform inversion analysis on the resistivity profiles of the two measurement areas in step 1, quantitatively explain the distribution of pollutant concentrations, and provide a basis for targeted remediation and management of contaminated sites.

Claims

1. A NAPLs pollutant concentration inversion method based on resistivity method, characterized in that: The steps include: Step 1: select a NAPLs contaminated survey area and an uncontaminated survey area of ​​equal area at the survey site, obtain corresponding resistivity profiles in the two survey areas, and measure relevant parameters at different depths in the two survey areas; the relevant parameters include resistivity, porosity, water content, pollutant content and pore fluid resistivity; Step 2: Take an appropriate amount of soil from the survey site as indoor test soil, measure the resistivity of the sample, and based on the Archie extended model, introduce the pollutant influence coefficient b and correct the soil porosity to obtain the NAPLs contaminated soil resistivity model; Step 3, using the relevant parameters in the two measurement areas in step 1 to correct the NAPLs contaminated soil resistivity model in step 2; Step 4, using the modified NAPLs contaminated soil resistivity model to perform inversion analysis on the resistivity profiles of the two measurement areas in step 1, to quantitatively explain the distribution of pollutant concentrations; The resistivity model expression of NAPLs contaminated soil in step 2 is: p = ap w n w m R w -p (1+bR O ) Where a is the coefficient of pore distribution heterogeneity, ρ w is the pore fluid resistivity; n w is the soil water porosity; m is the cementation index; R w is the volumetric water content; p is the saturation index; R o is the volume contamination rate, which is the ratio of the volume of NAPLs pollutants in the soil to the volume of soil particles; The four parameters of the NAPLs contaminated soil resistivity model in step 2, namely, the cementation index m, the saturation index p, the pollutant influence coefficient b, and the pore distribution heterogeneity coefficient a, were measured and calculated respectively: Calculate the cementation index m: measure the water-saturated soil resistivity of the indoor test soil at different porosities, and plot the measurement results into a relationship curve between the formation factor and the porosity in double logarithmic coordinates. The slope of the relationship curve is the cementation index. Calculate the saturation index p: The saturation index p is a function of the porosity n and the volume moisture content β, and the expression is: p=f(n)·f(β) In the formula, the volume moisture content β is the ratio of the sum of the volume of water and NAPLs pollutants in the pores to the volume of soil particles; Calculate the pollutant impact coefficient b: Use the control variable method, when the volume contamination rate R o When the resistivity is constant, the volume water content R w It decreases in the form of a power function; when the volume water content R w When the resistivity is constant, the volume contamination rate R o It increases linearly, and the expression of the resistivity model is: ρ=cR w -p (1+bR O ) In the formula, b and c are fitting parameters, and the value of parameter b is obtained by fitting; Calculation of pore distribution inhomogeneity coefficient a: The expression of pore distribution inhomogeneity coefficient a is calculated by the resistivity model expression of NAPLs contaminated soil: The step 3 comprises: Step 31, when the pollutant content of the indoor test soil is measured to be zero, the resistivity theoretical value under the same conditions is corrected for the first time according to the resistivity of the uncontaminated test area at different depths in step 1, and the ratio of the measured resistivity value of the uncontaminated sample to the theoretical value is defined as the correction coefficient, and the average of the correction coefficients corresponding to different depths is taken as the a1 value; the same conditions refer to the same porosity, water content, and pore fluid resistivity conditions of the indoor test soil and the uncontaminated test area; Step 32, when the pollutant content of the indoor test soil is not zero, the resistivity theoretical value under the same conditions is corrected for the second time based on step 31 according to the resistivity at different depths of the NAPLs contaminated measurement area, and the ratio of the measured resistivity value to the theoretical value of the contaminated sample is defined as the correction coefficient, and the average of the correction coefficients corresponding to different depths is taken as the a2 value to obtain the final NAPLs contaminated soil resistivity model; the same conditions refer to the indoor test soil and the NAPLs contaminated measurement area having the same porosity, water content, pollutant content, and pore fluid resistivity conditions; The final NAPLs contaminated soil resistivity model expression is: p=aa1a2p w n w m R w -p (1+bR O )。

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