A rapid screening technology method for volatile organic compounds in groundwater

Abstract

The present invention belongs to the technical field of environmental pollution prevention and control, and discloses a rapid screening technology method for volatile organic compounds in groundwater. The monitoring method of the present invention uses the volatility and distribution coefficient of VOCs to establish the correlation between the concentration of VOCs in groundwater and soil gas, and inverts the pollution status of groundwater VOCs through on-site rapid monitoring of soil gas VOCs with simpler operation, lower cost and higher monitoring efficiency; it can reduce the cost of site investigation and shorten the investigation cycle.

Description

Technical Field

[0001] The invention relates to the technical field of environmental pollution prevention and control, and in particular to a technical method for rapid screening of volatile organic compounds in groundwater. Background Art

[0002] With the acceleration of urbanization and the adjustment of industrial structure, the contaminated sites left behind by the relocation of urban industrial enterprises have become a new environmental problem that restricts the reuse of land resources, and the problem of volatile organic compounds (VOCs) pollution in groundwater is becoming increasingly serious. VOCs generally refer to organic substances with a boiling point range of 50℃~260℃, a saturated vapor pressure of more than 133.3Pa at room temperature, and exist in the form of vapor at room temperature. They mainly include: low molecular weight aromatic hydrocarbons, aliphatic hydrocarbons, halogenated hydrocarbons, ketones, acetic acids, nitriles, acrylic acids, ethers, etc. VOCs mainly come from coal chemical industry, petrochemical industry, fuel coating manufacturing, solvent manufacturing and use, and are one of the typical pollutants in groundwater. Many incidents related to contaminated plots that have occurred in recent years are closely related to the respiratory exposure caused by the volatilization of VOCs in groundwater, and the further health hazards caused. Therefore, conducting environmental monitoring of VOCs in groundwater on site is of great practical significance for assessing the pollution status of the site, carrying out pollution remediation work, and promoting secondary land development, and has become one of the focuses of environmental protection work.

[0003] At present, the method used for monitoring groundwater VOCs is to build groundwater monitoring wells to collect groundwater samples and send them to the laboratory for analysis. However, this method has high well construction costs and a long cycle. The collected samples can only be sent to the laboratory for analysis, and the concentration of VOCs in groundwater cannot be determined on-site. It is not convenient to adjust the point distribution method in time according to the test results during the preliminary site investigation stage, which may affect the representativeness of the samples collected; the collected VOCs water samples are not easy to store and transport, and it is difficult to find out the groundwater pollution status in a short time. In addition, the samples are prone to leakage or decomposition during storage, transportation, and testing, which affects the authenticity of the test results. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a rapid screening technology method for volatile organic compounds in groundwater.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] In a first aspect, the present invention provides a method for rapid screening of volatile organic compounds in groundwater, comprising the following steps:

[0007] (1) drilling holes at points in the vertical direction of the soil to collect soil gas at the points;

[0008] (2) recording the soil depths above and below the distribution points respectively, detecting the concentrations of volatile organic compounds in the soil gas above and below the distribution points; and calculating the concentrations of volatile organic compounds in the groundwater gas according to formulas (II) to (V);

[0009] Formula (II) to Formula (V) are as follows:

[0010]

[0011]

[0012]

[0013]

[0014] In formula (II) to formula (V), d1 is the depth of the upper soil gas sampling point, in m; d2 is the depth of the lower soil gas sampling point, in m; C1 is the concentration of volatile organic compounds in the soil gas at the upper sampling point, in μg / L; C2 is the concentration of volatile organic compounds in the soil gas at the lower sampling point, in μg / L; α is the depth coefficient; H is the Henry coefficient of volatile organic compounds; C w is the volatile organic compound concentration of groundwater vertically corresponding to the sampling point; C a is the concentration of volatile organic compounds in the soil gas at the sampling point, in μg / L.

[0015] The method of the present invention utilizes the volatility and distribution coefficient of VOCs to establish a correlation between the concentrations of VOCs in groundwater and soil gas, and inverts the pollution status of groundwater VOCs through on-site rapid monitoring of soil gas VOCs with simpler operation, lower cost and higher monitoring efficiency. The method has wide applicability and the applicable depth can be adjusted according to the needs of on-site investigation (such as the burial depth of the sampling area and the vertical changes of the soil medium).

[0016] As a preferred embodiment of the method for rapid screening of volatile organic compounds in groundwater according to the present invention, in step (1), a soil gas VOCs rapid monitoring system is used during the collection; the soil gas VOCs rapid monitoring system includes a drill bit, a hammering device, a gas extraction device, an enrichment device and an analysis device; the hammering device is located above the drill bit; the hammering device, the gas extraction device, the enrichment device and the analysis device are connected in sequence.

[0017] As a preferred embodiment of the method for rapid screening of volatile organic compounds in groundwater described in the present invention, the hammering device is a hammer drill; the vacuum device is a sampling air pump of an active extraction method; the enrichment device includes a sampling tube containing a solid adsorbent 2,6-dibenzofuran porous polymer and a portable thermal desorber; and the analysis device is an online gas chromatograph.

[0018] Preferably, a flow meter is provided between the gas extraction device and the enrichment device.

[0019] Furthermore, the flow meter reading adjusts the well washing rate to no more than 200 mL / min; the well washing volume is 3 to 5 times the volume of the drill bit and drill pipe. Sampling should be started immediately after the well washing is completed, and the sampling flow rate should be no more than 200 mL / min.

[0020] As a preferred embodiment of the method for rapid screening of volatile organic compounds in groundwater of the present invention, in step (1), when the vertical texture of the soil is homogeneous, the method of distributing the points adopts any one of the following methods:

[0021] a. When the vertical burial depth of soil is less than 3m, according to the thickness of unsaturated zone D, and Deploy sampling points at depth;

[0022] b. When the vertical burial depth of soil is ≥3m and <6m, according to the thickness of unsaturated zone D, and Deploy sampling points at depth;

[0023] c. When the vertical burial depth of soil is ≥6m, sampling points shall be arranged within 3m above the phreatic water surface according to the thickness D of the unsaturated zone, and the depths of the sampling points shall be D-2 and D-1 respectively.

[0024] As a preferred embodiment of the method for rapid screening of volatile organic compounds in groundwater of the present invention, in step (1), when the vertical texture of the soil is heterogeneous and vertically stratified, the method of point distribution adopts any one of the following:

[0025] a. When the thickness of the deepest homogeneous layer of soil is h<3m, according to the thickness of the unsaturated zone D, at the depth and Sampling points were set up at various locations;

[0026] b. When the thickness of the deepest vertical homogeneous layer of soil is h≥3m, sampling points shall be arranged at depths D-2 and D-1 according to the thickness of the unsaturated zone D.

[0027] As a preferred embodiment of the method for rapid monitoring of groundwater VOCs of the present invention, in step (2), the Henry coefficient H of volatile organic compounds at 0 to 90° C. is as shown in formula (VI);

[0028] ln(H)=AB / T+C×ln(T)——Formula (VI);

[0029] In formula (VI), A, B, and C are all constants, and T is the absolute temperature.

[0030] As a preferred embodiment of the method for rapid screening of volatile organic compounds in groundwater according to the present invention, in step (2), when detecting the concentration of volatile organic compounds in the soil gas at the upper and lower parts of the distribution points, the concentration of volatile organic compounds in the soil gas is continuously detected until stable data is obtained, and then the peak value and steady-state concentration data are taken.

[0031] In a second aspect, the present invention applies the method for rapid screening of volatile organic compounds in groundwater to the prevention and control of groundwater pollution.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The present invention uses the volatility and distribution coefficient of VOCs to establish the correlation between the concentration of VOCs in groundwater and soil gas, and inverts the pollution status of groundwater VOCs through on-site rapid monitoring of soil gas VOCs with simpler operation, lower cost and higher monitoring efficiency. The monitoring method of the present invention can reduce the cost of site investigation and shorten the investigation cycle. For example, the cost of traditional groundwater monitoring well investigation is hundreds of thousands of yuan, and the cycle is several months, while the cost of the monitoring method of the present invention is only tens of thousands of yuan, and the cycle only takes about one day.

[0034] The present invention is applicable to sites where the groundwater flow field is stable and VOCs pollution exists in the soil, thereby improving the investigation efficiency and having wide applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a schematic diagram of the structure of the on-site rapid monitoring device for soil gas VOCs. DETAILED DESCRIPTION

[0036] To better illustrate the purpose, technical solutions and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments. It should be understood by those skilled in the art that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0037] Unless otherwise specified, the experimental methods used in the examples are all conventional methods; the materials, reagents, etc. used, unless otherwise specified, can be obtained from commercial channels.

[0038] Example 1: A soil gas VOCs rapid monitoring system

[0039] like Figure 1 As shown, the soil gas VOCs rapid monitoring system includes a drill bit, a hammering device, a gas extraction device, an enrichment device and an analysis device; the hammering device, the gas extraction device, the enrichment device and the analysis device are connected in sequence.

[0040] The hammering device is a hammering drill, which drives a high-frequency vibration head through the power of a four-stroke gasoline engine to hammer the drill rod nearly ten thousand times per minute; the suction device is a sampling air pump of an active extraction method; a flow meter is provided between the suction device and the enrichment device; the enrichment device is a Tenax sampling tube of a solid adsorbent 2,6-dibenzofuran porous polymer connected to a portable thermal desorber HandyTD TD265 device; the analysis device is an online gas chromatograph.

[0041] Example 2: Point layout for rapid monitoring of VOCs in groundwater

[0042] The vertical point layout plan should comprehensively consider the groundwater burial depth and the vertical changes of the soil medium. Two sample points need to be arranged in the vertical direction at the same coordinate point.

[0043] The soil gas diffusion process is closely related to the homogeneity of the unsaturated zone and the diffusion coefficient of soil volatile organic matter. In order to ensure the uniformity of soil texture and water content in the monitoring layer, the interval between sampling points should not be too large, and the spacing should be controlled within 1m. According to the groundwater burial depth and the vertical heterogeneity of soil texture, it is divided into vertical homogeneous and vertical heterogeneous conditions. The sampling depth is further determined according to the groundwater burial depth and vertical layer thickness.

[0044] The depth and interval of sample point layout depend on the site groundwater depth (D) and the thickness of the deepest homogeneous layer in the unsaturated zone (d), see Table 1 for details.

[0045] Table 1 Vertical layout plan

[0046]

[0047] Example 3: Inversion formula for rapid monitoring of VOCs in groundwater

[0048] There is a gas-liquid distribution equilibrium between VOCs in groundwater and the adjacent unsaturated zone soil gas, as shown in formula (I):

[0049]

[0050] In formula (I): C w is the concentration of volatile organic compounds in groundwater vertically corresponding to the sampling point, in μg / L; H is the Henry coefficient of volatile organic compounds (dimensionless); C a is the concentration of volatile organic compounds in soil air at the sampling point, in μg / L.

[0051] Since the migration process of volatile organic compounds between groundwater and adjacent unsaturated soil layers can be approximated to a steady-state diffusion process, according to Fick's first law, the following equations (II) to (V) are further derived:

[0052]

[0053]

[0054]

[0055]

[0056] In formula (II) to formula (V): d1 is the depth of the upper soil gas sampling point, in m; d2 is the depth of the lower soil gas sampling point, in m; C1 is the concentration of volatile organic compounds in the soil gas at the upper sampling point, in μg / L; C2 is the concentration of volatile organic compounds in the soil gas at the lower sampling point, in μg / L; α is the depth coefficient (dimensionless); the other parameters have the same meanings as formula (I).

[0057] Example 4: A method for rapid monitoring of VOCs in groundwater

[0058] (1) Pilot sand box test

[0059] The sand box is 5.3m×1.12m×2.5m in size and filled with 1m high sand (pure sand: red soil = 1:3). The permeability coefficient is 10.23m / d. The water level of the upstream and downstream of the sand box is adjusted to 1m and 0.9m respectively to keep the flow field stable. Prepare 3L of benzene (BTEX) solution with concentrations of 1000mg / L benzene, 300mg / L toluene, and 100mg / L ethylbenzene, and inject the solution into the sand box. After migrating in the groundwater of the sand box for 3 days, turn off the flow field control system so that the diffusion of BTEX in the groundwater into the soil gas gradually reaches equilibrium.

[0060] (2) Rapid monitoring method of VOCs in groundwater

[0061] Drill holes for sampling in the vertical direction, position the sampling drill bit to the sampling depth; keep the drill bit and drill rod in place, remove the hammer device, connect the suction device, enrichment device and analysis device, and check the air tightness of the connections of each device. Start the suction device to start pumping, adjust the well washing rate to no more than 200mL / min according to the flow meter reading, and the well washing volume is generally 3 to 5 times the volume of the drill bit and drill rod. Sampling should be started immediately after the well washing is completed, and the sampling flow rate should not be higher than 200mL / min. The sample collection volume should be determined according to the required detection limit and analysis method, but should not be greater than 1.0L. The exhaust port of the suction device is connected in series with the Tenax sampling tube, the portable thermal desorber HandyTD TD265, and the portable gas chromatograph (Frog-5000) to continuously detect the soil gas VOCs concentration until stable data is obtained, and then take the peak and steady-state concentration data.

[0062] In this experiment, the soil gas VOCs rapid monitoring system of Example 1 was used to drill holes for sampling in the vertical direction. The sampling depths of each borehole were 2 / 3D and 5 / 6D, i.e., 1.33m and 1.67m, respectively. Six sampling points were set, and their sampling depths and soil gas VOCs concentrations were shown in the following table: Note: The upper soil layer is represented by u, the lower soil layer is represented by d, the symbol for benzene is B, the symbol for toluene is T, the symbol for ethylbenzene is E, and the concentration unit is μg / L.

[0063] Table 2 Soil gas concentrations in the upper and lower layers at sampling points

[0064] Point uB dB uT dT E dE A1 716.40 429.68 250.57 183.66 232.52 131.90 A2 12.06 6.86 4.37 3.70 20.49 11.45 A3 157.79 105.51 41.89 47.14 126.31 78.94 A4 11.59 9.93 2.73 2.00 12.98 10.04 A5 157.79 105.51 41.89 47.14 126.31 78.94 A6 11.59 9.93 2.73 2.00 12.98 10.04

[0065] The Henry coefficient equation of benzene series under the conditions of 0-90℃ is shown in formula (VI), and the constants are shown in Table 3 with reference to the literature "Henry's law constants of volatile organic compounds between 0and 95℃-Data compilationand complementation in context of urban temperature increases of the subsurface".

[0066] ln(H)=AB / T+C×ln(T)——Formula (VI);

[0067] In formula (VI), A, B, and C are all constants, and T is the absolute temperature.

[0068] Table 3 Henry coefficient equation parameters for benzene series

[0069] Benzene A B C ln(H) H benzene 246.963 14043 -35.34 -1.49052072 0.22525533 Toluene 322.281 17633.4 -46.425 -1.37264092 0.253436769 Ethylbenzene 571.229 29076.8 -83.347 -1.17265852 0.30954292

[0070] Under the experimental conditions (T=298.15K), the Henry coefficients H of benzene, toluene and ethylbenzene were calculated to be 0.2253, 0.2534 and 0.3095 respectively.

[0071] Since the upper layer sampling depth d1 of the six points is 1.33m, the lower layer sampling depth d2 is 1.67m, and the unsaturated zone thickness D is 2m, the depth coefficient α calculated by substituting it into formula (III) in Example 3 is 2.

[0072] Combining the soil gas concentrations and related Henry coefficients in the upper and lower layers in Tables 2 and 3, the VOCs concentration in groundwater was calculated by substituting them into Formula (V) in Example 3. The concentration units are all μg / L, as shown in Table 4:

[0073] Table 4 VOCs concentration in groundwater

[0074]

[0075]

[0076] The present invention uses the volatility and distribution coefficient of VOCs to establish the correlation between the concentration of VOCs in groundwater and soil gas, and inverts the pollution status of groundwater VOCs through on-site rapid monitoring of soil gas VOCs with simpler operation, lower cost and higher monitoring efficiency. The monitoring method of the present invention can reduce the cost of site investigation and shorten the investigation cycle. For example, the cost of traditional groundwater monitoring well investigation is hundreds of thousands of yuan, and the cycle is several months, while the cost of the monitoring method of the present invention is only tens of thousands of yuan, and the cycle only takes about one day.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A method for rapid screening of volatile organic compounds in groundwater, characterized in that: The following steps are involved: (1) drilling holes at points in the vertical direction of the soil to collect soil gas at the points; When the soil texture is homogeneous in vertical direction, the point distribution method may be any of the following: a. When the vertical burial depth of soil is less than 3m, according to the thickness of unsaturated zone D, and Deploy sampling points at depth; b. When the vertical burial depth of soil is ≥3m and <6m, according to the thickness of unsaturated zone D, and Deploy sampling points at depth; c. When the vertical burial depth of soil is ≥6m, sampling points are arranged within 3m above the phreatic surface according to the thickness D of the unsaturated zone, and the depths of the sampling points are D-2 and D-1 respectively; (2) respectively recording the soil depths above and below the distribution points, detecting the concentrations of volatile organic compounds in the soil gas above and below the distribution points; and calculating the concentrations of volatile organic compounds in the groundwater gas according to formulas (II) to (V); Formula (II) to Formula (V) are as follows: ——Formula (II); ——Formula (III); ——Formula (IV); ——Formula (V); In formula (II) to formula (V): d1 is the depth of the upper soil gas sampling point, in m; d2 is the depth of the lower soil gas sampling point, in m; C 1 is the concentration of volatile organic compounds in soil gas at the upper sampling point, in μg / L; C 2 is the concentration of volatile organic compounds in soil gas at the lower sampling point, in μg / L; α is the depth coefficient; H is the Henry coefficient of volatile organic compounds; C w The volatile organic compound concentration of groundwater vertically corresponding to the sampling point; C a is the concentration of volatile organic compounds in the soil gas at the sampling point, in μg / L.

2. The method for rapid screening of volatile organic compounds in groundwater according to claim 1, characterized in that: In step (1), a soil gas VOCs rapid monitoring system is used during the collection; the soil gas VOCs rapid monitoring system includes a drill bit, a hammering device, a gas extraction device, an enrichment device and an analysis device; the hammering device is located above the drill bit; the hammering device, the gas extraction device, the enrichment device and the analysis device are connected in sequence.

3. The method for rapid screening of volatile organic compounds in groundwater according to claim 2, characterized in that: The hammering device is a hammer drill; the air extraction device is a sampling air pump of an active extraction method; the enrichment device includes a sampling tube containing a solid adsorbent 2,6-dibenzofuran porous polymer and a portable thermal desorber; the analysis device is an online gas chromatograph.

4. The method for rapid screening of volatile organic compounds in groundwater according to claim 3, characterized in that: A flow meter is provided between the gas extraction device and the enrichment device.

5. The method for rapid screening of volatile organic compounds in groundwater according to claim 4, characterized in that: The flow meter reading is used to adjust the well washing rate to no higher than 200 mL / min; the well washing volume is 3 to 5 times the volume of the drill bit and drill pipe. Sampling should be started immediately after the well washing is completed, and the sampling flow rate should be no higher than 200 mL / min.

6. The method for rapid screening of volatile organic compounds in groundwater according to claim 1, characterized in that: In step (2), the Henry coefficient H of the volatile organic matter under the condition of 0-90°C is as shown in formula (VI); ——Formula (VI); In formula (VI), A, B, and C are all constants, and T is the absolute temperature.

7. The method for rapid screening of volatile organic compounds in groundwater according to claim 1, characterized in that: In step (2), when detecting the concentration of volatile organic compounds in the soil gas at the upper and lower parts of the distribution point, the concentration of volatile organic compounds in the soil gas is continuously detected until stable data is obtained, and then the peak value and steady-state concentration data are obtained.

8. Application of the rapid screening method for volatile organic compounds in groundwater according to any one of claims 1 to 7 in the prevention and control of groundwater pollution.

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