Prediction method for groundwater contamination concentration

Abstract

To provide a method for predicting the concentration of contaminants in underground water when a remaining contaminant elutes into the underground water in a permeable layer after a clay layer (impermeable layer) is decontaminated.SOLUTION: Disclosed is a method for predicting a pollutant concentration in the optional point in an aquifer in the case where the pollutant remaining in a part of a clay layer after a pollutant removal treatment diffuses in the aquifer of a clay layer upper side. The prediction is performed based on: a distance of the above point from a pollutant remaining place in an underground water and height of the point from the upper face of the clay layer; the actual flow rate of underground water; the dispersion coefficient of the pollutant in the downstream direction of the underground water; that coefficient of the pollutant in a ground surface direction; delay coefficient; and the effective porosity of the aquifer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for predicting the concentration of contaminants in groundwater after the purification of contaminated ground, and more particularly to a method for predicting the concentration of contaminants in a permeable layer of an upper ground layer made of highly permeable sandy soil or the like and an impermeable layer made of low permeability clay or the like, after volatile contaminants such as organic chlorine compounds present in the lower ground layer (impermeable layer) have been removed by electrothermal methods or the like. [Background technology]

[0002] Patent Document 1 describes a purification technique for removing volatile contaminants from an impermeable layer using an electric heating method. The method involves pumping up groundwater in the permeable layer to make the layer unsaturated, then applying a voltage between electrodes inserted into the impermeable layer saturated with groundwater to heat the contaminated area in the impermeable layer. The contaminant gas vaporized from the contaminated area and introduced into the gas phase between the soil particles in the permeable layer is then vacuum-suctioned with a vacuum pump through a gas suction hole. The method then involves pumping up groundwater in the permeable layer 12 to make the layer unsaturated with groundwater. Then, applying a voltage between electrodes 5, 5 inserted into the impermeable layer 13 saturated with groundwater to heat the contaminated area 14b in the impermeable layer 13. The contaminant gas G vaporized from the contaminated area 14b and introduced into the gas phase between the soil particles in the permeable layer 12 is then vacuum-suctioned with a vacuum pump VP through a gas suction hole 6 to collect the contaminant gas G. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-26492 Summary of the Invention [Problem to be solved by the invention]

[0004] After the clay layer (impermeable layer) is purified using the electrothermal method, there is a possibility that remaining contaminants may leach into the groundwater in the permeable layer. When this happens, it is necessary to understand the level of groundwater concentration, but there were no tools available to calculate this. Furthermore, in order to proceed with purification efficiently, it is necessary to plan the depth and concentration of the clayey soil that must be purified to ensure that the groundwater concentration complies with the standard. However, there was no easy tool to predict this, and the operation control of the electrothermal method was left to trial and error.

[0005] An objective of the present invention is to provide a method for predicting the concentration of contaminants in groundwater when the remaining contaminants are eluted into the groundwater in the permeable layer after the clay layer (impermeable layer) has been purified. [Means for solving the problem]

[0006] The present invention solves the above problems by the following means.

[0007] [1] A method for predicting the concentration of a contaminant at any point in an aquifer when a contaminant remaining in a part of a clay layer after contaminant removal treatment diffuses into the aquifer above the clay layer, comprising: A method for predicting groundwater contamination concentrations based on the distance in the groundwater flow direction from the location where the contaminant remains at the site and the height from the top of the clay layer, the actual flow rate of the groundwater, the dispersion coefficient of the contaminant in the downstream direction of the groundwater, the dispersion coefficient of the contaminant in the direction toward the surface of the ground, the delay coefficient, and the effective porosity of the aquifer.

[0008] [2] A method for predicting groundwater contamination concentration according to [1], wherein the prediction is made according to the following formula:

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[0009] According to the present invention, if contaminants remain in the clay layer after the clay layer has been purified by electrothermal heating, it is possible to estimate the contaminant concentration when the contaminants leach out of the purified clay layer and re-contaminate the aquifer. This prediction method makes it possible to determine to what depth the clay layer should be heated and to what concentration the heating should reach when applying electrothermal heating, thereby enabling the electrothermal heating to be implemented in a planned manner.

[0010] Furthermore, according to the present invention, it is possible to set a purification target for the concentration of pollutants in clayey soil, which increases the accuracy of purification period and cost calculations (electrode installation depth and heating period). [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic vertical cross-sectional view of contaminated ground. [Figure 2] 10 is a graph showing the predicted results of pollutant concentrations. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, an embodiment will be described with reference to the drawings.

[0013] FIG. 1 is a schematic longitudinal cross-sectional view of the contaminated ground, in which a permeable layer exists above a clay layer (impermeable layer), and an aquifer exists within the permeable layer.

[0014] Part of the clay layer was contaminated with organic chlorine compounds, etc., and has been treated by electrothermal decontamination. Contaminants remain in part of the treatment area without being purified. These remaining contaminants dissolve into the groundwater in the aquifer, and are dispersed downstream (to the right in Figure 1) and upward (towards the ground surface; upward in Figure 1) by the transport action of the groundwater flow and the action of concentration diffusion.

[0015] The actual velocity of groundwater flow is v X In the clay layer, the actual flow velocity is treated as zero.

[0016] In the present invention, the contamination (contaminant concentration) c at any point downstream (downstream in the direction of groundwater flow) from the remaining contaminated area at a distance x and at a height z from the top surface of the clay layer is calculated using the following formula.

[0017]

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[0018] R d : delay coefficient, n e :effective porosity, c: Contaminant concentration, D X : dispersion coefficient in the flow direction, D z : vertical dispersion coefficient, v x : Actual flow rate in the aquifer, α L :Vertical dispersion length, α T :lateral dispersion length, D M : Molecular diffusion coefficient

[0019] In this embodiment, α T =α L / 100, but α L and α T The ratio is not limited to 100, but may be selected from the range of 10 to 100.

[0020] The above partial differential equation is the fundamental equation for water-soluble contamination in advection-dispersion analysis, with the first term on the right-hand side being the dispersion term in the x-axis direction (downstream direction of the groundwater), the second term being the dispersion term in the z-axis direction (towards the ground surface), and the third term being the advection term (advection due to the actual groundwater flow velocity). Note that it is assumed that there is no decomposition of contaminants during diffusion. It is also assumed that the contaminant concentration in the clay layer other than the remaining contaminant portion is zero, and that there is no elution of contaminants from other than the remaining contaminant portion. It is also assumed that the hydraulic conductivity, effective porosity (total porosity), and volumetric water content of the aquifer and clay layer are uniform and constant.

[0021] The partial differential equations are solved using commercially available software, or are solved analytically or numerically using Fortran or C language, etc., using a finite difference method or other such methods.

[0022] In the above partial differential equation, the values ​​of each coefficient are as follows, the pollutant is a volatile organic compound such as PCE (tetrachloroethylene), and the area of ​​the polluted area is 1 m 2 The area is (5m x 0.2m), and the remaining amount (weight) of pollutants at the polluted site is 0.1mg / L. Figure 2 shows the calculation results of the change in pollutant concentration over time at a point (observation point) 5m downstream and 2m above the polluted site.

[0023] R d (Delay factor): 1.0 n e (Effective porosity, but total porosity n in the clay layer): Effective porosity of the aquifer: 0.3, total porosity of the clay layer: 0.4 v x (Actual flow rate of the aquifer): 0.1 m / day α L (Vertical dispersion length): 3.0m α T (lateral dispersion length): 3.0×10 -2 m D M (Molecular diffusion coefficient in aquifer and clay layer): 8.64 x 10 -5 m 2 / day

[0024] As shown in Figure 2, the change in pollutant concentration over time at the observation point can be predicted. Therefore, when applying the electric heating method, it is possible to determine to what depth the clayey soil layer needs to be heated and to what concentration level, making it possible to implement the electric heating method in a planned manner.

Claims

1. A method for predicting a contaminant concentration at an arbitrary point in an aquifer when a contaminant remaining in a part of a clay layer after contaminant removal treatment diffuses into the aquifer above the clay layer, comprising: A method for predicting groundwater contamination concentrations based on the distance in the groundwater flow direction from the location where the contaminant remains at the site and the height from the top of the clay layer, the actual flow rate of the groundwater, the dispersion coefficient of the contaminant in the downstream direction of the groundwater, the dispersion coefficient of the contaminant in the direction toward the surface of the ground, the delay coefficient, and the effective porosity of the aquifer.

2. 2. The method for predicting groundwater contamination concentration according to claim 1, wherein the prediction is carried out according to the following formula: [Equation 1]

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