JP2010064002A - Method for estimating risk of groundwater contamination - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for estimating risk of underground water contamination, the method for predicting possibility that soil pollution may cause groundwater contamination in the future, and for examining the necessity of measures for each site in accordance with health risk of people or risk of groundwater contamination in the site. <P>SOLUTION: A conceptual model of the underground is prepared by grasping a site in excess of soil elusion criteria obtained by the survey of soil in a ground contaminated with heavy metal. A parameter of moisture movement and substance (heavy metal) movement including soil moisture characteristic curve, saturated hydraulic conductivity, soil particle density, dry density, molecular diffusion factor, diffusion length and sorptive property is calculated or obtained on the basis of contaminated conditions and moisture geological conditions. Using an advection diffusing equation, a heavy metal concentration profile is determined for the underground from the time of occurrence of contamination until now and compared with the present contaminated condition. After the reasonableness is assured, a heavy metal concentration profile in the future is determined to estimate the risk of groundwater contamination. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for estimating the risk of groundwater contamination, and in particular, predicts the possibility that contaminated soil will cause groundwater contamination in the future. It relates to a method for estimating the risk of groundwater contamination so that the need for aggressive measures can be examined.

  In recent years, soil investigations conducted as part of redevelopment of factory sites and environmental management at factories in operation have revealed cases of soil contamination due to heavy metals such as lead, arsenic, cadmium, hexavalent chromium, mercury, and selenium. is increasing. Since soil contamination by such heavy metals causes groundwater contamination, the maximum amount of elution allowed in Japan is regulated by the soil elution standard in the Soil Contamination Countermeasures Law.

  General countermeasures against soil contamination include containment methods, insolubilization / fixation methods, purification methods, excavation and removal. In addition, a management method called groundwater monitoring is permitted in situations where groundwater contamination does not occur. Among these methods, there are water-blocking and blocking methods for the containment method, both of which are methods that contain contaminated soil with a water-blocking wall or barrier wall, making it extremely difficult to reuse the land and gaining the understanding of neighboring residents. There is a difficulty that cannot be done.

  The insolubilization / fixation method includes a chemical insolubilization method, a cement solidification method, and a geochemical solidification method. Among them, the chemical insolubilization method is a method of chemically detoxifying using chemicals such as ferric chloride, and there is a possibility of re-elution, and the cement solidification method may also be re-eluted, Both have problems with long-term stabilization. The geochemical solidification method is a method of immobilizing specific hazardous substances in new crystalline minerals. It is geochemically stable and has excellent long-term stability.

  The purification method includes a pumping extraction method, an electrolysis method, a heat treatment method, a cleaning method, and the like. The pumping extraction method is a method of extracting and removing contaminants by forcibly circulating water using a borehole. The electrolysis method is a method in which pollutants are electrolyzed by electric current and metal ions are recovered, and the heat treatment method is a method in which the pollutants are heated to volatilize or burn, and are recovered or diffused in the air. The cleaning method is a method to reduce the content and elution amount by excavating contaminated soil and strata after excavation, or by high-pressure cleaning in situ and removing heavy metals together with fine-grained deposits. These purification methods have the effect of reducing harmful substances in any method, but there is a problem that the removal rate of heavy metal content is at most 30 to 70%, and several tens of percent or more remain.

  In addition, excavation and removal is a method of excavating contaminated soil and transporting it to the outside. Although contaminated soil is almost completely removed from the target site, the cost of soil contamination countermeasures becomes very high and the economic burden on the operator increases. . In addition, there is a problem of how to discard the removed soil, and if it is disposed in a place where groundwater is shallow, there is a possibility that contamination will occur due to the disposal.

  In this way, there are advantages and disadvantages to each method of dealing with contaminated soil, but there is no way to quantify the possibility that soil that exceeds the standard will cause groundwater contamination in the future, so the risk of groundwater contamination cannot be denied. Therefore, the current situation is that measures for excavation and removal are mainly adopted. As a result, as described above, there is a problem of the burden of countermeasure costs for contaminated soil and the disposal destination of contaminated soil.

  On the other hand, the risk of groundwater contamination due to soil exceeding the soil elution amount due to heavy metals includes many targets such as soil quality, groundwater level, type and amount of pollutants, mechanism of contamination, rainfall, etc. It varies according to the characteristics peculiar to the earth. In other words, even if there is soil that exceeds the soil elution amount standard, it is fully conceivable that groundwater contamination will not occur in the future. Therefore, instead of applying a uniform standard value, we consider the necessity of countermeasures for each land (hereinafter referred to as a site where there is a possibility of soil contamination) according to the risk of groundwater contamination. It is being sought to determine the need for purification.

  With regard to the purification of such contaminated soil, for example, in Patent Document 1, a washing process for washing heavy metal contaminated soil and separating it into washed muddy water and washed treated soil, and a heavy metal elution component from the washed treated soil are immobilized. The washing turbid water separated in the washing step is preferably separated into solids containing treated water and heavy metals in the water treatment step, and the content of pollutants in the soil is below the standard. A method for purifying contaminated soil that can suppress the amount of elution to below the standard is shown.

JP 2005-238207 A

  However, the contaminated soil purification method disclosed in Patent Document 1 is the above-described purification method, and the values in which the content of lead and arsenic in the contaminated soil is reduced are shown as samples. The necessity of countermeasures for each site according to human health risks and groundwater contamination risk has not been studied.

  Therefore, in the present invention, it is predicted that there is a possibility that contaminated soil will cause groundwater contamination in the future, and the necessity of measures for each site can be examined according to the groundwater contamination occurrence risk at that site. The challenge is to provide a method for estimating the risk of contamination.

In order to solve the above problems, the method for estimating the risk of occurrence of groundwater contamination according to the present invention is as follows.
A method for estimating the risk of future groundwater contamination due to these pollutions in heavy metal contaminated land,
A conceptual model of the target site is created based on the pollution status and hydrogeological status at the point where the soil elution amount exceeds the soil elution standard exceeded in the soil contaminated with heavy metals, and the soil moisture characteristic curve, saturation hydraulic conductivity, soil Calculate or obtain parameters for moisture and mass (heavy metal) migration, including particle density, dry density, molecular diffusion coefficient, dispersion length, adsorption properties,
Using the advection diffusion equation with the conceptual model and the calculated or acquired parameters, calculating the heavy metal concentration distribution status in the ground during the period from the predicted occurrence of contamination to the present, and the contamination status obtained by the soil survey In comparison, in the state where the difference between the two is below a predetermined threshold value, the future heavy metal concentration distribution situation is calculated with the conceptual model and the parameter calculated or obtained, and the advection diffusion equation, in the land contaminated with heavy metal It is characterized by estimating the risk of future groundwater contamination due to these pollutions.

  By collecting the parameters of water movement and substance (heavy metal) movement in the ground based on the pollution status and hydrogeological status obtained from soil surveys in this way, it has been difficult in the past. It is possible to accurately estimate the degree of contamination of groundwater by existing heavy metals. Also, first determine the dispersion state of heavy metal concentration during the period from the occurrence of contamination to the present, and the difference between the two is less than a predetermined threshold compared to the contamination status obtained from the soil survey, that is, the obtained contamination status is reasonable. It is possible to estimate the risk of occurrence of groundwater contamination with higher accuracy by calculating the future contamination risk after verifying whether or not it is a thing.

  In the present invention, the soil moisture characteristic curve, saturated hydraulic conductivity, soil particle density, dry density, and adsorption characteristic stored in advance in the database are selected based on the soil particle size distribution and organic matter content obtained by the soil survey. If there is no corresponding data, the parameters are calculated by laboratory experiments, and it takes a lot of time and money to calculate the parameters by experiments. However, if there is data calculated in the past, it can be used effectively. Can save money and time.

In addition, the calculation by the laboratory experiment of the adsorption characteristics of the heavy metal to the soil,
As adsorb heavy metal adsorption per gram of dry _soil (mg g ^-1 )
As _total is the amount of total heavy metals per gram of dry soil (mg g ^-1 )
Dry soil weight (g) using W _soil for analysis
As _solute concentration of heavy metals in soil solution in distilled water extraction test (mg cm ⁻³ )
v the volume of water used in distilled water extraction test _water (cm ³⁾
As _total (before) is the total amount of heavy metals per gram of dry soil before the laboratory experiment (mg g ^-1 )
Then, the heavy metal adsorption amount in the soil can be calculated accurately by calculating the heavy metal adsorption amount in the soil by the following equation (1) and the total heavy metal amount in the soil before the test by the equation (2).

Furthermore, the estimation of the risk of occurrence of groundwater contamination is as follows:
Risk _{GW, year} is the target period (year), groundwater contamination risk of the target heavy metal (subscript year is the target period for risk assessment)
C _{GWcal, year} is the target period (year), calculated maximum concentration of the target substance in groundwater (mgl ⁻¹ , subscript ^max is the maximum value)
C _Gwcriteria is the groundwater environmental standard for target substances (mgl ^-1 )
Then, the ratio of the maximum value of the pollutant concentration in the groundwater calculated during the target period with respect to the groundwater environmental standard is calculated by the following equation (3).

  As described above, the method for estimating the risk of groundwater contamination according to the present invention can accurately estimate the degree of groundwater contamination by heavy metals, which has been difficult in the past, and the period from the time of occurrence of contamination to the present time. Since the future pollution risk is calculated after obtaining the heavy metal concentration dispersion status and verifying whether it is appropriate or not, it is possible to estimate the risk of occurrence of groundwater contamination with higher accuracy.

  In addition, the soil moisture characteristic curve, saturated hydraulic conductivity, soil particle density, dry density, adsorption characteristics are stored in the database in advance, and the corresponding parameters are selected according to the soil particle size distribution and organic matter content obtained by soil survey. By using the data calculated in the past, you can save money and time.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention unless otherwise specified, but are merely illustrative examples. Not too much.

  First, an outline of the method for estimating the risk of occurrence of groundwater contamination according to the present invention will be described with reference to the flowchart shown in FIG. In the present invention, first, a soil survey such as a surface soil survey, a boring survey, a geological survey, a groundwater survey, a soil analysis, etc. is performed at a site with soil contamination in step S11, and a risk assessment point, a target substance (contamination target) in step S12. Heavy metal).

  Then, based on the investigation results such as the underground pollution of the target site and the hydrogeological condition obtained, the soil elution amount of the target heavy metal and the concentration distribution in the depth direction of the total content are ascertained in step S13. Simplification as a conceptual model based on the geological classification, groundwater level and thickness of the target aquifer, and surrounding groundwater use points.

  Further, in step S14, water movement, material (heavy metal) movement parameters, that is, soil moisture characteristic curve (soil water retention curve necessary for simulation of movement of unsaturated zone water and material (heavy metal) in the local hydrogeological situation are shown. Curve, expressed by suction force and soil moisture content (water retention)), saturated hydraulic conductivity (apparent water velocity passing through saturated soil), soil particle density (soil particle density), drying Density (density of soil (which includes three phases of soil particles, soil moisture, soil air)), molecular diffusion coefficient (proportional constant that moves heavy metals by concentration gradient), dispersion length (in water movement in soil) Parameters such as the coefficient of influence due to hydraulic dispersion) and adsorption characteristics (ratio of the concentration of heavy metals dissolved in soil solution and heavy metals adsorbed on soil particles (depending on soil and material)) Literature There retrieve and a particle size distribution obtained by laboratory test, a method of obtaining from a database storing historical data using the organic content. At this time, items that cannot be acquired from the database are calculated by conducting a laboratory test or the like.

  Based on the collected data and parameters, the advection diffusion equation is used in step S15, and the movement of heavy metals in the ground for the predicted period from the occurrence of pollution to the present (hereinafter referred to as the pollution period). The simulation 1 is performed, and in step S16, the ground metal concentration profile (distribution state in the depth direction) predicted by the simulation 1 is compared with the current profile, and the validity of the simulation 1, that is, the prediction is made. The difference between the heavy metal concentration profile in the ground and the current profile is obtained, and it is confirmed whether or not the difference is not more than a predetermined threshold value. At this time, the dispersion length is adjusted. As a result of the validity, if it is determined that the predicted heavy metal concentration profile in the ground is significantly different from the current profile and there is no validity, the process returns to step S14 and the parameters are reviewed. Repeat the process.

  Thereafter, in step S17, the advection diffusion equation is used on the basis of these results. From now, for example, 100 years, 300 years, 500 years, and 1000 years later (the number of years in which the simulation is performed depends on the number of years to reach the peak. (Different) simulation 2 is performed, and the concentration profile of how heavy metals in the soil permeate through the soil and the concentration of heavy metals in the groundwater are calculated.

  Based on this result in step S18, if the countermeasures are not implemented, the groundwater contamination risk in that area is evaluated, and if the countermeasures are not implemented in step S19, the risk of outflow of contaminated groundwater outside the site Each of the evaluations is carried out, and based on these, the countermeasure method is examined in step S20, the groundwater contamination occurrence risk in that case and the cost-effectiveness of the countermeasure are examined, and the process ends.

  The groundwater contamination occurrence risk is estimated in this way. Next, an apparatus for carrying out the groundwater contamination occurrence risk estimation method according to the present invention will be described with reference to the block diagram shown in FIG.

  In FIG. 2, 10 is a groundwater contamination occurrence risk estimation device, and 11 is an input device for inputting data necessary for this groundwater contamination occurrence risk estimation device 10, and is a site with soil contamination carried out in step S11 of FIG. , Geological classification of risk assessment points obtained by soil survey, groundwater level and thickness of the target aquifer, surrounding groundwater use point, target substance (heavy metal to be contaminated), amount of soil elution of target heavy metal and The concentration distribution in the depth direction of the total content, and the moisture movement and heavy metal movement parameters obtained from the database in step S14, that is, soil moisture characteristic curve, saturated hydraulic conductivity, soil particle density, dry density, molecular diffusion Enter the soil particle size distribution, organic matter content, etc. to obtain parameters such as coefficient, dispersion length, and adsorption characteristics. Reference numeral 12 denotes an output device that uses a display device, a printer, or the like. For example, the output device 12 may be directly distributed to a user who has requested calculation of the risk of occurrence of groundwater contamination using a communication line such as the Internet.

  The groundwater contamination occurrence risk estimation device 10 includes a control device 101, a mathematical formula program storage device 102, a storage device 103 that stores parameters necessary for simulation calculation, and the like. The control device 101 further includes a CPU 1011 and an arithmetic device 1012. Consists of. Further, the mathematical formula program storage device 102 stores various mathematical formulas necessary for the simulation calculation, which are a finite element numerical analysis program 1021 for solving the advection diffusion equation described later, Van Genuchten's formula 1022, formula for calculating the amount of heavy metal adsorbed on soil. 1023, Freundlich type adsorption isotherm 1024, risk evaluation calculation formula 1025, countermeasure cost calculation formula 1026, and the like. The storage device 103 that stores parameters necessary for the simulation calculation stores the calculation result storage device 1031 that stores the calculation result, the soil moisture characteristic curve storage device 1032 that stores the soil moisture characteristic curve, and the saturated hydraulic conductivity of various soils. A saturated water permeability coefficient storage device 1033, a dry density storage device 1034 that similarly stores a dry density, an adsorption characteristic storage device 1035 that also stores an adsorption characteristic, and the like.

  The operation of the apparatus shown in FIG. 2 will be described in correspondence with the flow chart described in FIG. 1. In step S12, the risk assessment point, the target substance (contamination) is confirmed by the soil survey performed in step S11 in FIG. When the target heavy metal) is selected and a conceptual model is created in step S13, the data and the result of the soil survey are first input to the groundwater contamination occurrence risk estimation device 10 by the input device 11.

  Then, the control apparatus 101 determines the moisture movement necessary for the simulation calculation, the substance (heavy metal) movement parameter, that is, the soil moisture characteristic curve, from the particle size distribution of the soil, the organic matter content, and the data of the soil investigation result input in step S14. Saturated hydraulic conductivity, soil particle density, dry density, molecular diffusion coefficient, dispersion length, adsorption characteristics, etc., soil moisture characteristic curve storage device 1032 of storage device 103, saturated hydraulic conductivity storage device 1033, dry density storage device 1034, adsorption characteristics Acquired from the storage device 1035 or the like. At this time, parameters not stored in the database are calculated by performing a separate laboratory test, and the calculation results are added to these parameter databases as parameter data.

  When the parameters necessary for the simulation calculation are obtained in this way, the control device 101 uses those data and parameters, uses the advection diffusion equation (finite element numerical analysis program 1021) stored in the mathematical formula program storage device 102, and performs step S15. In the period from the occurrence of contamination predicted to the present time (hereinafter referred to as a contamination period), the computer 1012 is caused to perform a simulation 1 on the movement of heavy metal in the ground. In step S16, the ground heavy metal concentration profile predicted by the simulation 1 is compared with the profile obtained by the soil survey, and the validity of the simulation 1 is confirmed. This confirmation is performed by comparing the value obtained by the simulation calculation with the profile obtained by the soil survey, and automatically determining whether the difference is greater than a predetermined threshold or by causing the output device 12 to perform the simulation. The calculation result and the profile obtained from the soil survey may be displayed so that a person can judge.

  If it is determined that the result of the simulation calculation is appropriate, in the same manner, in step S17, the advection diffusion equation is used based on these results, and for example, 100 years later, 300 years later, 500 years later. After that, the simulation 2 up to 1000 years later is carried out, the concentration profile of how heavy metals in the soil penetrate into the soil and the concentration of heavy metals in the groundwater are calculated and output to the output device 12. In addition, the risk assessment formula 1025 is used to evaluate the risk of groundwater contamination, the risk of groundwater contamination in the area when no countermeasures are taken, the risk of outflow of contaminated groundwater outside the site, the examination of countermeasures and the cost of countermeasures Cost effectiveness is examined using the calculation formula 1026 and each result is output to the output device 12.

  The above is the outline of the present invention and the explanation of the apparatus for carrying out the groundwater contamination risk estimation method. Next, referring to FIG. 3 to FIG. 6, it is obtained by the soil survey of the target site in step S13 of FIG. Soil water characteristic curve related to water movement, which is a parameter model used in simulation, and a conceptual model (Fig. 3) such as geological classification, groundwater level and thickness of the target aquifer, and surrounding groundwater use points. An example graph (FIG. 4), adsorption characteristics (FIG. 5) of heavy metal (in this case, arsenic is taken as an example) to the embankment, and FIG. 6 listing the parameters used in these simulations will be described.

  First, FIG. 3, which is a conceptual model of contaminated land, was obtained by surface soil survey, boring survey, geological survey, groundwater survey, soil analysis, etc. carried out in step S11 in the flow diagram of FIG. Based on data such as pollution and hydrogeological conditions of the target area, in step S13, the soil elution amount and total content of the target heavy metal in the depth direction, the geological classification of the target area, and the groundwater level of the target aquifer , Thickness, surrounding groundwater use point, etc. are simplified as a conceptual model. In the following description, the case where the heavy metal is arsenic will be described as an example.

In the figure, the vertical axis shows the depth from the ground surface (unit: cm), and the horizontal axis shows the amount of soil elution of heavy metal (arsenic) in the upper part of the figure (unit: mgl ^-1 ) and the lower part. The scale is also the total content of arsenic (unit: mgkg ^-1 ), point A is the point where the highest concentration of arsenic was confirmed, point B is the point showing the average contamination concentration distribution of the target site, right side The figure of “Site conceptual model” shows the soil and groundwater level of the site. In points A and B, □ indicates the total content of arsenic, and ● indicates the amount of soil leaching.

The land on which the groundwater contamination occurrence risk estimation shown in FIG. 3 was created, for example, around 1985, and as shown in FIG. 3, there is an embankment layer of embankment from the ground surface to about 6 m. There is a clay layer in the lower layer. The groundwater level was confirmed about 6m from the ground surface, and arsenic exceeding the soil elution standard was confirmed by surface soil survey and boring survey in the entire target area, but the arsenic concentration in the groundwater is below the lower limit of quantitation. (0.001 mgl ⁻¹ ).

As described above, the maximum concentration of arsenic is confirmed at point A, and the soil elution amount is 0.058 mgl ⁻¹ (soil elution amount standard 0.001 mgl ⁻¹ ), and the total content is 70 mgkg ⁻¹ . Point B showing the average contamination concentration distribution of the target site has a soil elution amount of 0.02 mgl ⁻¹ and a total content of 26 mgkg ⁻¹ . Further, the total amount of arsenic which penetrate into the soil due to contamination, the results calculated from the total arsenic content of each depth at each point, about 27Mgkg ^-1 at point A, was about 6Mgkg ^-1 at point B. Note that this conceptual model is input by the input device 11 in the block diagram of FIG. 2 and used in the simulation calculation.

FIG. 4 is an example of a soil moisture characteristic curve relating to moisture movement in the ground. This was created in step S14 in the flow chart of FIG. 1, and a laboratory experiment was performed to determine soil moisture characteristics for the embankment constituting the site, and the values indicated by ● in FIG. 4 were obtained. In FIG. 4, the horizontal axis represents the moisture content, and the vertical axis represents the logarithmic value (cmH ₂ O) of the pressure head. FIG. 4 is obtained by using van Genuchten's formula (the formula shown by 1022 in the block diagram of FIG. 2), which is generally used to obtain the slope of the soil moisture characteristic curve from the data indicated by ● in FIG. Were interpolated and identified as indicated by a broken line to obtain parameters as shown in FIG. 6 as a soil moisture characteristic curve. The result obtained here is stored in the soil moisture characteristic curve storage device indicated by 1032 in the block diagram of FIG.

  The arsenic adsorption characteristics on the embankment are obtained by one of the following methods. One is a method of analyzing the total arsenic content of the local soil and the arsenic concentration in the soil solution in a distilled water extraction test by shaking for 6 hours at a solid-liquid ratio of 1:10. The other is to measure the total content (Astotal (before)), elution amount (CO), and pH of the target arsenic in the target soil. Add 5 times the amount of distilled water, shake and let stand for more than 1 hour, mix gently before measurement to make it suspended, attach a pH meter electrode, and stabilize pH after 30 seconds or more. At that point, read the value.

The concentration of the solution to which arsenic is added is a reference value, and a solution having four concentrations of 10 times, 100 times, and 1000 times the reference is prepared. Prepare 20g (Wsoil) of air-dried soil and 200ml (Vwater) of concentration adjustment solution, seal it in a beaker etc. and mix for 24 hours with a shaker (number of shaking: normal temperature, normal pressure, 200 times per minute) (Equal width 4-5 cm), and after completion of shaking, the solution was centrifuged and the solution was filtered with a 0.45 μm membrane filter, and the concentration (mg / l) (As solute), pH, and ORP in the equilibrium solution were measured. . And
As _adsorb : Arsenic adsorption amount per 1g of dry _soil (mg g ^-1 )
As _total : _Total amount of arsenic per 1 g of dry soil (mg g ^-1 )
W _soil : Dry soil weight (g) used for analysis
As _solute : Arsenic concentration in soil solution in distilled water extraction test (mg cm ⁻³ )
v _water : Volume of water used for distilled water extraction test (cm ³ )
As _total (before): Total amount of arsenic per gram of dry soil before testing (mg g ⁻¹ )
Then, the amount of arsenic adsorption in the soil is calculated by the following formula (1) shown by 1023 in FIG. 2, the total amount of arsenic in the soil before the test is calculated by the formula (2), and calculated by the formula (1). The arsenic adsorption amount in the soil thus obtained was obtained as indicated by ● in FIG. In FIG. 5, the horizontal axis represents the amount of arsenic adsorbed to the soil (mg / g), and the vertical axis represents the arsenic concentration (mg / cm ³ ) in the soil solution.

  These data were interpolated by the Freundlich type adsorption isotherm shown by 1024 in FIG. 2 to obtain the relationship shown by the solid line in FIG. The identified parameters are shown in FIG. The calculation result is stored in the adsorption characteristic storage device 1035 in the block diagram of FIG. 2, and the above formulas (1) and (2) are stored in the calculation formula 1023 of the heavy metal adsorption amount to the soil in the mathematical formula program storage device 102. ing.

For other parameters shown in FIG. 6, for example, the saturated permeability coefficient was obtained by a saturated permeability test. As a result, the saturated hydraulic conductivity of the embankment was 36 cm day ⁻¹ (4 × 10 ⁻⁴ cm s ⁻¹ ). Moreover, the measured value was shown as the dry density of the embankment, the molecular diffusion coefficient of arsenic was shown from the literature value, and the dispersion length was shown as 60 cm as 1/10 of the simulation distance. In addition, for the period from 1986 to 2006, rainfall data was obtained daily from AMeDAS near the target site. For the simulations after 2007, rainfall data from 1986 to 2006 was used repeatedly.

  The arsenic concentration in the groundwater was the arsenic concentration in the soil solution at the depth (5 m from the ground surface) in the center of the saturation zone (4 m to 6 m from the ground surface). Actually, there is lateral groundwater movement in the saturation zone, and uncontaminated groundwater flows from the upstream side, so it is assumed that the concentration in the groundwater will be lower than the predicted value. This concentration was set.

  Next, in the land of the conceptual model shown in FIG. 3, a graph (FIG. 7) showing the calculated value and the result of actual measurement by simulation of the arsenic content in the situation (current state) after 20 years from the start of pollution. As a result of the simulation 2 after 100 years, 300 years, 500 years, and 1000 years after the occurrence of contamination, a graph (FIG. 8) showing the predicted value of the calculated arsenic concentration in the soil solution. The present invention will be described in more detail using a graph (FIG. 9) showing the predicted arsenic concentration in groundwater.

The area of the groundwater contamination occurrence risk estimation simulation in the land shown in FIG. 3 covers the range from the ground surface to 6 m from the unsaturated / saturated ground. Rain was given to the upper boundary condition of the water movement simulation, the evapotranspiration from the bare ground surface was small, and the evaluation was made on the safe side against groundwater contamination, and was ignored here. The lower boundary condition was set such that the constant water head was 200 cm (ground water surface 4 m from the ground surface). In the mass transfer simulation, it was assumed that arsenic was not included in the rain, the upper boundary was the concentration flux condition, and the lower boundary was the concentration gradient zero condition. Arsenic was added from the vicinity of the surface layer. The initial condition was 20 mgkg ⁻¹ for point A and 8 mgkg ⁻¹ for point B, and the initial condition included all arsenic present in the soil layer in the surface layer 10 cm.

The advection diffusion equation (finite element numerical analysis program) was used for the calculation of the groundwater contamination risk estimation simulation. This program is stored in 1021 in the block diagram of FIG. FIG. 7 shows the result of the calculated value and the actual measurement value of the arsenic content in the situation (current state) after 20 years from the start of contamination, as a result of the current state simulation 1 in step S15 in the flow chart described in FIG. is there. In FIG. 7, the vertical axis represents the depth from the ground surface (unit: cm), the horizontal axis represents the total content (unit: mgkg ⁻¹ ), ● represents the actually measured value, and the solid line represents the calculated value. The peak of arsenic concentration in the soil is observed around 25 cm from the ground surface at both point A and point B, and is well reproduced. Further, the calculated value of the maximum concentration was slightly smaller than the actual measurement value.

For example, simulation 2 after 100 years, 300 years, 500 years, and 1000 years after the occurrence of contamination in step S17 in the flowchart of FIG. 1 is also calculated using the advection diffusion equation (finite element numerical analysis program). FIG. 8 shows the predicted value of the arsenic concentration in the soil solution. In FIG. 8, the vertical axis represents the depth from the ground surface (unit: cm), and the horizontal axis represents the total content (unit: mgkg ⁻¹ ), after 100 years, 300 years, 500 years, and 1000 years later. The content of each is indicated by letters. At any point, arsenic at a concentration exceeding the groundwater environmental standard was not detected in the saturation zone (4 to 6 m from the ground surface).

Moreover, both the point A and the point B moved to the deeper peak concentration depth, and a peak was confirmed in the vicinity of 170 cm from the ground surface after 500 years and 220 cm from the ground surface after 1000 years. In addition, the peak concentration decreases with the passage of time, and at point B, after 500 years, it is 0.01 mg ⁻¹ or less, which is the soil elution amount standard and the groundwater environment standard. On the other hand, at point A, although the peak concentration decreased, a concentration exceeding 0.01 mg ⁻¹ was confirmed even after 1000 years.

The predicted arsenic concentration in groundwater up to 1000 years later is shown in FIG. In FIG. 9, the horizontal axis represents the year, the vertical axis represents the arsenic concentration in the groundwater, the 0.01 mgl ⁻¹ horizontal line represents the groundwater environmental standard, and the 0.001 mgl ⁻¹ horizontal line represents the lower limit of quantification. There is no significant change in concentration at any point until 700 years later. At point A, the concentration begins to increase after 700 years and at point B after 950 years. However, even after 1000 years, the groundwater environmental standard (0.01 mgl ⁻¹ ) and the lower limit of quantification (0.001 mgl ⁻¹ ) were not reached.

In this way, it was confirmed that even if the soil leaching amount exceeding the soil elution standard was left as it is, the possibility of groundwater contamination is low until 1000 years later. However, at point A where the maximum concentration was confirmed, the maximum value of arsenic concentration in the soil solution exceeded 0.01 mgl ⁻¹ even after 1000 years (see FIG. 8). It is judged that the risk of occurrence of this remains. On the other hand, since the maximum value of the arsenic concentration in the soil solution is lower than 0.01 mgl ⁻¹ after 500 years at point B, the risk of causing groundwater contamination in the target site is considered to be extremely low.

  Of course, since simulation has uncertainty, it is considered necessary to periodically monitor groundwater and check the arsenic concentration in the groundwater when continuing management with contaminated soil remaining.

Finally, the risk assessment of groundwater contamination due to the failure to implement steps S18, 19, and 20 in the flow chart of FIG. 1, the risk assessment of the outflow of contaminated groundwater, the examination of countermeasure methods based on the risk assessment, and the cost effectiveness First, the risk of groundwater contamination is
Risk _{GW, year} : Groundwater contamination risk of the target heavy metal (subscript year is the target period for risk assessment)
C _GWcal : Calculated concentration of the target substance in groundwater (mgl ⁻¹ , subscript ^max is the maximum value)
C _Gwcriteria : Groundwater environmental standards for target substances (mgl- ¹ )
The following formula (3) is used, assuming that it is calculated by the ratio of the maximum concentration of pollutant concentration in the groundwater, which is calculated during the target period against the groundwater environmental standards. This equation (3) is stored in 1025 of the block diagram shown in FIG.

  Based on the results of this risk assessment, if the calculation result by equation (3) is large, the calculated value will exceed groundwater environmental standards in the target period. Therefore, it is determined that the possibility of groundwater contamination is high. There is a need for countermeasures. Although the calculation results do not exceed the groundwater environmental standards, it is desirable to take positive measures even when there is a possibility of groundwater contamination from the viewpoint of soil heterogeneity. Also, if the calculation result is small, the possibility of groundwater contamination is low. Therefore, monitoring should be performed without taking aggressive measures, and the monicing frequency should be changed according to the value of the calculation result. Then, the cost-effectiveness of the countermeasure in each case is calculated by the countermeasure cost calculation formula stored in 1026 in FIG.

  Based on the contamination status and hydrogeological status obtained from soil surveys in this way, it is difficult to collect the parameters of water movement and material (heavy metal) movement in the ground and estimate the risk of groundwater contamination. It is possible to accurately estimate the risk of contamination of groundwater by heavy metals. In addition, by obtaining a heavy metal concentration profile for the period from the time of the occurrence of contamination to the present, verifying whether it is appropriate, and calculating the future contamination risk, a more accurate occurrence of groundwater contamination will occur. Risk estimation is possible.

  According to the present invention, it is possible to accurately estimate the risk of groundwater contamination by heavy metals, so it is possible to implement countermeasures only when necessary, thereby effectively utilizing land without incurring expensive countermeasure costs. Can do.

It is a schematic flowchart of the estimation method of the groundwater contamination generation | occurrence | production risk which becomes this invention. It is a schematic block diagram of the apparatus which implements the estimation method of the groundwater contamination generation | occurrence | production risk which becomes this invention. It is the figure which showed typically the geological condition of the land to which the actual example of the estimation method of the groundwater contamination occurrence risk which becomes this invention is applied. It is an example of the soil moisture characteristic curve regarding moisture movement. It is the adsorption characteristic of arsenic to the embankment. It is an example of the parameter used for simulation. It is the graph which showed the result of the calculated value and the actual value of the content of arsenic in the situation (present condition) after 20 years from the start of contamination. It is the graph which showed the predicted value of the arsenic density | concentration in the soil 100 years, 300 years later, 500 years later, and 1000 years after pollution generation by the simulation 2. It is the graph which showed the arsenic density | concentration in the groundwater estimated to 1000 years later.

Explanation of symbols

10 Groundwater contamination risk estimation device 101 Control device 1011 CPU
1012 Arithmetic Unit 102 Formula Program Storage Unit 1021 Finite Element Numerical Analysis Program 1022 for Solving the Advection Diffusion Equation 1022 Van Genuchten's Formula 1023 Heavy Amount Adsorption Amount Formula 1024 Freundlich Type Adsorption Isotherm 1025 Risk Evaluation Calculation Formula 1026 Countermeasure Cost Calculation Formula 103 Storage device 1031 Calculation result storage device 1032 Soil moisture characteristic curve storage device 1033 Saturated hydraulic conductivity storage device 1034 Dry density storage device 1035 Adsorption characteristic storage device 11 Input device 12 Output device

Claims (4)

A method for estimating the risk of future groundwater contamination due to these pollutions in heavy metal contaminated land,
A conceptual model of the target site is created based on the pollution status and hydrogeological status at the point where the soil elution amount exceeds the soil elution standard exceeded in the soil contaminated with heavy metals, and the soil moisture characteristic curve, saturation hydraulic conductivity, soil Calculate or obtain parameters for moisture and mass (heavy metal) migration, including particle density, dry density, molecular diffusion coefficient, dispersion length, adsorption properties,
Using the advection diffusion equation with the conceptual model and the calculated or acquired parameters, calculating the heavy metal concentration distribution status in the ground during the period from the predicted occurrence of contamination to the present, and the contamination status obtained by the soil survey In comparison, in the state where the difference between the two is below a predetermined threshold value, the future heavy metal concentration distribution situation is calculated with the conceptual model and the parameter calculated or obtained, and the advection diffusion equation, in the land contaminated with heavy metal A method for estimating the risk of occurrence of groundwater contamination, characterized by estimating the risk of future groundwater contamination caused by these pollutions.   Select and obtain the soil moisture characteristic curve, saturated hydraulic conductivity, soil particle density, dry density, adsorption characteristics stored in advance in the database based on the soil particle size distribution and organic matter content obtained by the soil survey The method for estimating the risk of occurrence of groundwater contamination according to claim 1, wherein, when there is no data to perform, parameters are calculated by an indoor experiment. Calculation of the adsorption characteristics of heavy metals on soil by laboratory experiments
As adsorb heavy metal adsorption per gram of dry _soil (mg g ^-1 )
As _total is the amount of total heavy metals per gram of dry soil (mg g ^-1 )
Dry soil weight (g) using W _soil for analysis
As _solute concentration of heavy metals in soil solution in distilled water extraction test (mg cm ⁻³ )
v the volume of water used in distilled water extraction test _water (cm ³⁾
As _total (before) is the total amount of heavy metals per gram of dry soil before the laboratory experiment (mg g ^-1 )
The amount of heavy metal adsorption in the soil is calculated by the following equation (1), and the total amount of heavy metal in the soil before the test is calculated by the equation (2). A risk estimation method.

The estimation of the groundwater contamination risk is as follows:
Risk _{GW, year} is the target period (year), groundwater contamination risk of the target heavy metal (subscript year is the target period for risk assessment)
C _{GWcal, year} is the target period (year), calculated maximum concentration of the target substance in groundwater (mgl ⁻¹ , subscript ^max is the maximum value)
C _Gwcriteria is the groundwater environmental standard for target substances (mgl ^-1 )
The ratio of the maximum value of the pollutant concentration in the groundwater calculated during the target period with respect to the groundwater environmental standard is calculated by the following equation (3). Method for estimating the risk of groundwater contamination described in 1.

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