PURIFICATION METHOD OF POPs-CONTAINING SOIL

Abstract

To provide an efficient purification method of POPs-containing soil, capable of reducing an amount of water to be used, and being executed in a short term, in purifying the POPs-containing soil.SOLUTION: There is provided a purification method of POPs-containing soil comprising: a step for embedding and installing a water block wall for surrounding a section including the POPs-containing soil in plan view; a step for installing a withdrawal well and a water pouring well in the section; a step for pouring wash water through the water pouring well; a step for withdrawing ground water in the POPs-containing soil, through the withdrawal well while changing the water intake depth.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for cleaning soil containing POPs.

Background Art

[0002] There is growing concern about the problem of contamination caused by the residue of a group of compounds called POPs (Persistent Organic Pollutants) released into the environment in soil and groundwater. Although many compounds have been pointed out as having the potential to behave as POPs, among them, a group of organic fluorine compounds called PFAS (Perfluoroalkyl Substances and Polyfluoroalkyl Compounds) has attracted particular attention in recent years.

[0003] PFAS is pointed out as a causative substance that causes environmental problems because it is hardly decomposable and remains for a long time. There is a demand to remove those released into the environment and remaining in soil or groundwater. PFAS diffused near the soil surface can be cleaned by excavating the surface soil and performing a washing treatment. However, it is not practical to excavate and clean all of such soil for PFAS accumulated in medium to deep layer soil.

[0004] Since PFAS is a soluble molecule having surface activity, for those diffused below the groundwater level in the underground soil, it is possible to purify them by collecting the groundwater in which PFAS is dissolved and removing PFAS. However, PFAS diffused in the soil above the groundwater level is adsorbed and retained on the surfaces of clay and silt in the soil and at the gas-liquid interface between the interstitial water and voids in the soil due to its own surface activity.

[0005] Figure 1 schematically shows the state of PFAS molecules diffused in the soil above the groundwater level, i.e., in the unsaturated zone. The clay particles P (clay and silt particles) that make up the soil aggregate to form lumps, and the lumps of clay particles P are adjacent to each other with gaps (only 2 lumps of clay particles P are shown in Figure 1, but similar lumps are in contact with each other to form a three-dimensional network with gaps). The lumps of clay particles P are covered by interstitial water W, and the voids are filled with air.

[0006] Under such circumstances, some of the PFAS molecules S adsorb on the surface of the clay particles P with their hydrophobic groups in the molecule facing the clay particles P and their hydrophilic groups facing the interstitial water W. Also, most of the rest are considered to be adsorbed on the gas-liquid interface of the interstitial water W with their hydrophilic groups facing the interstitial water W and their hydrophobic groups facing the voids. In Figure 1, the hydrophobic groups of the PFAS molecules are shown as black circles, and the hydrophilic groups are shown as thick black lines. Also, to avoid clutter in the illustration, reference numerals are attached only to some of them.

[0007] In this state, simply sucking, for example, using a pumping pump can pump up the interstitial water W, but the recovery of PFAS in the soil cannot be expected much. This is because the air bubbles in the unsaturated zone of the soil have a strong affinity for the soil particles, form a meniscus between the soil particles, and tend to stay in place against the flow of the interstitial water W. Even if the interstitial water W is pumped up, the air bubbles in the soil cannot be removed much, and the gas-liquid interface formed in the soil remains. Furthermore, since the PFAS molecules S are strongly adsorbed on the gas-liquid interface due to their surface activity, as long as the gas-liquid interface in the soil remains after pumping up the interstitial water W in the soil, the PFAS molecules adsorbed on such a gas-liquid interface are considered to remain in place. That is, simply pumping up the interstitial water W using a pumping pump cannot eliminate or significantly remove the air bubbles in the soil, and the recovery of PFAS adsorbed on the gas-liquid interface is uncertain. Such a situation is not necessarily limited to PFAS alone and is considered to occur commonly for all POPs having surface activity. Therefore, hereinafter, this specification targets PFAS and POPs having PFAS-like surface activity.

[0008] In view of the above circumstances, it has been proposed to provide injection wells and pumping wells, inject washing water into the soil, and then pump up groundwater after the soil pores are filled with water, thereby purifying the soil and recovering pollutants in the soil.

[0009] Patent Document 1 describes a method for washing and cleaning contaminated soil in which a water barrier is constructed in the contaminated soil so as to surround or sandwich a contaminated area where pollutants are distributed, the ground surface of the contaminated soil surrounded or sandwiched by the water barrier is covered with an airtight material, injection wells and pumping wells are installed through the airtight state, the lower end of the water barrier is separated from the upper surface of an impermeable layer or a semi-permeable layer, water is injected into the permeable layer through the injection well, and water is pumped by a suction pump connected to the pumping well.

[0010] Patent Document 2 describes a method for purifying contaminated soil in a circulation system in which pumping holes and injection holes are drilled in the ground, groundwater is pumped from the pumping holes, pollutants are recovered by a pollutant recovery device, and then water is injected into the injection holes.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0012] In the above method of pumping water after filling the soil with water, the water flow generated in the soil is not constant over the entire range, and a main flow is locally formed. To wash and remove pollutants over the entire soil to be treated, a large amount of water and a long period of time are required, the purification efficiency is low, and there is a high risk that the pollution cannot be sufficiently removed.

[0013] The present invention has been made in view of such circumstances, and an object thereof is to provide a highly efficient purification method that can reduce the amount of water used and can be carried out in a short period of time when purifying POPs-containing soil.

Means for Solving the Problems

[0014] The invention disclosed in the present application to solve the above problems has various aspects, and an outline of typical ones of these aspects is as follows.

[0015] (1) A step of burying and installing a water-blocking wall that surrounds a section containing POPs-containing soil in a plan view, a step of installing a pumping well and an injection well in the section, a step of injecting washing water from the injection well, and a step of pumping groundwater in the POPs-containing soil from the pumping well while changing the pumping depth. A method for purifying POPs-containing soil.

[0016] (2) In (1), the injection well is installed so as to surround the pumping well in a plan view. A method for purifying POPs-containing soil.

[0017] (3) In (2), a step of performing an airtight treatment on the ground surface in the section. A method for purifying POPs-containing soil.

[0018] (4) In (2), a step of filling the ground surface in the section with water. A method for purifying POPs-containing soil.

[0019] (5) In (2), the pumping depth is changed so as to decrease the depth from near the ground surface toward the groundwater level depth. A method for purifying POPs-containing soil.

[0020] (6) In (5), the injection well injects the washing water at a depth near the ground surface. A method for purifying POPs-containing soil.

[0021] In (7)(5), the injection depth of the washing water from the injection well is changed according to the extraction depth, a method for cleaning POPs-containing soil.

[0022] In (8)(1), the water barrier reaches an impermeable layer or a semi-impermeable layer, and the injection well injects the washing water at a depth deeper than the groundwater level depth, a method for cleaning POPs-containing soil.

[0023] In (9)(8), the extraction depth is changed so as to increase the depth from the groundwater level depth toward the vicinity of the ground surface, a method for cleaning POPs-containing soil.

[0024] In any one of (10)(1) to (9), the method for cleaning POPs-containing soil includes a step of adding nanobubbles to the washing water.

[0025] In any one of (11)(1) to (9), the method for cleaning POPs-containing soil includes a step of applying vibration to the POPs-containing soil in parallel with the step of injecting washing water from the injection well.

Brief Description of the Drawings

[0026]

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Embodiments for Carrying Out the Invention

[0027] FIG. 2 is a cross-sectional view of soil schematically showing the state of executing the method for cleaning POPs-containing soil according to the first embodiment of the present invention.

[0028] Soil, although having fine differences depending on its topography, is generally considered to form layers in the order of the unsaturated zone VZ, the saturated zone SZ, and the impervious layer or semi-impervious layer AC from the ground surface GP downward. The height of the ground surface is the ground surface height GL, and the height of the surface of the saturated zone SZ coincides with the groundwater level depth WL, which is the surface where groundwater becomes atmospheric pressure. In the unsaturated zone VZ, there are voids between the particles constituting the soil, and interstitial water and air are mixed. In the saturated zone SZ, the space between the particles constituting the soil is filled with interstitial water.

[0029] Since POPs diffuse from their emission sources on the ground surface downward, the POPs-containing soil PS, which is contaminated soil, is considered to be distributed so as to diffuse from the ground surface downward as shown in Fig. 2. In the purification of the POPs-containing soil PS, the distribution of the POPs-containing soil PS should be grasped in advance by means of borehole surveys or the like.

[0030] In the method for purifying POPs-containing soil according to this embodiment, first, the water cutoff wall 1, the pumping well 2, and the injection well 3 are installed. The water cutoff wall 1 is buried and installed so as to surround the section containing the POPs-containing soil PS in plan view, that is, to define a closed area surrounding the POPs-containing soil PS. The water cutoff wall 1 can be of any type as long as it can block the movement of water in the soil, and the installation method thereof is also arbitrary. Driving of steel sheet piles, construction of a soil-cement wall from trench excavation, etc. may be performed, and a waterproof sheet such as a resin-made one may be used in combination. The lower end of the water cutoff wall 1 needs to reach a sufficient depth, but it is not necessarily required to reach the impervious layer or semi-impervious layer AC. As long as it reaches near the groundwater level depth WL, as shown in Fig. 2, it does not necessarily need to reach the groundwater level depth WL. The upper end of the water cutoff wall 1 is located above the ground surface height GL and is exposed from the ground surface GP.

[0031] The extraction well 2 is installed within the section so as to penetrate the POPs-containing soil PS included in the section. The installation position of the extraction well 2 is preferably near the center in the plan view of the distribution of the POPs-containing soil PS within the section. However, depending on the distribution of the POPs-containing soil PS, or when the distribution of the POPs-containing soil PS is not necessarily clear, this is not always the case. Usually, the installation position of the extraction well 2 is near the center of the section.

[0032] The drilling depth of the extraction well 2 needs to reach near the groundwater level depth WL. More specifically, it is provided so as to reach at least 50% or more, more preferably 75% or more, with respect to the groundwater level depth WL at the installation position of the extraction well 2. It is not necessary for the drilling depth of the extraction well 2 to reach a position deeper than the groundwater level depth WL, but it is acceptable.

[0033] In this embodiment, at least two or more injection wells 3 are provided and installed so as to surround the extraction well 2 in the plan view. The meaning of "the injection well 3 surrounds the extraction well 2" will be described later. The drilling depth of the injection well 3 does not need to be deep, and it is sufficient if it reaches at least a position deeper than the ground surface height GL. The injection well 3 shown in FIG. 2 has a drilling depth close to the ground surface height GL. However, alternatively, it may be deeper and reach near the groundwater level depth WL. The handling when the injection well 3 is drilled deep will be described later.

[0034] Also, a monitoring well 4 may be drilled and installed within the section. The monitoring well 4 is installed so as to reach at least deeper than the groundwater level depth WL. Its main purpose is to monitor whether each process is being carried out normally in the implementation of the purification method, and to detect the fluctuation of the groundwater level, the water injection from the injection well 3, and the situation where the water injection penetrates into the unsaturated zone VZ. The installation position and the number of installation of the monitoring well 4 are arbitrary.

[0035] In addition, the ground surface GP within the compartment is subjected to airtight treatment and covered with an airtight layer 5. The airtight layer 5 may be created by fixing a rubber or resin sheet to the ground surface GP, or by forming a thin layer of cement or concrete, or by a combination thereof or by another method. Also, the joints between the airtight layer 5 and the extraction well 2, the injection well 3, and the monitoring well 4 may be appropriately sealed to maintain airtightness.

[0036] Figure 3 is a ground plan view schematically showing a state in which the method for purifying POPs-containing soil according to the first embodiment of the present invention is being carried out. In the figure, the existence range of the POPs-containing soil PS distributed in the ground is shown shaded.

[0037] As shown in the figure, the water barrier 1 is installed so as to contain and surround the POPs-containing soil PS inside thereof, forming a compartment. In the example of Figure 3, the shape of the compartment is substantially hexagonal, but this shape is not particularly limited and may be different depending on the topography, the arrangement of above-ground buildings, and the presence or absence of underground rocks and other obstacles. Also, when the POPs-containing soil PS is widely distributed and it is difficult to surround all of it with a single compartment, a plurality of compartments are arranged adjacent to each other. In Figure 3, the adjacent compartments installed as necessary are shown by broken lines. The arrangement of the plurality of compartments may be arranged in a honeycomb pattern, a checkerboard pattern, or any other arbitrary planar laying pattern, as shown in Figure 3.

[0038] The pumping well 2 is installed near the center of the section at a position where it is estimated that the POPs-containing soil PS is distributed. In contrast, at least two or more injection wells 3, three in the example of FIG. 3, are installed so as to surround the pumping well 2. Then, in the process of purifying the POPs-containing soil PS, the water injected from the injection well 3 penetrates into the underground soil and is pumped up from the pumping well 2, thereby dissolving and recovering the POPs in the POPs-containing soil PS in water. Therefore, the flow of water in the soil is as shown by F in the figure, and the water injected from the injection well 3 forms a flow so as to wash away the POPs from the periphery of the pumping well 2 toward the pumping well 2.

[0039] In order for such a flow to be formed, the injection well 3 must be installed so as to surround the pumping well 2. For example, the water injected from a specific injection well 3 cannot reach the POPs-containing soil PS distributed on the opposite side of the pumping well 2 as seen from the injection well 3 and cannot wash it. Therefore, in this specification, "the injection well 3 surrounds the pumping well 2" means that for any injection well 3 in the section, at least one other injection well 3 is installed in the section on the opposite side of the pumping well 2 as seen from the injection well 3.

[0040] For example, FIG. 4 shows the state when attention is paid to the injection well 3 located on the right side in the figure of the section shown in FIG. 3. When the injection well 3 being focused on is the injection well 3-1 and the other injection wells 3 are the injection wells 3-2 and 3-3 respectively, as seen from the injection well 3-1, the opposite side of the pumping well 2 means the other side of the straight line passing through the pumping well 2 and perpendicular to the line segment connecting the pumping well 2 and the injection well 3 (shown by the broken line) in plan view, which is the region shown by the hatching in the figure. At least one other injection well 3 is installed in such a region, and here, the two injection wells 3-2 and 3-3 are applicable. The same can be said for the injection wells 3-2 and 3-3.

[0041] Returning to Fig. 2, in order to form the flow of groundwater from the injection well 3 towards the pumping well 2, the negative pressure formed by the suction pump attached to the pumping well 2 must be able to form a pressure gradient from the injection well 3 towards the pumping well 2 in the soil. However, as shown in Fig. 1, the soil in which the flow of groundwater is formed from the injection well 3 towards the pumping well 2 is the unsaturated zone VZ, and there are voids between the soil particles. Therefore, the negative pressure acting through the pumping well 2 acts on the ground surface through the voids between the soil particles, sucking in the atmosphere from the ground surface, preventing the formation of a pressure gradient from the injection well 3 towards the pumping well 2.

[0042] Therefore, the ground surface GP within the section is covered with the airtight layer 5 so that the negative pressure in the soil does not transmit to the atmosphere, thereby correctly forming the pressure gradient in the soil and enabling the formation of the groundwater flow as shown in Fig. 3.

[0043] Subsequently, in the method for cleaning POPs-containing soil according to the present embodiment, washing water is injected from the injection well 3 into the soil, and pumping is performed by the suction pump attached to the pumping well 2 to dissolve and recover the POPs in the POPs-containing soil PS in the washing water. The pumped and recovered washing water may be reused as the washing water to be injected again from the injection well 3 after removing the POPs in the solution by appropriate treatment.

[0044] Here, the pumping well 2 is configured to be able to change its water intake depth, and the groundwater in the POPs-containing soil PS is pumped while changing the water intake depth. In the present embodiment, the water intake depth is changed from a shallow position close to the ground surface height GL towards the groundwater level depth WL, intermittently or continuously becoming deeper. This operation will be described with reference to Figs. 5 to 7.

[0045] Figs. 5 to 7 are cross-sectional views of the soil for schematically explaining the procedure of the method for cleaning POPs-containing soil according to the present embodiment. First, as shown in Fig. 5, the water intake position 20 of the pumping well 2 is set at a shallow position relatively close to the ground surface height GL, and pumping by a suction pump connected to the pumping well 2 and injection of washing water from the injection well 3 are performed simultaneously. At this time, the washing water injected from the injection well 3 penetrates into the soil in the unsaturated zone VZ to form a temporary saturated region TS, and due to the pressure gradient generated in the soil by the negative pressure generated by the suction force at the water intake position 20 of the pumping well 2, a water flow WF as indicated by the arrow in Fig. 5 is generated. At this time, since the airtight layer 5 is provided on the ground surface GP within the section, the negative pressure applied to the soil is prevented from escaping from the ground surface GP into the atmosphere. In the temporary saturated region TS, sufficient washing water injected from the injection well 3 flushes away the bubbles in the soil pores and causes them to disappear, and the POPs molecules present in the region can no longer adsorb to the gas-liquid interface. While a part of them adsorbs to the soil particle surface, most of the rest dissolve in the groundwater or washing water in the soil.

[0046] The water flow WF becomes a flow dominated by the horizontal component that flows from around the pumping well 2 toward the pumping well 2 around the water intake depth where the water intake position 20 is set. And since most of the POPs distributed near the water intake depth in the POPs-containing soil PS are in a state of being dissolved and released in the groundwater or washing water, they easily move together with the water flow WF, are hardly diffused, and are flushed toward the pumping well 2 and recovered and removed.

[0047] Pumping and injection are continued for a time sufficient to sufficiently remove the POPs distributed near the water intake depth. This time may be continued until the concentration of POPs in the pumped groundwater is measured and confirmed to be less than a predetermined concentration, or may be determined in advance based on the measurement result of the POPs concentration near the water intake depth in the pre-existing POPs-containing soil PS.

[0048] Subsequently, as shown in FIG. 6, the pumping depth of the water intake position 20 of the pumping well 2 is changed to a deeper position. Similarly, pumping by the suction pump connected to the pumping well 2 and injection of cleaning water from the injection well 3 are performed simultaneously. The water flow WF generated at this time becomes a flow dominated by the horizontal component, flowing from the periphery of the pumping well 2 toward the pumping well 2 around the changed pumping depth, as shown in the figure. Therefore, no main flow occurs in the shallow region that has already been purified, and the flow mainly passes through the unpurified region in the POPs-containing soil PS, and the POPs distributed near the pumping depth can be efficiently washed away and collected and removed into the pumping well 2.

[0049] Similarly, as shown in FIG. 7, the pumping depth of the water intake position 20 of the pumping well 2 is changed to an even deeper position, and the pumping depth is set near the groundwater level depth WL. Similarly, pumping by the suction pump connected to the pumping well 2 and injection of cleaning water from the injection well 3 are performed simultaneously. The water flow WF generated at this time also becomes a flow dominated by the horizontal component, flowing from the periphery of the pumping well 2 toward the pumping well 2 around the changed pumping depth, as shown in the figure. In this way, by appropriately changing the pumping depth from near the ground surface toward the groundwater level depth while performing pumping from the pumping well 2 and injection from the injection well 3, POPs in the POPs-containing soil PS can be efficiently recovered and removed at each depth, so that the purification of the POPs-containing soil PS is made more efficient, the time required for purification is shortened, and the total amount of cleaning water required for purification is reduced.

[0050] In addition, in the method described with reference to FIGS. 5 to 7, the pumping depth of the water intake position 20 of the pumping well 2 was changed in three steps, but it is not limited to this, and it may be four steps or more, or two steps. Alternatively, instead of making these intermittent steps, it may be changed continuously.

[0051] Any method for changing the water intake position 20 of the pumping well 2 may be used. As an example, a pumping well 2 with a structure as shown in FIG. 8 can be used. The pumping well 2 may be formed by vertically drilling through the unsaturated zone VZ soil and inserting a casing pipe 21 to prevent the collapse of soil into the wellbore. Groundwater may flow into the casing pipe 21 intermittently or through a mesh screen 22 provided on the entire surface or on the side surface of the casing pipe 21.

[0052] At the water intake position 20 of the pumping well 2, a suction pump 23, which is a submersible pump, is inserted and installed from the upper part of the pumping well 2. Packings 24 are provided above and below the suction pump 23 to seal the space between the inner surface of the casing pipe 21 in a liquid-tight manner. As a result, the negative pressure generated by the operation of the suction pump 23 does not spread in the vertical direction of the casing pipe 21, but acts on the unsaturated zone VZ soil through the mesh screen 22 in the section sandwiched between the upper and lower packings 24.

[0053] The groundwater sucked by the suction pump 23 is sent to the ground surface through the pumping pipe 25. The suction pump 23 is fixed to the tip of the pumping pipe 25, and by changing the insertion depth of the pumping pipe 25, the installation depth of the suction pump 23, that is, the water intake depth, can be arbitrarily changed.

[0054] Also, as a modification of the first embodiment, the injection depth of the injection well 3 may be changed according to the water intake depth of the pumping well 2. FIG. 9 is a cross-sectional view of the soil schematically illustrating the state of performing the method for cleaning POPs-containing soil according to the modification of this embodiment.

[0055] FIG. 9 shows the state of performing the procedure corresponding to the procedure in FIG. 6 in the previous description. That is, it represents the situation where the recovery and removal of POPs contained in a relatively shallow position of the POPs-containing soil PS close to the ground surface height GL are completed, and the water intake position 20 of the pumping well 2 is changed to a deeper position.

[0056] In this modification example, the injection well 3 is installed at the same depth as the extraction well 3, that is, near the groundwater level WL, and its injection position 30 can be changed. The structure of the injection well 3 can adopt the same structure as that shown in the example of the structure of the extraction well 2 above. By providing a packing on the injection pipe inserted into the casing pipe so that the washing water flows out only at a specific depth into the unsaturated zone VZ soil through the mesh screen provided on the side surface of the casing pipe.

[0057] In this modification example, as shown in FIG. 9, by making the injection depth of the injection well 3 and the extraction depth of the extraction well 3 approximately the same, most of the water flow WF in the temporarily formed saturated region TS can be made into a horizontal component, and more efficiently, the POPs in the POPs-containing soil PS are washed away to the extraction position 20 of the extraction well 2 and recovered and removed.

[0058] As a further modification example of the present embodiment, instead of creating the airtight layer 5 as the airtight treatment applied to the ground surface GP within the section, the ground surface GP within the section may be filled with water. The water filled on the ground surface GP penetrates into the soil, but by managing the water level of the water filling the ground surface GP and appropriately adding water, the ground surface GP is kept in a flooded state, preventing the intrusion of the atmosphere from the ground surface into the soil, and preventing the negative pressure acting on the soil by the extraction well 3 from escaping into the atmosphere.

[0059] The water penetrating from the ground surface GP merges with the washing water injected from the injection well 3 and is used as the washing water for washing the POPs-containing soil PS. At this time, since the water flow WF in the saturated region TS generates a flow caused by the water penetrating from the ground surface GP, it is considered that the horizontal component flowing from around the extraction well 2 toward the extraction well 2 becomes weaker compared to the example described with reference to FIGS. 5 to 7. However, it is effective when the situation of the ground surface GP within the section, for example, the presence of a building, etc., includes areas where it is difficult or partially difficult to create the airtight layer 5.

[0060] FIG. 10 is a cross-sectional view of the soil schematically showing the state of implementing the method for cleaning POPs-containing soil according to the second embodiment of the present invention.

[0061] In the second embodiment, different from the previous embodiment, the water barrier 1 is provided to reach the impervious layer or semi-impervious layer AC, and blocks the movement of groundwater within the section from the outside of the section. Further, the injection well 3 is provided to reach a sufficient depth so as to inject the washing water at a depth deeper than the groundwater level depth WZ. The injection well 3 may be a single one, and it is not always necessary that a plurality of injection wells 3 are installed so as to surround the pumping well 2. The pumping well 2 and the monitoring well 4 are the same as those in the previous embodiment. The creation of the airtight layer is not necessarily required.

[0062] Figs. 11 to 13 are cross-sectional views of the soil schematically explaining the procedure of the method for cleaning POPs-containing soil according to this embodiment. First, as shown in Fig. 11, the washing water is injected into the saturated zone SZ from the injection well 3. At this time, since the washing water is blocked by the water barrier 1 and cannot flow out of the section, as shown by the water flow WF, it propagates in the groundwater and raises the groundwater level. The injection from the injection well 3 is performed until all the soil within the section is saturated as shown in the figure. The saturation state of the soil within the section may be monitored via the monitoring well 4.

[0063] When the soil within the section is saturated, as shown in Fig. 12, the groundwater is pumped from the pumping well 2. Here, the pumping well 2 is configured to be able to change its pumping depth in the same manner as in the previous embodiment, and the depth of the pumping position 20 is set near the groundwater level depth WL. At the same time, in order to maintain the saturated state of the soil within the section, the injection from the injection well 3 is continuously performed. The water flow WF generated at this time becomes a flow dominated by the horizontal component that flows from around the pumping well 2 toward the pumping well 2 near the pumping depth, as shown in the figure. Therefore, the POPs distributed near the pumping depth of the pumping well 2 can be efficiently washed away and collected by the pumping well 2.

[0064] If the purification of the POPs-containing soil PS distributed near the set pumping depth has been sufficiently performed, the pumping depth of the pumping well 2 is changed stepwise or continuously from the groundwater level depth WL toward the vicinity of the ground surface, and the groundwater is pumped in the same manner to purify the POPs-containing soil PS.

[0065] Figure 13 shows the state of purification of the POPs-containing soil PS when the water intake depth of the pumping well 2 is changed stepwise or continuously and the water intake position 20 is set near the ground surface. The POPs distributed at deeper positions have already been recovered and removed. If the purification of the POPs-containing soil PS remaining near the ground surface is sufficiently carried out, the purification of the soil within the section is completed.

[0066] In this embodiment, although the change is made to increase from the groundwater level depth WL towards the vicinity of the ground surface, alternatively, the change may be made to decrease from the vicinity of the ground surface towards the groundwater level depth WL. However, with the pumping by the pumping well 2, the washing water flowing from the groundwater level depth WL towards the vicinity of the ground surface brings about the effect of diffusing the POPs diffused at deeper positions upwards. Therefore, it is necessary to pay attention so that such POPs can also be sufficiently recovered and removed.

[0067] Figure 14 is a cross-sectional view of the soil schematically explaining the state of executing the method for purifying the POPs-containing soil according to the modified example of the second embodiment. In this modified example, the pumping well 2 is not configured to be able to change the water intake height, and simply pumps the groundwater flowing into the well from the wall surface of the pumping well 2. At this time, after saturating the soil within the section by the water injection from the injection well 3, pumping is performed from the pumping well 2, and as shown in the figure, regardless of the depth in the soil, a water flow WF from the periphery of the pumping well 2 towards the pumping well 2 is formed to purify the POPs-containing soil PS.

[0068] In this case, due to various conditions such as the distribution of the soil and the interstitial density in the ground, the water flow WF does not occur evenly. Therefore, in order to sufficiently purify the POPs-containing soil PS, sufficient time of water injection and pumping may be performed, or the amount of water injection may be changed, or the water injection may be performed while changing the presence or absence of water injection to raise and lower the groundwater level within the section for purification.

[0069] In each of the embodiments described above and their modifications, the washing water injected from the injection well 3 may be simply water, or may be one to which nanobubbles are added in order to more easily adsorb and remove POPs in the POPs-containing soil PS. POPs molecules having surface activity such as PFAS have the property of being easily adsorbed to the gas-liquid interface between water and air. When bubbles are contained in the washing water and a gas-liquid interface exists, POPs molecules are trapped at the gas-liquid interface of such bubbles and carried along with the water flow. Therefore, it is considered that the recovery and removal efficiency of POPs increases dramatically.

[0070] Generally, bubbles in water cannot exist stably for a long time. In the case of bubbles introduced into the soil, it is considered that they merge with the air existing in the gaps in the soil particles and collapse, and thus do not function as a transport medium for POPs molecules. However, in the case of so-called nanobubbles in which the size of the bubbles is sufficiently small, it is known that the bubbles hardly merge naturally and can stably exist in water for an extremely long period of time.

[0071] Therefore, by adding nanobubbles to the washing water injected from the injection well 3, POPs molecules can be trapped at the gas-liquid interface, and after permeating through the soil, they can be recovered by pumping from the pumping well 2. The recovered nanobubbles trapping POPs molecules can be collapsed, for example, by applying ultrasonic waves, etc., to recover the POPs molecules.

[0072] Similarly, in each of the embodiments described above and their modifications, when injecting cleaning water from the injection well 3, vibration may be applied to the POPs-containing soil within the compartment in parallel. When implementing the method for cleaning POPs-containing soil in each of the embodiments of the present invention and their modifications, it is essential to push out and remove the air bubbles in the soil pores of the unsaturated zone VZ by injecting sufficient cleaning water from the injection well 3 to form a temporary saturated zone TS. As already explained, the air bubbles in the soil have a strong affinity for the soil particles and tend to stay in place, requiring time for their removal, and it is assumed that there may be cases where it is difficult to obtain a temporary saturated zone TS. In such cases, by applying appropriate vibration to the soil where the water injection is being carried out, defoaming from the soil can be promoted, and it can be expected that more efficient cleaning of the POPs-containing soil can be achieved.

[0073] The method of applying vibration to the soil is not particularly limited. For the soil near the ground surface GL, general equipment such as a concrete vibrator or a vibration compactor can be used. For soil at a deeper depth, vibration compaction methods such as the vibrofloatation method or the rod compaction method, which are generally used for soil compaction in soft ground improvement works, can be employed. The depth of the soil to which vibration is applied may be changed according to the pumping depth of the pumping position 20 of the pumping well 2 in the method for cleaning POPs-containing soil according to each of the above-described embodiments and their modifications. That is, the depth at which vibration is applied is appropriately changed to be the same as or near the pumping depth. Alternatively, it is also conceivable to use the casing pipe 21 in the pumping well 2 or the injection well 3 or both as a vibration transmission body and apply vibration by a vibrator to the part exposed on the ground, and these may be combined.

[0074] Figure 15 is a flowchart showing the procedure of the method for cleaning POPs-containing soil according to the first embodiment of the present invention.

[0075] First, in plan view, an impervious wall surrounding the section containing the POPs-containing soil is buried and installed (S11). Subsequently, a pumping well and an injection well are installed within the section (S12). At this time, the injection well is installed so as to surround the pumping well in plan view. Also, a monitoring well may be installed. Thereafter, an airtight treatment is performed on the ground surface within the section, or a water seal treatment is performed by filling the ground surface within the section with water (S13). Up to this point is the preliminary preparation for purification.

[0076] Thereafter, cleaning water is injected from the injection well, and groundwater in the POPs-containing soil is pumped from the pumping well (S14). At this time, nanobubbles may be added to the cleaning water, or the soil may be vibrated. As the purification of the POPs-containing soil progresses, groundwater is pumped while changing the pumping depth of the pumping well (S15). At this time, the injection well may be one that injects the cleaning water at a depth near the ground surface, and as in the modification of the first embodiment, the injection depth of the injection well may be changed simultaneously. Also, the pumping depth is changed so as to decrease from near the ground surface toward the groundwater level depth.

[0077] If the planned purification of the POPs-containing soil is completed, the injection of cleaning water from the injection well and the pumping of groundwater from the pumping well are stopped (S16). Thereafter, it is inspected whether the purification of the POPs-containing soil is carried out as intended (S17). This inspection may be performed, for example, by measuring the POPs concentration in the pumped groundwater, or by collecting soil at one or a plurality of specific locations within the section and measuring the amount of POPs contained therein.

[0078] As a result of the inspection, if the purification of the POPs-containing soil is insufficient, cleaning water is injected again from the injection well, and groundwater in the POPs-containing soil is pumped from the pumping well, and the process is repeated thereafter (return to S14). Also, as a result of the inspection, if the purification of the POPs-containing soil is sufficient, each facility installed in the section, that is, the impervious wall, the pumping well, the injection well, the airtight layer, and the monitoring well are removed, and the purification of the POPs-containing soil is terminated (S18).

[0079] FIG. 16 is a flowchart showing the procedure of the method for cleaning POPs-containing soil according to the second embodiment of the present invention.

[0080] First, in plan view, a water barrier surrounding the section containing the POPs-containing soil is buried and installed (S21). At this time, the water barrier reaches an impervious layer or a semi-impervious layer. Subsequently, a pumping well and an injection well are installed in the section (S22). At this time, the injection well injects the cleaning water at a depth deeper than the groundwater level depth. Also, a monitoring well may be installed. Up to this point is the preliminary preparation for cleaning.

[0081] Thereafter, cleaning water is injected from the injection well until the soil in the section is saturated (S23). At this time, nanobubbles may be added to the cleaning water, or the soil may be vibrated. If the soil in the section is saturated, pumping from the pumping well 2 is started (S24). And as the cleaning of the POPs-containing soil progresses, the pumping depth of the pumping well is changed (S25). At this time, the pumping depth may be changed so as to increase the depth from the groundwater level depth toward the vicinity of the ground surface.

[0082] If the planned cleaning of the POPs-containing soil is completed, the injection of the cleaning water from the injection well and the pumping of the groundwater from the pumping well are stopped (S26). Thereafter, it is inspected whether the cleaning of the POPs-containing soil is performed as intended (S27). This inspection may be performed, for example, by measuring the POPs concentration in the pumped groundwater, or by collecting the soil at one or a plurality of specific locations in the section and measuring the amount of POPs contained therein.

[0083] As a result of the inspection, if the cleaning of the POPs-containing soil is insufficient, cleaning water is injected again from the injection well, and groundwater in the POPs-containing soil is pumped from the pumping well, and the process is repeated thereafter (return to S24). Also, as a result of the inspection, if the cleaning of the POPs-containing soil is sufficient, each facility installed in the section, that is, the water barrier, the pumping well, the injection well, and the monitoring well are removed, and the cleaning of the POPs-containing soil is terminated (S28).

Description of Reference Numerals

[0084] 1 Water shielding wall, 2 Lift well, 3 Injection well, 4 Monitor well, 5 Airtight layer, 20 Water intake position, 21 Casing pipe, 22 Mesh screen, 23 Suction pump, 24 Packing, 25 Lift pipe, 30 Injection position, S PFAS molecules, P Mud particles, W Interstitial water, GP Ground surface, VZ Unsaturated zone, SZ Saturated zone, AC Impervious layer or semi-impervious layer, GL Ground surface height, WL Groundwater level depth, TS Saturated region, WF Water flow.

Claims

1. In plan view, a step of burying and installing a water barrier surrounding a section containing POPs-containing soil; A step of installing a pumping well and an injection well in the section; A step of injecting washing water from the injection well; A step of pumping groundwater in the POPs-containing soil while changing the pumping depth from the pumping well; A method for purifying POPs-containing soil having the above steps.

2. The injection well is installed so as to surround the pumping well in plan view. The method for purifying POPs-containing soil according to Claim 1.

3. A step of performing an airtight treatment on the ground surface in the section is included. The method for purifying POPs-containing soil according to Claim 2.

4. A step of filling water on the ground surface in the section is included. The method for purifying POPs-containing soil according to Claim 2.

5. The pumping depth is changed so as to decrease the depth from near the ground surface toward the groundwater level depth. The method for purifying POPs-containing soil according to Claim 2.

6. The injection well injects the washing water at a depth near the ground surface. The method for purifying POPs-containing soil according to Claim 5.

7. The injection depth of injecting the washing water from the injection well is changed according to the pumping depth. The method for purifying POPs-containing soil according to Claim 5.

8. The water barrier reaches an impermeable layer or a semi-permeable layer. The injection well injects the washing water at a depth deeper than the groundwater level depth. The method for purifying POPs-containing soil according to Claim 1.

9. The pumping depth is changed so as to increase the depth from the groundwater level depth toward near the ground surface. The method for purifying POPs-containing soil according to Claim 8.

10. A step of adding nanobubbles to the washing water is included. The method for purifying POPs-containing soil according to any one of Claims 1 to 9.

11. A step of applying vibration to the POPs-containing soil in parallel with the step of injecting washing water from the injection well is included. The method for purifying POPs-containing soil according to any one of Claims 1 to 9.

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