A method for repairing residual saline water in a coastal underground reservoir based on groundwater extraction

Abstract

The present invention provides a method for repairing residual saline water in a coastal underground reservoir based on groundwater extraction. The present invention utilizes the commonly existing groundwater extraction wells to establish an optimized well extraction technology that can not only ensure the safe extraction of underground fresh water but also repair the pollution of residual saline water by optimizing the form, distribution, and capacity of the extraction wells, achieving the dual goals of utilizing and protecting the fresh water resources in the coastal underground reservoir. Aiming at the problem of residual saline water pollution in the underground reservoir, the present invention utilizes the commonly existing groundwater extraction wells to establish an optimized well extraction technology that can not only ensure the safe extraction of underground fresh water but also repair the pollution of residual saline water by optimizing the form, distribution, and capacity of the extraction wells.

Description

Technical Field

[0001] The present invention relates to the technical field of coastal saline water remediation, and more particularly, to a method for remediating residual saline water in a coastal underground reservoir based on groundwater extraction. Background Art

[0002] In the late 1980s, due to consecutive years of drought, the extraction of groundwater increased significantly, and environmental geological problems such as large-area groundwater depression funnels and seawater intrusion emerged in many areas. A new way of groundwater development and utilization, the "underground reservoir", began to be applied in the Shandong Peninsula. A water conservancy project built using natural underground water storage spaces with the functions of intercepting, regulating, and utilizing groundwater flow has advantages that cannot be compared with surface projects. The development of underground reservoirs not only effectively increases the supply of water resources but also basically eliminates the increasingly serious seawater intrusion problem in the region, and has important application and promotion value for the development, utilization, and protection of groundwater resources.

[0003] When building an underground reservoir in the coastal area, it is necessary to consider the dual needs of regulating water resources and preventing seawater intrusion, and generally, an underground cut-off dam needs to be constructed to prevent the seaward intrusion of seawater downstream. The commonly used construction materials for underground cut-off dams are concrete, cement slurry, bentonite, etc. The bottom of the dam body is embedded in the aquifer impermeable floor, and the height of the dam body is generally close to the ground or the 0 m contour line, thus forming a full-section physical barrier to the seaward advance of saline water and basically isolating the hydraulic connection between the underground reservoir and the downstream. According to geological conditions, common construction techniques include in-situ mixing method, steel sheet pile method, high-pressure jet grouting method, diaphragm wall method, etc. After the project is implemented, the residual saline water in the dam inevitably remains in the underground reservoir. Due to the isolated hydraulic connection, it cannot be removed through seaward drainage.

[0004] The remaining residual saline water continuously migrates inland, polluting the underground fresh water and becoming a non-negligible pollution source in the underground reservoir. The engineering practice of the Qingdao Dagu River Underground Reservoir shows that 20 years after the construction of the cut-off dam, the polluted area of the residual saline water has increased to 15.17 km 2 . The construction of the underground reservoir is to better utilize and protect fresh water resources, but the accompanying problem of residual saline water pollution goes against the original intention of the project. It is urgent to develop low-cost and effective in-situ remediation technologies to remove the residual saline water and achieve the goal of protecting the fresh water resources of the underground reservoir.

[0005] After the construction of the coastal underground reservoir, due to the isolated hydraulic connection with the downstream, the residual saline water in the underground cut-off dam inevitably remains in the underground reservoir and cannot be removed through seaward drainage. The remaining residual saline water continuously migrates inland, polluting the underground fresh water and becoming a non-negligible pollution source in the underground reservoir. Existing technical solutions are either not universal or are too theoretical and divorced from actual working conditions, and cannot solve the pollution problems of existing underground reservoirs.

[0006] Existing technical solutions

[0007] 1) Publication number of the application: CN114319404A: The present invention provides a subsurface curtain for preventing seawater intrusion and purifying residual brackish water, and its construction method. According to the hydrogeological conditions in coastal areas, relevant areas such as seawater intrusion and underground aquifers are determined; the combined subsurface curtain includes two walls and is distributed front and back along the direction of groundwater flow, including a hanging wall on the side of the inland underground aquifer and a sunken wall on the side close to the sea; the sunken wall is arranged at the bottom of the aquifer, and the lower part is embedded in the impermeable bedrock; the upper opening of the wall is located between the local sea level and the 10% seawater salinity isohaline of the brackish water wedge; the hanging wall is arranged at the upper part of the aquifer, built from the ground surface, with an opening left at the bottom of the aquifer, and the bottom end of the hanging wall needs to be lower than the top end of the sunken wall; the distance between the hanging wall and the sunken wall is 1 / 5 to 4 / 5 of the height of the sunken wall; through the technical solution of the present invention, seawater intrusion can be effectively prevented, and the residual brackish water in the upstream groundwater source area after the establishment of the subsurface curtain can be removed.

[0008] 2) Academic paper "Study on the Desalination Process of Residual Brackish Water behind a Dam in a Phreatic Aquifer under Non-Isothermal Conditions" (Journal of Ocean University of China (Natural Science Edition), 2024, 54(05): 115 - 124): This paper uses a two-dimensional groundwater-salt-heat coupling numerical model at the field scale to study the influence of the temperature difference between fresh and brackish water on the retreat (desalination) process of residual brackish water, and quantitatively evaluates the dynamic changes in the length of the residual brackish water wedge and the total residual salt content after the construction of the dam. The paper assumes that there is a hydraulic connection between the underground reservoir and the downstream. By reducing the height of the underground cut-off dam (53% of the aquifer thickness), it allows fresh water and residual brackish water in the underground reservoir to bypass the dam top and discharge to the sea. Driven by the hydraulic gradient of fresh water, the residual brackish water gradually retreats, achieving the goal of removing the residual brackish water. This research provides a scientific reference for the future construction of underground reservoirs and the design of the height of cut-off dams.

[0009] 3) Academic paper "Utilization of pit lake on the cleaning process of residual saltwater in unconfined coastal aquifers" (Science of the Total Environment, 2021, 770: 144670): In the Dagu River groundwater reservoir in Qingdao, sand mining formed an artificial sand pit with a depth of 8 - 26 m. The groundwater in the groundwater reservoir flowed into the sand pit through lateral recharge, forming a surface pit lake with an area close to 94 hectares. This paper proposed to use the artificial pit lake to accelerate the discharge of residual saltwater, thereby gradually removing the residual saltwater in the groundwater reservoir, and predicted the efficiency of the artificial pit lake in discharging and repairing the residual saltwater through site monitoring and numerical simulation techniques. It was found that when the discharge rate of the pit lake increased to 1.6×10 5 m 3 / d, the repair efficiency increased by 17 times, and the repair time was shortened from 20 years to 5 years.

[0010] Both of the prior arts (1) and (2) assume that the hydraulic connection between the groundwater reservoir and the downstream exists. By reducing the height of the underground cut-off dam, the fresh water and residual saltwater in the groundwater reservoir are allowed to bypass the dam top and discharge to the sea. Driven by the fresh water hydraulic gradient, the residual saltwater gradually retreats to achieve the goal of removing the residual saltwater. Especially for the prior art (1), a suspended cut-off dam with a guiding function is installed upstream of the underground physical dam to divert the groundwater flow into the residual saltwater area and accelerate the purification of the residual saltwater. These methods provide scientific references for the future construction of groundwater reservoirs and the design of the height of cut-off dams, but they are not applicable to the currently established groundwater reservoirs because the currently built underground physical dams often penetrate the aquifer and their height is greater than the thickness of the aquifer, thus completely isolating the hydraulic connection between the groundwater reservoir and the downstream, and the residual saltwater simply cannot discharge to the sea.

[0011] The prior art (3) uses the discharge of groundwater to an artificial pit lake to accelerate the purification of residual saltwater and has achieved good repair effects. However, this method is special. There just happens to be a relatively deep artificial pit lake above the residual saltwater area of the Dagu River groundwater reservoir, allowing groundwater to discharge to the surface water. But this does not mean that such artificial pit lakes exist in all coastal groundwater reservoirs. Therefore, this method is not universal and is not suitable for all coastal groundwater reservoirs.

[0012] In addition to the underground cut-off dam, the groundwater reservoir system also includes groundwater extraction projects, mainly referring to various extraction wells, water collection corridors and auxiliary structures for pumping groundwater. Building a groundwater reservoir is for better water use, and the water intake project plays an important role in the effectiveness of the groundwater reservoir. Summary of the Invention

[0013] In order to make up for the deficiencies of the existing technology, the present invention provides a method for repairing residual saline water in a coastal underground reservoir based on groundwater extraction. The present invention utilizes the commonly existing groundwater extraction wells, and by optimizing the form, distribution and capacity of the extraction wells, an extraction well optimization technology is established that can not only ensure the safe extraction of underground fresh water but also repair the pollution of residual saline water, achieving the dual goals of utilizing and protecting the fresh water resources of the coastal underground reservoir. Aiming at the problem of residual saline water pollution in the underground reservoir, the present invention utilizes the commonly existing groundwater extraction wells, and by optimizing the form, distribution and capacity of the extraction wells, an extraction well optimization technology is established that can not only ensure the safe extraction of underground fresh water but also repair the pollution of residual saline water.

[0014] The present invention is realized through the following technical solutions: A method for repairing residual saline water in a coastal underground reservoir based on groundwater extraction, characterized in that it specifically includes the following steps:

[0015] Step S1: Conduct hydrogeological investigation and monitoring on the coastal aquifer to determine the average sea level position, ground surface, unsaturated zone, water table, and aquifer confining floor. Seawater intrusion will form a saline water intrusion zone at the bottom of the coastal aquifer; by building an impermeable underground physical dam in the saline water intrusion zone to intercept the intruding seawater, the upstream of the underground physical dam is the underground reservoir; due to the isolation effect of the underground physical dam body, part of the saline water remains in the underground reservoir, forming an initial residual saline water area, which can be further divided into an initial high-concentration residual saline water area and an initial low-concentration residual saline water area;

[0016] Step S2: In the underground reservoir, the density of the residual saline water is higher than that of the surrounding underground fresh water. Driven by the density difference between fresh and saline water, the residual saline water continuously intrudes inland. The instantaneous residual saline water area surrounded by the 0.25 g / L isoconcentration line of chloride ions is the polluted area; the range of the polluted area gradually increases with time. The area surrounded by the 17.5 g / L isoconcentration line of chloride ions is the instantaneous high-concentration residual saline water area, and the area surrounded by the 17.5 g / L isoconcentration line of chloride ions and the 0.25 g / L isoconcentration line of chloride ions is the instantaneous low-concentration residual saline water area; the instantaneous high-concentration residual saline water area transports salt to the instantaneous low-concentration residual saline water area, resulting in the range of the instantaneous high-concentration residual saline water area gradually decreasing with time until it disappears. At this time, the entire instantaneous residual saline water area is composed of the instantaneous low-concentration residual saline water area;

[0017] Step S3: Extraction wells are vertically arranged on the underground reservoir. Through the extraction wells, the fresh water in the underground reservoir is pumped. During the pumping process, if the chloride ion concentration in the well water exceeds 0.25 g / L, it means that the pumped water has been polluted by residual saline water, and the pumping is stopped, and the extraction well fails;

[0018] Step S4: The form of the extraction well adopts continuous pumping;

[0019] Step S5: The repair effect of continuous pumping on residual saline water is measured by the total salt content within the instantaneous residual saline water area. SM t The change is measured; the repair efficiency DSE is defined as:

[0020] (1)

[0021] In the formula, SM 0 is the total salt content within the initial residual saline water area.

[0022] Step S6: The potential positions of the production wells are divided into an effective pumping area and a failure area by the failure boundary line. The vertical distance between the failure boundary line and the initial residual saline water area is about 10 m; in the effective pumping area, the closer the production well is to the failure boundary line, the higher its repair efficiency; the repair efficiency of continuous pumping for residual saline water DSE increases by more than 20%; the vertical distance of 50 m from the initial residual saline water area 10-1 is the target line for a 20% increase in repair efficiency; the area enclosed between the target line for a 20% increase in repair efficiency and the failure boundary line is the target area where the repair efficiency of the production well increases by more than 20%;

[0023] Step S7: The capacity of the production well affects the effect of continuous pumping on the repair of residual saline water; the target line for a 20% increase in repair efficiency moves inland. For every 0.5 m 3 / d increase in the pumping flow rate, the target line for a 20% increase in repair efficiency moves 10 m inland, and the target area where the repair efficiency of the production well increases by more than 20% increases;

[0024] Step S8: The production well adopts the form of continuous pumping, and its distribution, i.e., position and capacity, i.e., pumping flow rate, are selected within the target area where the repair efficiency increases by more than 20%.

[0025] The present invention provides a new technology for repairing residual saline water in an underground reservoir. By using continuous production wells located in the optimized target area, it can not only ensure the safe extraction of underground fresh water, but also reduce the polluted area of residual saline water, remove residual salts, and achieve the goal of repairing residual saline water pollution.

[0026] Although the optimized target areas and repair efficiencies of production wells in different coastal underground reservoirs may not be the same, the design ideas of the form, distribution, and capacity of the production wells provided by the present invention can help designers solve the problem of residual saline water pollution in underground reservoirs.

[0027] Due to the adoption of the above technical solutions, the present invention has the following beneficial effects compared with the prior art:

[0028] 1. The present invention provides a new idea for the remediation of residual saline pollution based on groundwater extraction. The production wells located within the optimization target area can, through continuous pumping, not only ensure the safe extraction of fresh groundwater but also reduce the area of residual saline pollution and remove residual salts, thereby achieving the goal of remediating residual saline pollution.

[0029] 2. Universality. The construction of underground reservoirs is for better water use, and production wells are very common in the operation and management of underground reservoirs. Utilizing this common water intake facility to remediate residual saline pollution makes the present invention universal.

[0030] The additional aspects and advantages of the present invention will become apparent in the following description section or be learned through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The above and / or additional aspects and advantages of the present invention will become apparent and be readily understood from the description of the embodiments in conjunction with the following drawings, in which:

[0032] Figure 1 is a schematic diagram of the technology for remediating residual saline water in a coastal underground reservoir based on groundwater extraction;

[0033] Figure 2 is a diagram of the process of landward pollution by residual saline water;

[0034] Figure 3 is the process of suppressing landward pollution by residual saline water through continuous pumping;

[0035] Figure 4 is the distribution of the groundwater flow field in the underground reservoir under continuous pumping;

[0036] Figure 5 is the efficiency of the production well in remediating residual saline water at different pumping rates;

[0037] Figure 6 is the instantaneous change in the area of the residual saline water area and the intrusion distance;

[0038] Figure 7 is the instantaneous change in the salt content in the residual saline water area. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0039] In order to more clearly understand the above objects, features, and advantages of the present invention, the present invention will be further described in detail below in conjunction with the drawings and specific embodiments. It should be noted that, without conflict, the embodiments of the present application and the features in the embodiments can be combined with each other.

[0040] Many specific details are set forth in the following description in order to fully understand the present invention. However, the present invention can also be implemented in other ways different from those described herein. Therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0041] The following will specifically describe Figures 1 to 3 the method for repairing residual brackish water in a coastal underground reservoir based on groundwater extraction in the embodiments of the present invention.

[0042] As Figure 1 shown, the present invention proposes a method for repairing residual brackish water in a coastal underground reservoir based on groundwater extraction, which specifically includes the following steps:

[0043] Step S1: For the technology of repairing residual brackish water in a coastal underground reservoir based on groundwater extraction described in the present invention, the composition schematic diagram is as Figure 1 shown. Conduct hydrogeological investigation and monitoring on the coastal aquifer 1 to determine the average sea level position 2, the ground surface 3, the unsaturated zone 4, the water table 5, the aquifer confining floor 6. Seawater intrusion will form a brackish water intrusion zone 7 at the bottom of the coastal aquifer 1; by building an impermeable underground physical dam 8 in the brackish water intrusion zone 7 to intercept the intruding seawater, the purpose of preventing and controlling seawater intrusion is achieved. The upstream of the underground physical dam 8 is the underground reservoir 9; due to the isolation effect of the dam body of the underground physical dam 8, part of the brackish water is retained in the underground reservoir 9, forming an initial residual brackish water area 10-1, which can be further divided into an initial high-concentration residual brackish water area 11-1 and an initial low-concentration residual brackish water area 12-1;

[0044] Step S2: In the underground reservoir 9, the density of the residual brackish water is higher than that of the surrounding fresh groundwater. Driven by the density difference between the brackish water and the fresh water, the residual brackish water continuously intrudes inland ( Figure 2 ), and the instantaneous residual brackish water area 10-2 surrounded by the isoconcentration line 13 of chloride ion 0.25 g / L is the polluted area; the range of the polluted area gradually increases with time, becoming a serious threat to the fresh water quality in the underground reservoir. The area surrounded by the isoconcentration line 14 of chloride ion 17.5 g / L is the instantaneous high-concentration residual brackish water area 11-2, and the area surrounded by the isoconcentration line 14 of chloride ion 17.5 g / L and the isoconcentration line 13 of chloride ion 0.25 g / L is the instantaneous low-concentration residual brackish water area 12-2; the instantaneous high-concentration residual brackish water area 11-2 transports salt to the instantaneous low-concentration residual brackish water area 12-2, resulting in the range of the instantaneous high-concentration residual brackish water area 11-2 gradually decreasing with time until it disappears. At this time, the entire instantaneous residual brackish water area 10-2 is composed of the instantaneous low-concentration residual brackish water area 12-2;

[0045] Step S3: The purpose of building an underground reservoir is to better utilize water, and the water intake project plays an important role in the performance of the underground reservoir. Extraction wells 15 are vertically arranged on the underground reservoir 9. Through the extraction wells 15, the fresh water in the underground reservoir 9 is pumped out, which is the main groundwater discharge method. During the pumping process, if the chloride ion concentration in the well water exceeds 0.25 g / L, it means that the pumped water has been polluted by the residual brackish water, and the pumping stops, and the extraction well fails;

[0046] Step S4: The form, distribution, and capacity of the production well 15 affect the process of residual saline water landward pollution. The form of the production well 15 includes continuous pumping and intermittent pumping. Intermittent pumping will increase the area of residual saline water pollution and is more likely to cause the production well to fail. Continuous pumping will inhibit the landward intrusion of residual saline water ( Figure 3 ), reducing the area of residual saline water pollution. Therefore, continuous pumping has a positive environmental effect on the problem of residual saline water pollution. The form of the production well 15 adopts continuous pumping;

[0047] Step S5: The remediation effect of continuous pumping on residual saline water is measured by the total salt content SM t change within the instantaneous residual saline water area 10-2; the remediation efficiency DSE is defined as:

[0048] (1)

[0049] In the formula, SM 0 is the total salt content within the initial residual saline water area 10-1.

[0050] Step S6: The distribution of the production well affects the remediation effect of continuous pumping on residual saline water; when the production well is close to the initial residual saline water area 10-1, it is easy to cause the well to fail. Figure 1 In the figure, area 16 is the production well failure area. The potential positions of the production wells are divided into an effective pumping area 18 and a failure area 16 by the failure boundary 17. The vertical distance between the failure boundary 17 and the initial residual saline water area 10-1 is about 10 m; in the effective pumping area 18, the closer the production well 15 is to the failure boundary 17, the higher its remediation efficiency color;

[0051] Without pumping, the landward intrusion of residual saline water increases the instantaneous residual saline water area 10-2. During this process, the salt in the instantaneous residual saline water area migrates to the surrounding fresh water area through convective dispersion, thus achieving the effect of desalination. Taking the remediation efficiency in this natural scenario as a reference, the design requirement of the present invention is that the remediation efficiency of continuous pumping on residual saline water DSE is increased by more than 20%;

[0052] Figure 1 In the figure, 19 is the target line for a 20% increase in remediation efficiency, and the vertical distance of 50 m from the initial residual saline water area 10-1 is the target line 19 for a 20% increase in remediation efficiency; the area enclosed between the target line 19 for a 20% increase in remediation efficiency and the failure boundary 17 is the target area 20 where the remediation efficiency of the production well is increased by more than 20%; for the production well 15 in the target area 20, through continuous pumping, it can not only ensure the safe extraction of underground fresh water, but also reduce the area of residual saline water pollution and remove residual salts, achieving the goal of repairing residual saline water pollution.

[0053] Under the continuous pumping of the production wells in the target area 20, the groundwater flow field distribution in the underground reservoir is as Figure 4 shown. The salt water in the instantaneous high-concentration residual salt water area 11-2 migrates to the instantaneous low-concentration residual salt water area 12-2, while the groundwater flow in the fresh water area on the left side of the 0.25 g / L isoconcentration line 13 of chloride ions is directed towards the instantaneous low-concentration residual salt water area 12-2, thus inhibiting the process of pollution of the instantaneous low-concentration residual salt water area 12-2 towards the inland. At the same time, the groundwater flow in the salt water area on the right side of the 0.25 g / L isoconcentration line 13 of chloride ions is directed towards the production wells, gradually transferring the salt transferred from the high-salt area to the production wells and taking it away from the underground reservoir by pumping, achieving the goal of repairing the residual salt water pollution. During the whole process, the chloride ion concentration in the well water is lower than 0.25 g / L, and the production wells do not fail.

[0054] Step S7: The capacity of the production wells affects the effect of continuous pumping on the repair of residual salt water; Figure 5 shows the efficiency of the production wells in repairing residual salt water under different pumping rates. As the pumping rate increases, the repair efficiency of the production wells gradually increases, and the 20% target line 19 for the improvement of the repair efficiency moves towards the inland. For every 0.5 m 3 / d increase in the pumping rate, the 20% target line 19 for the improvement of the repair efficiency moves 10 m towards the inland, and the target area 20 where the repair efficiency of the production wells increases by more than 20% expands; when the pumping rate is very low, such as 0.2 m 3 / d, the repair efficiency of the production wells does not meet the target requirements, and the target area 20 where the repair efficiency of the production wells increases by more than 20% does not exist.

[0055] Step S8: The production wells adopt the form of continuous pumping, and their distribution, i.e., location and capacity, i.e., pumping rate, are selected within the target area provided for an improvement in the repair efficiency of more than 20%, and the requirements for safely extracting fresh groundwater and repairing the residual salt water pollution can be met. Figure 5 For different coastal underground reservoirs, the boundary conditions of the aquifer, hydrogeological parameters, and the location of the underground physical dam are all different, and the scope of the residual salt water pollution formed is also different. Therefore, for different engineering sites, the optimized target area for the production wells to repair the residual salt water may deviate from that provided by the present invention, and readers can make fine-tuning according to the actual engineering conditions.

[0056] For different coastal underground reservoirs, the boundary conditions of the aquifer, hydrogeological parameters, and the location of the underground physical dam are all different, and the scope of the residual salt water pollution formed is also different. Therefore, for different engineering sites, the optimized target area for the production wells to repair the residual salt water may deviate from that provided by the present invention, and readers can make fine-tuning according to the actual engineering conditions. Embodiment

[0057] Using the professional groundwater numerical simulation software FEFLOW, a variable-density model for the migration of residual salt water in the coastal underground reservoir is established, and the processes of groundwater flow and solute transport are controlled by the Richards equation coupled with the convective-dispersion equation. The length of the entire aquifer is 300 m, and the thickness is 30 m. The average permeability coefficient, porosity, longitudinal dispersion coefficient, transverse dispersion coefficient, and molecular diffusion coefficient of the aquifer are 6×10 -4m / s, 0.4, 1 m, 0.1 m, 1×10 -9 m 2 / s. The sea - level height at the ocean boundary is 28.5 m, and the groundwater level height at the inland fresh - water boundary is 29.6 m. The hydraulic gradient between the two is 0.37%, which is a common hydraulic gradient in coastal aquifers.

[0058] Seawater intrusion occurs under the density difference drive. When it reaches stability, the seawater intrusion distance is 100 m. A subsurface physical dam with a height of 30 m is built 50 m away from the ocean boundary to control seawater intrusion. Inside the subsurface physical dam, that is, in the subsurface reservoir, the initial residual saline - water area 10 - 1 has an area of 510 m 2 . Two cases are defined, namely the no - pumping case and the continuous - pumping case. The no - pumping case is used as a control to observe the landward pollution process of the residual saline water under natural conditions. In the continuous - pumping case, the pumping rate of the production well is 1.0 m 3 / d. In the target area 20 where the repair efficiency of the production well is increased by more than 20%, a location (200 m, 12.5 m) is selected to install the production well to observe the process of continuous - pumping for repairing the residual saline - water pollution.

[0059] The areas and intrusion distances of the instantaneous residual saline - water area 10 - 2 (i.e., the polluted area) and the internal high - salinity area 11 - 2 are statistically analyzed, and the results are as Figure 6 shown. For the pumping and no - pumping cases, the changes in the area and intrusion distance of the instantaneous high - concentration residual saline - water area 11 - 2 with time are basically the same, indicating that pumping has little impact on the instantaneous high - concentration residual saline - water area 11 - 2. Without pumping, the area and intrusion distance of the instantaneous residual saline - water area surrounded by the 0.25 g / L isoconcentration line of chloride ions gradually increase with time. The intrusion distance reaches up to 100% (invading the inland boundary), and the polluted area reaches 20% (occupying the area of the subsurface reservoir) at 20,000 days. However, under continuous pumping, the area and intrusion distance of the instantaneous residual saline - water area surrounded by the 0.25 g / L isoconcentration line of chloride ions first increase and then decrease, and the polluted area decreases to 4% at 20,000 days, showing the inhibitory effect of continuous pumping on the residual saline - water pollution.

[0060] Figure 7It shows the variations of the total salt content (greater than 0.25 g / L) in the instantaneous residual saline area 10-2, the salt content (greater than 17.5 g / L) in the instantaneous high-concentration residual saline area 11-2, and the salt content (0.25 - 17.5 g / L) in the instantaneous low-concentration residual saline area 12-2 over time. The variation trends of the salt content in the high-concentration residual saline area are the same under the pumping and non-pumping conditions, indicating that pumping has little impact on the high-concentration area. The salt content in the high-concentration residual saline area reduces to 0 in 2000 days, and the entire instantaneous residual saline area becomes the low-concentration residual saline area. The total salt content in the instantaneous residual saline area 10-2 gradually decreases over time, but the reduction rate of the residual salt content significantly accelerates under continuous pumping. The restoration efficiency is as high as 95% in 20000 days, which is 28% higher than the natural restoration efficiency without pumping.

[0061] In the description of the present invention, the term "a plurality of" refers to two or more, unless otherwise clearly defined. The orientation or positional relationship indicated by terms such as "upper", "lower", etc. is based on the orientation or positional relationship shown in the drawings, and is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to the present invention; the terms "connection", "installation", "fixation", etc. should all be understood in a broad sense. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be directly connected or indirectly connected through an intermediate medium. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0062] In the description of this specification, the descriptions of terms such as "one embodiment", "some embodiments", "specific embodiments", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or instance. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0063] The above are only the preferred embodiments of the present invention and are not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for repairing residual brackish water in a coastal underground reservoir based on groundwater extraction, characterized in that , specifically including the following steps: Step S1: Conduct hydrogeological surveys and monitoring on the coastal aquifer (1) to determine the mean sea level position (2), the ground surface (3), the unsaturated zone (4), the water table (5), the aquifer confining bedrock (6). Seawater intrusion will form a saltwater intrusion zone (7) at the bottom of the coastal aquifer (1). By constructing an impermeable underground physical dam (8) in the saltwater intrusion zone (7) to intercept the intruding seawater, the area upstream of the underground physical dam (8) is the underground reservoir (9). Due to the isolation effect of the underground physical dam (8), part of the saltwater remains in the underground reservoir (9) to form an initial residual saltwater zone (10-1), which can be further divided into an initial high-concentration residual saltwater zone (11-1) and an initial low-concentration residual saltwater zone (12-1); Step S2: In the underground reservoir (9), the density of the residual saltwater is higher than that of the surrounding fresh groundwater. Driven by the density difference between saltwater and fresh water, the residual saltwater continuously intrudes inland. The instantaneous residual saltwater zone (10-2) surrounded by the 0.25 g / L isochlorine line (13) of chloride ions is the polluted area. The scope of the polluted area gradually increases with time. The area surrounded by the 17.5 g / L isochlorine line (14) of chloride ions is the instantaneous high-concentration residual saltwater zone (11-2). The area surrounded by the 17.5 g / L isochlorine line (14) and the 0.25 g / L isochlorine line (13) of chloride ions is the instantaneous low-concentration residual saltwater zone (12-2). The instantaneous high-concentration residual saltwater zone (11-2) transports salt to the instantaneous low-concentration residual saltwater zone (12-2), resulting in the gradual reduction of the scope of the instantaneous high-concentration residual saltwater zone (11-2) with time until it disappears. At this time, the entire instantaneous residual saltwater zone (10-2) is composed of the instantaneous low-concentration residual saltwater zone (12-2); Step S3: A production well (15) is vertically installed above the underground reservoir (9). Through the production well (15), the fresh water in the underground reservoir (9) is pumped. If the chloride ion concentration in the well water exceeds 0.25 g / L during the pumping process, it indicates that the pumped water has been polluted by the residual saltwater, and the pumping stops, and the production well fails; Step S4: The production well (15) adopts continuous pumping; Step S5: The restoration effect of continuous pumping on the residual saline water is measured by the change in the total salt content within the instantaneous residual saline area (10-2). SM t The restoration efficiency DSE is defined as: （1） In the formula, SM 0 is the total salt content inside the initial residual saline area (10 - 1); Step S6: Divide the potential locations of the production wells into an effective pumping area (18) and a failure area (16) by means of a failure boundary (17). The vertical distance between the failure boundary (17) and the initial residual saline water area (10-1) is approximately 10 m. In the effective pumping area (18), the closer the production well (15) is to the failure boundary (17), the higher its repair efficiency. The repair efficiency of continuous pumping for residual saline water DSE is increased by more than 20%; the vertical distance of 50 m from the initial residual saline water area (10-1) is the 20% target line (19) for the increased repair efficiency; the area enclosed between the 20% target line (19) for the increased repair efficiency and the failure boundary (17) is the target area (20) where the repair efficiency of the production well is increased by more than 20%; Step S7: The capacity of the production well affects the effect of continuous pumping on the remediation of residual saline water; the 20% target line (19) for the improvement of the remediation efficiency moves inland, and for every 0.5 m 3 / d increase in the pumping flow rate, the 20% target line (19) for the improvement of the remediation efficiency moves 10 m inland, and the target area (20) where the remediation efficiency of the production well is increased by more than 20% increases; Step S8: The production well adopts the form of continuous pumping, and its distribution, namely location and capacity, namely pumping flow rate, is selected within the target area where the remediation efficiency is increased by more than 20%.

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