Waste uranium mine underground water radionuclide in-situ PRB restoration method

Abstract

The invention provides a waste uranium mine underground water radionuclide in-situ PRB remediation method, which comprises the following steps: determining pollution plume distribution and key hydraulic parameters through site hydrogeological survey and system monitoring; the key migration form of radionuclide in underground water is accurately diagnosed by utilizing geochemical simulation; based on the key migration form of radionuclides, targeted screening is conducted on efficient reaction media matched with the radionuclides, the needed hydraulic retention time is determined through a dynamic column experiment, the core thickness of the PRB wall is quantitatively calculated in combination with the underground water flow velocity, and precise design and construction are achieved; after the system is operated, effect verification is carried out according to double standards of pollutant concentration standard reaching and human health risk controllability. According to the method, the crossing from empirical repair to data-driven quantitative targeted repair is realized, and the method has the outstanding advantages of high removal efficiency, scientific and reliable design, high long-term predictability and health guarantee as an ultimate target.

Description

Technical Field

[0001] This invention relates to the field of in-situ remediation technology for groundwater pollution, and in particular to a method for in-situ remediation of radioactive nuclides (PRB) in groundwater from abandoned uranium mines. Background Technology

[0002] During the long period following mining, beneficiation, and closure of uranium mines, tailings, waste rock, and other waste materials continuously release radioactive nuclides such as uranium (U) and radium (Ra) through precipitation leaching. This forms a high-concentration pollution plume that migrates downstream into groundwater, posing a long-term and serious threat to regional water environment safety and human health. Currently, traditional "extraction-treatment" technologies for this type of pollution generally have inherent limitations, such as high operating costs, susceptibility to tailing and rebound effects, and potential disturbance to natural groundwater flow fields. These limitations make it difficult to meet the long-term needs for efficient and economical remediation of complex uranium mining sites.

[0003] Permeable Reactive Barrier (PRB) technology, as an efficient in-situ passive remediation method, constructs a permeable wall filled with active media along the groundwater flow path to intercept and transform pollutants. It has significant advantages such as good treatment effect, low long-term maintenance cost, and minimal environmental disturbance, and is considered a promising alternative.

[0004] However, when PRB technology is directly applied to groundwater remediation in abandoned uranium mines, a series of key bottlenecks still exist: First, in the typical neutral to weakly alkaline, high-carbonate groundwater environment of the mining area, uranium mainly forms highly soluble and highly mobile uranyl carbonate anions (such as UO2(CO3)3). 4- Conventional PRB (Pollution Remediation Bypass) designs are mostly based on general heavy metal pollution models. The media used (such as ordinary zero-valent iron and zeolite) are not sufficiently targeted to specific forms of uranium, resulting in limited removal efficiency and easy saturation and failure of the media. Secondly, uranium mining sites have highly heterogeneous hydrogeological structures (e.g., well-developed fissures and strong heterogeneity), and groundwater flow direction and rate are variable. Existing designs rely heavily on experience and fail to integrate key hydrogeological parameters (such as permeability coefficient K) with the three-dimensional spatial distribution of the pollution plume for quantitative design. This results in a lack of scientific basis for key parameters such as wall location and thickness, easily leading to pollution plume "bypassing" or incomplete treatment. Furthermore, in current technical practices, site investigation, pollutant morphology diagnosis, engineering design, and effect evaluation are often disconnected, failing to form a data-driven, interconnected systematic technical process, which restricts the integrity, specificity, and predictability of remediation solutions. Therefore, there is an urgent need for an innovative systematic approach to overcome the above-mentioned shortcomings and achieve efficient, stable, and predictable in-situ remediation. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a systematic, quantitative, and highly targeted method for in-situ PRB remediation of radionuclides in groundwater from abandoned uranium mines.

[0006] This invention provides a method for in-situ remediation of radioactive nuclides in groundwater from abandoned uranium mines using radioactive rbore (PRB), comprising the following steps: S1. Conduct hydrogeological drilling downstream of the target uranium mine contaminated area to construct a monitoring well network; obtain the permeability coefficient K of the aquifer through on-site pumping tests, and simultaneously measure the stable water level of each monitoring well to determine the hydraulic gradient I and groundwater flow direction; systematically collect groundwater samples, measure the hydrochemical parameters of each sample, and comprehensively draw a three-dimensional spatial distribution map of the pollution plume. S2. Based on the hydrochemical parameters obtained in step S1, perform geochemical simulation to identify the key migration chemical forms of target radionuclides in groundwater during different hydrological periods and their proportions. S3. Based on the parameters determined in step S1, calculate the actual average flow velocity of groundwater. v Based on the key migration patterns diagnosed in step S2, highly efficient reaction media are targeted for screening, and the hydraulic retention time t required for the reaction media to achieve the preset removal standard for the target pollutant is determined through dynamic column experiments; according to the formula Calculate the core thickness L of the PRB wall, and comprehensively determine the layout, total length and depth of the PRB wall; S4. Based on the PRB wall design in step S3, excavate a trench at the selected location and construct a seepage-proof curtain; fill the trench with the selected reaction medium according to the design sequence and proportion to form the PRB wall; wherein, the polluted groundwater flows naturally through the PRB wall under the drive of the natural hydraulic gradient. S5. After the system is in operation, the concentration of the target radionuclides in the downstream effluent of the PRB wall should be monitored regularly to assess whether the radionuclides have reached the remediation target value based on health risks.

[0007] Further, the water chemical parameters mentioned in step S1 include pH value, redox potential Eh, major ion concentration, and target radionuclide concentration; the major ions include, but are not limited to, K+. + Na + Ca² + Mg² + Cl - SO4² - HCO3 - The target radionuclide is 2³ 8 U and²² 6 Ra.

[0008] Furthermore, the different hydrological periods mentioned in step S2 include the high-water period, the normal-water period, and the low-water period; the key migrating chemical form includes uranyl carbonate complex; wherein, , This is the dominant form.

[0009] Furthermore, the formula for calculating the actual average flow velocity v of groundwater in step S3 is as follows: , where n is the effective porosity of the aquifer.

[0010] Furthermore, the highly efficient reaction medium for targeted screening in step S3 includes at least anion exchange resins that have a specific adsorption effect on anionic uranium.

[0011] Furthermore, the reaction medium also includes zero-valent iron composite material and activated carbon; the filling order of the reaction medium in the PRB wall along the direction of groundwater flow is as follows: anion exchange resin layer, zero-valent iron composite material layer, and activated carbon layer.

[0012] Furthermore, in step S4, the seepage-proof curtain is composed of at least one layer of compacted clay lining and at least one layer of polymer geomembrane.

[0013] Furthermore, the health risk-based repair target value mentioned in step S5 is: radionuclide²³ 8 The concentration of U is below 30 μg / L,²² 6 The concentration of Ra is less than 1 Bq / L, and the annual effective dose due to drinking water is less than 0.1 mSv.

[0014] Compared with the prior art, the present invention has the following beneficial effects: 1) High targeting and efficiency: Through geochemical simulation and correlation analysis, the migration form of uranium in groundwater is accurately identified, mainly in the form of uranyl carbonate anions. Based on this, specific reaction media (such as anion exchange resins) are screened, breaking through the limitations of blind selection and low efficiency of traditional media, and achieving a leap from "extensive treatment" to "morphology-targeted remediation".

[0015] 2) Parameter-driven, reliable design: Innovatively, the hydrogeological parameters such as permeability coefficient (K) and hydraulic gradient (I) measured on site are deeply integrated with the spatial distribution of the pollution plume and the dynamic parameters of the medium (hydraulic residence time t), and the thickness of the PRB wall is quantitatively designed, so that the engineering dimensions have a clear scientific basis, significantly improving the reliability of interception and effectively avoiding the risks of "bypassing" and incomplete treatment.

[0016] 3) Closed-loop process with strong predictability: A complete technical system encompassing "investigation, diagnosis, quantitative design, construction, and verification" is established to achieve data integration and decision-making linkage across all stages, forming a closed-loop workflow. This method enhances the systematic nature, replicability, and predictability of the remediation plan, overcoming the drawbacks of fragmented processes and reliance on experience in traditional technologies. Attached Figure Description

[0017] Figure 1 A flowchart illustrating the steps of an in-situ PRB remediation method for radioactive nuclides in groundwater of an abandoned uranium mine, provided as an embodiment of the present invention.

[0018] Figure 2 This is a drilling layout diagram for the working area of ​​an abandoned uranium mine.

[0019] Figure 3 The graph shows the relationship between descent depth and logarithm of time for ZK1, ZK2, ZK4, and ZK5; among them, Figure 3 In the diagram, A represents the logarithmic relationship between the drawdown of borehole ZK1 and time, B represents the logarithmic relationship between the drawdown of borehole ZK2 and time, C represents the logarithmic relationship between the drawdown of borehole ZK1 and time, and D represents the logarithmic relationship between the drawdown of borehole ZK1 and time.

[0020] Figure 4 Plots showing the relationship between drawdown and logarithm of time for ZK7 and ZK10; where, Figure 4 Figure B shows the logarithmic relationship between the drawdown and time for borehole ZK7, and Figure C shows the logarithmic relationship between the drawdown and time for borehole ZK10.

[0021] Figure 5 This is a schematic diagram of the groundwater isobars before the main dam in the work area was repaired.

[0022] Figure 6 For groundwater in the work area at different times 238 U content.

[0023] Figure 7 For groundwater in the work area at different times 226 Ra content.

[0024] Figure 8 Groundwater levels before and after work area remediation 238 A diagram comparing the annual average concentration of U.

[0025] Figure 9 Groundwater levels before and after work area remediation 226 A diagram showing the comparison of annual average Ra activity concentrations. Detailed Implementation

[0026] To enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this specification, and not all embodiments. Based on the embodiments in this specification, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this specification.

[0027] Please see Figure 1 , Figure 1 A flowchart illustrating the steps of an in-situ PRB remediation method for radioactive nuclides in groundwater from abandoned uranium mines, provided in this embodiment of the invention, specifically includes: S1. Conduct hydrogeological drilling downstream of the target uranium mine contaminated area to construct a monitoring well network; obtain the permeability coefficient K of the aquifer through on-site pumping tests, and simultaneously measure the stable water level of each monitoring well to determine the hydraulic gradient I and groundwater flow direction; systematically collect groundwater samples, measure the hydrochemical parameters of each sample, and comprehensively draw a three-dimensional spatial distribution map of the pollution plume. S2. Based on the hydrochemical parameters obtained in step S1, perform geochemical simulation to identify the key migration chemical forms of target radionuclides in groundwater during different hydrological periods and their proportions. S3. Based on the parameters determined in step S1, calculate the actual average flow velocity of groundwater. v Based on the key migration patterns diagnosed in step S2, highly efficient reaction media are targeted for screening, and the hydraulic retention time t required for the reaction media to achieve the preset removal standard for the target pollutant is determined through dynamic column experiments; according to the formula Calculate the core thickness L of the PRB wall, and comprehensively determine the layout, total length and depth of the PRB wall; S4. Based on the PRB wall design in step S3, excavate a trench at the selected location and construct a seepage-proof curtain; fill the trench with the selected reaction medium according to the design sequence and proportion to form the PRB wall; wherein, the polluted groundwater flows naturally through the PRB wall under the drive of the natural hydraulic gradient. S5. After the system is in operation, the concentration of the target radionuclides in the downstream effluent of the PRB wall should be monitored regularly to assess whether the radionuclides have reached the remediation target value based on health risks.

[0028] To better understand the present invention, the present invention will be further described below with reference to specific implementation examples.

[0029] Example 1 1. Overview of the uranium mine remediation site: The target site in this embodiment is a closed uranium mine, whose tailings dam is located in a valley. The main aquifer at the uranium mine remediation site is confined aquifer in the weathered limestone fissures beneath the Quaternary overburden. For many years, the seepage water rich in radioactive nuclides such as uranium and radium in the uranium mine tailings dam has been continuously migrating downstream into the groundwater, posing a potential threat to the drinking water safety of downstream residential areas.

[0030] 2. Specific implementation steps: (1) Site characteristic investigation and diagnosis: This embodiment obtains all the basic data required for the quantitative design of the PRB system through surveys and tests. The specific process is as follows: ① Construction of hydrogeological drilling and monitoring well network: Based on the geological data of the work area and in accordance with the requirements of the hydrogeological survey, hydrogeological boreholes were arranged in a fan shape along the predicted pollution diffusion path within approximately 100 meters downstream of both the main and auxiliary tailings dams to establish a long-term monitoring well network. Five boreholes were drilled at each of the main and auxiliary dams, numbered ZK1 to ZK10. The arrangement of these boreholes on the hydrogeological profile is shown below. Figure 2 As shown.

[0031] The spatial location information of each borehole is shown in Table 1, and the exposed strata conditions are shown in Tables 2 and 3.

[0032] Table 1. List of boreholes in abandoned uranium mine working areas As shown in Table 1, the depth of each borehole ranges from 14 to 25 meters. Furthermore, the ground elevations of boreholes ZK1, ZK2, ZK3, ZK4, and ZK5 downstream of the main dam are 111.938m, 111.188m, 111.585m, 111.698m, and 110.528m, respectively, with depths of 24.85m, 20.10m, 23.50m, 14.20m, and 14.65m. The boreholes downstream of the secondary dam are ZK6, ZK7, ZK8, ZK9, and ZK10, with ground elevations of 110.638m, 109.898m, 109.788m, 109.738m, and 109.238m, respectively, and borehole depths of 21.50m, 15m, 21.80m, 16.60m, and 14.50m, respectively.

[0033] Table 2 Brief Description of Strata Exposed by Boreholes ZK1 to ZK5 in the Working Area Table 3 Brief Description of Strata Exposed by Boreholes ZK6 to ZK10 in the Working Area Tables 2 and 3 show that the main soil layers in the working area are Quaternary artificial fill, Quaternary alluvial-diluvial silty soil, clay, and gravelly clay, underlying completely weathered, strongly weathered, and moderately weathered limestone. The clay layer is exposed in all boreholes, has a dense structure, and is approximately 1.4–3.6 m thick. Due to the influence of regional tectonics and lithology, the groundwater in the working area is mainly confined water, with the main aquifers being gravelly clay layers and slab-weathered to completely weathered limestone layers.

[0034] ② Obtaining hydrogeological parameters: In this embodiment, there are 10 unconfined aquifer boreholes downstream of the tailings dam in the working area, numbered ZK1 to ZK10. To obtain groundwater hydrogeological parameters, groundwater pumping tests were conducted on each borehole. Borehole ZK3 of the main dam and borehole ZK8 of the auxiliary dam were selected as pumping wells, and adjacent boreholes were selected as observation wells. Single-hole steady-flow pumping tests were conducted, and the water level parameters of each borehole were observed and recorded in real time. The water level changes of boreholes ZK1, ZK2, ZK4, ZK5, ZK7, and ZK10 are shown in Table 4.

[0035] Table 4. Water level changes in each borehole during pumping tests. This embodiment calculates the hydrogeological parameters of the working area using the drawdown method. In the initial stage of the pumping test, due to the turbulent flow, data from the earlier stages are not used for calculations. After the pumping stabilizes in the middle stage, the drawdown of the pumping wells becomes linearly correlated with the logarithm of time. This embodiment calculates the hydrogeological parameters of the aquifer based on the slope of the linear fitting curve during the middle stage of pumping and related calculation formulas. The drawdown versus logarithm of time for boreholes ZK1, ZK2, ZK4, and ZK5 of the main dam is shown in the following diagram. Figure 3 As shown, the relationship between drawdown and logarithmic time in the secondary dam areas ZK7 and ZK10 is as follows: Figure 4 As shown.

[0036] pass Figure 3 as well as Figure 4 The graph of the relationship between the drawdown depth and time for each borehole and the formula (1) are used to calculate the permeability coefficient k and the radius of influence R of each borehole. The specific calculation results are shown in Table 5.

[0037] (1) In the formula, Q represents the flow rate of the pumping well; in the example, Q = 5.76 m³ / s. 3 / d, This represents the slope of the straight line segment in the logarithmic time-water level drawdown graph.

[0038] Table 5 Calculation results of parameters for the deceleration method The average value of the permeability coefficient calculation results of the above boreholes was taken, and the final permeability coefficient K of the underground aquifer was 2.23 m / d; the radius of influence R was 4.17 m.

[0039] Flow field measurement and average hydraulic gradient analysis: During the dry season, synchronous water level measurements were performed on all borehole monitoring wells. The water source for the repair area mainly originated from dam seepage. The water level depth data for each borehole before repair are shown in Table 6. A coordinate axis was established with the dam as the origin, and contour maps of the work area before repair were drawn based on the water level data. Figure 5 A schematic diagram of groundwater isostatics before the main dam repair. The average hydraulic gradient of the site, I = 0.01, was calculated based on the known borehole water level elevation and horizontal distance (obtained through coordinate calculation).

[0040] Table 6. Water level depth in the work area before restoration ③Water chemical sampling and plume mapping: This embodiment sampled groundwater from monitoring wells in each borehole during three periods: high water season, normal water season, and low water season. The pH value, Eh, and other indicators of the groundwater samples from each borehole in the work area were measured, and the main ion (K+) was analyzed. + Na + ,Ca² + ,Mg² + ,Cl - SO4² - HCO3 - ) and radionuclides (²³ 8 U,²² 6 Ra concentration. The hydrochemical parameters and basic components of groundwater in the working area at different periods are shown in Table 7. The units of hydrochemical parameters in the table are mg / L.

[0041] Table 7 Groundwater hydrochemical parameters in the work area at different times As shown in Table 7, the pH values ​​of groundwater in the working area during the normal water period, wet period, and dry period were 6.71–7.15, 6.96–7.79, and 6.52–9.67, respectively. Meanwhile, the average pH values ​​for each period were 6.93, 7.42, and 8.70, respectively. Furthermore, the redox conditions in the working area during the normal water period and dry period were relatively consistent, indicating weak oxidizing conditions with minimal spatial variability.

[0042] The groundwater cations in the work area are mainly Ca²⁺ + The main component is Ca², and its concentration relationship is as follows: + >Na + >Mg² + >K +Furthermore, the coefficients of variation were similar across different periods, indicating a stable source. Ion concentrations exhibited significant seasonal fluctuations: Ca²⁺ + The concentration peaked during the dry season (525.10 mg / L) and decreased to its lowest point during the wet season (99.60 mg / L); Na + The highest concentration (120.40 mg / L) was observed during the dry season, while the lowest concentration (13.90 mg / L) was observed during the normal water season. + With K + The extreme values ​​all occurred during the high-water season. Anions, primarily SO4²⁻ - It is absolutely dominant, with the concentration sequence being SO4². - >HCO3 - >Cl - Among them, SO4² - The concentration remained high (473.74–754.85 mg / L), with average values ​​for each period (480.23 mg / L during normal water period, 473.74 mg / L during wet water period, and 754.85 mg / L during dry water period) all far exceeding the Class III limit (250 mg / L) of the "Groundwater Quality Standard" (GB / T14848-2017). The highest exceedance at borehole ZK1 reached 3.16 times. This characteristic is directly related to the sulfuric acid leaching process previously used in the mine and is conclusive geochemical evidence of tailings leachate pollution. HCO3 - With Cl - The concentration is relatively stable.

[0043] All of the above characteristics indicate that the groundwater is in a highly mineralized state of calcium sulfate type.

[0044] Based on the test results of groundwater samples, radioactive nuclides were detected in the work area. 238 U、 226 The Ra content levels during the normal, wet, and dry seasons are as follows: Figure 6 and Figure 7 As shown.

[0045] from Figure 6 and Figure 7 It can be seen from this that the groundwater in the working area has been affected by 23 8 U and²² 6 Significant pollution of Ra. Monitoring data shows that²³ 8 The concentration of U ranged from 0 to 0.492 mg / L, with the ZK7 borehole reaching a concentration as high as 0.492 mg / L during the high-water season. This exceeded the WHO drinking water standard (0.03 mg / L) by more than 15 times, and was clearly identified as a "hotspot" for pollution spread.²² 6The Ra concentration ranged from 0 to 3.45 Bq / L, with the maximum exceedance of 2.45 times observed at point ZK1 during the normal water season. The pollution exhibited a clear spatiotemporal pattern: both concentration and exceedance rate followed a seasonal variation of "high water season > normal water season > low water season," revealing that rainfall leaching was the primary driving factor. Spatially, the pollution spread downstream (northwestward) from point ZK7. These characteristics collectively indicate that the remediation project must use the downstream section of ZK7 as the critical interception point and design the PRB (Plasma Remediation Block) based on the peak load of the high water season to ensure that the groundwater is remediated to meet the target values ​​for health risk control.

[0046] (2) Analysis of pollutant morphology and migration patterns: ① Geochemical Simulation: Based on the hydrogeochemical characteristics, using the characteristic pollutant U as the target, the existence form of U in the groundwater environment is analyzed to provide a basis for targeted remediation. All the aforementioned hydrochemical data (ion concentration, pH, Eh, total uranium concentration) are input into the professional geochemical simulation software PHREEQC to simulate the existence form and content of uranium in the groundwater of the working area (see Tables 8, 9, and 10 for details).

[0047] Table 8 Simulation results of uranium existence forms in ZK1, ZK2, and ZK3 Table 9 Simulation results of uranium existence forms in ZK4, ZK5, and ZK6 Table 10 Simulation results of uranium existence forms in ZK7, ZK9, and ZK10 Geochemical simulation (PHREEQC) results illustrate the migration nature of the target pollutant in the subsurface environment: Tables 8, 9, and 10 show that almost all (>99.9%) of uranium (U) in groundwater exists in the +6 valence state (U(VI)), while the contents of U(IV) and U(V) are negligible. Its specific speciation clearly shows that uranium mainly exists in the form of highly soluble and easily migrating uranyl carbonate anions, with UO2(CO3)3 being the most abundant. 4- UO2(CO3)2² is the dominant form (accounting for 36.83%~92.64% of total uranium). - It is the secondary form (accounting for 7.28%~53.14%), while other forms such as UO2CO3 and UO2(OH)2 account for a very small proportion (<0.47%). These results are consistent with the main forms of uranium in the acid leaching process (such as UO2²⁻). + The difference between the pollutants and UO2SO4 indicates a significant hydrogeochemical transformation after they enter natural groundwater. Therefore, when selecting the reaction medium for the subsequent PRB system, it is essential to choose a reaction medium that can efficiently remove anionic complexes.

[0048] ② Correlation analysis: Pearson correlation analysis was performed on the hydrochemical parameters of groundwater from 10 boreholes in the work area using SPSS software. The analysis revealed the correlations between the main ions and radionuclides in the groundwater at different time periods. 238 U、 226 The Pearson correlation coefficients of Ra and pH are shown in Table 11.

[0049] Table 11 Correlation coefficients of groundwater hydrochemical parameters in the work area at different periods As shown in Table 11, the Pearson correlation analysis results indicate that the groundwater²³ 8 U migration exhibits seasonal differences: during normal / high water periods, it differs from Cl migration. - Strongly correlated (r = 0.670 and 0.734 respectively), but during the dry season it is correlated with SO4²⁻. - Strong correlation, the above correlation also confirms the presence of groundwater²³ 8 U exists as a highly mobile dissolved complex, and its pathway is influenced by hydrological dynamics; furthermore, as shown in Table 11,²² 6 The migration of Ra is always related to SO4² - Ca² + Significantly positive correlation (e.g. with SO4²) - The r value reaches 0.781, indicating that... 6 Ra is controlled by sulfate coprecipitation or competitive adsorption with calcium ions. Therefore, when selecting adsorption media, it is necessary to screen for media with high selectivity for uranyl complex anions (such as anion exchange resins) and to consider radium removal in conjunction with these media.

[0050] (3) Quantitative design of PRB system: This embodiment determines the quantitative design of PRB walls through precise calculations. The specific process is as follows: ① Calculation of groundwater flow velocity: According to Darcy's law, the formula for calculating the average groundwater flow velocity is as follows: Based on the aforementioned data analysis, the aquifer's permeability coefficient K is 2.23 m / d, the average hydraulic gradient I is 0.01, and the effective porosity n of the aquifer, based on borehole core tests, is taken as an empirical value of 0.25. The calculated average groundwater flow velocity is approximately 0.089 m / d.

[0051] ② Targeted screening and parameter determination of reaction media: The key form of the radioactive nuclide uranium in groundwater is the uranyl carbonate complex anion. Therefore, a strongly basic anion exchange resin was selected as the core removal medium for the PRB reactive barrier. To ensure long-term effectiveness and consider the synergistic removal of radium, nano-zero-valent iron modified quartz sand composite material and granular activated carbon were selected as auxiliary media.

[0052] To determine the hydraulic retention time (HRT) during groundwater contamination removal, a dynamic column experiment was conducted in the laboratory using actual contaminated groundwater. The packing medium consisted of anion exchange resin, nano-zero-valent iron-modified quartz sand composite material, and activated carbon in a 1:1:1 ratio. The simulated flow rate was 0.089 m / d, and the effluent U concentration was continuously monitored. The dynamic column experiment determined that when the HRT reached 8 days, the effluent U concentration could be stably reduced to below 0.03 mg / L.

[0053] ③ Quantitative design of the wall: Based on the formula for calculating the thickness of the PRB reactive wall: The minimum required wall thickness was calculated to be approximately 0.712m. Considering a safety margin, the designed PRB wall thickness is 1.0 meter. Furthermore, to ensure complete interception of radioactive contaminants, based on the contamination plume distribution map, the total length of the PRB wall is designed to be 19 meters. Based on the depth of the target aquifer's top slab revealed by the borehole, the designed wall excavation depth is 7 meters.

[0054] Based on the design principles of the PRB remediation system, this embodiment involves on-site excavation of permeable reactive walls and filling with the reactive medium. One PRB reactive wall is deployed at a location downstream of the pollution plume's "neck" where the main ZK7 pollutant runoff can be completely intercepted. Three rectangular trenches, each 5m long, 2m wide, and 7m deep, are excavated perpendicular to the groundwater flow direction to serve as the three reactive walls of the PRB remediation system. The reactive medium filling width of the walls is 1m. During filling, the filling is kept as horizontal as possible, and the mixture is tamped after each certain thickness to ensure good mixing and avoid the formation of dominant channels. In addition, a rectangular trench 17m long, 2m wide, and 7m deep was dug along the direction of seepage water flow. At the port where the seepage water enters the rectangular trench, two rectangular trenches 4m long, 2m wide, and 7m deep were dug. A layer of clay and three layers of 1.5mm thick HDPE geomembrane were filled around the excavated trenches to simulate a curtain device of a permeable reactive wall to prevent the seepage water from the tailings dam from spreading to the surrounding area.

[0055] To ensure complete removal of radioactive nuclides, the three excavated reactive barriers were sequentially filled with strongly alkaline anion exchange resin, nano-zero-valent iron modified quartz sand composite material, and granular activated carbon along the direction of seepage water flow. During filling, the permeability coefficient of the materials within the barriers was maximized to be more than twice that of the aquifer to minimize clogging of the permeable reactive barriers. During backfilling of the remaining space in the excavated trenches, Φ160mm PVC pipes were buried behind each section of the permeable reactive barrier for water sampling. After completion, the PRB system began passive operation driven by the natural hydraulic gradient, requiring no external power; the contaminated groundwater was naturally treated as it flowed through the barriers.

[0056] (4) Verification of repair effect: Considering the impact of rainfall at different times on the concentration of radioactive pollutants in groundwater, this embodiment of the invention begins in July. After the system is put into operation, a long-term monitoring plan will be established. Considering the impact of rainfall at different times on the concentration of radioactive pollutants in groundwater, water samples will be collected from monitoring wells upstream of the PRB wall (background value), inside the wall (process monitoring), and downstream of the wall (effect verification) during the wet season (September) and the dry season (December). The water samples will be sealed, preserved, and sent for analysis.

[0057] ① Annual assessment of the remediation effect in uranium mining areas: To evaluate the effectiveness of permeable reactive barrier (PRB) technology in remediating radioactive groundwater contamination, during system operation, 23 8 U and²² 6 Ra was continuously monitored, and groundwater samples before and after remediation were collected annually during system operation. 238 The annual average concentration of U and 226 The annual average activity concentrations of Ra are as follows: Figure 8 and Figure 9 As shown in the figure, the dashed lines represent the World Health Organization (WHO) drinking water quality standards: 238 U is 30 μg / L 226 Ra (1 Bq / L). The site investigation prior to remediation indicated that... 8 U is the main pollutant, with the most prominent exceedance points and magnitudes. The PRB system has shown significant effectiveness since its implementation, as evidenced by long-term monitoring data (…). Figure 8 , Figure 9 The display shows that all monitoring points have been repaired. 8 The annual average concentration of U has significantly decreased from a state of widespread exceedance and has stabilized below the World Health Organization (WHO) drinking water quality guidance value (0.03 mg / L); at the same time,²² 6 The annual average activity concentration of Ra has also decreased from exceeding the standard at some sites (such as ZK1 and ZK2) to below the WHO guidance value (1 Bq / L), achieving comprehensive compliance with pollutant concentration standards and fundamentally improving groundwater quality.

[0058] ② Quantitative assessment of the effectiveness of uranium mine restoration: To further evaluate the remediation efficacy of the PRB system for groundwater in the uranium mine area in this embodiment, the pollution status of groundwater in the uranium mine area before and after remediation was compared to assess the remediation effect. The World Health Organization (WHO) drinking water quality guidelines (²³) were used as the benchmark. 8 U: 30 μg / L,²² 6Using Ra (1 Bq / L) as the standard, the pollution level of water bodies in each monitoring well in the working area was analyzed by the single pollution index method and the modified Nemerow comprehensive pollution index method. The pollution level was divided into the following categories according to the comprehensive pollution index (PI): clean (PI<0.80), relatively clean (0.81≤PI<2.50), lightly polluted (2.50≤PI<4.25), moderately polluted (4.5≤PI<7.20) and severely polluted (PI≥7.20).

[0059] The state of groundwater pollution in the uranium mining area before remediation: The Nemerow Integrated Pollution Index method was used to evaluate the pollution level of 10 monitoring wells in the site before remediation. The results of the water quality evaluation of the radionuclide pollution in the work area before remediation are shown in Table 12.

[0060] Table 12 Results of Water Quality Assessment Before Radionuclide Remediation in Uranium Mine Working Area As can be seen from Table 12, the uranium mine remediation work area is most heavily polluted and covers the widest area during the normal water level period, with point ZK7²³. 8 U levels were severely exceeded, classifying the site as "severely polluted" (comprehensive index reached 10.958), making it the core of the pollution. Most other sites showed light to moderate pollution. Pollution was highly concentrated during the high-water season, with only site ZK7 showing severely excessive levels. 8 U (single-factor index as high as 16.405) indicates "severe pollution" (comprehensive index 14.372). Although it was detected at all points during the dry season, and the comprehensive index showed "clean to relatively clean," the single-factor index confirmed the widespread presence of pollution. In summary, the groundwater in the mining area (especially during the dry and wet seasons) is heavily contaminated with radionuclides, posing a clear risk to human health.

[0061] Current status of groundwater pollution in the uranium mining area after remediation: Based on the monitoring data after remediation, the remediation effect of the PRB system was evaluated using the single-factor pollution index and the Nemerow comprehensive pollution index method. Table 13 shows the water quality evaluation results of the uranium mine working area after radionuclide remediation during the wet and dry seasons.

[0062] Table 13. Results of Water Quality Assessment Before Radionuclide Remediation in Uranium Mine Working Area Table 13 shows that during the high-water season, the pollution level at all sampling points reached "clean" or "relatively clean". However, points ZK1 and ZK6, located upstream of the PRB wall, showed a lower pollution level due to insufficient reaction time of the medium.²³ 8The U-factor index was slightly higher than the background value (1.123 and 1.177 respectively), but the comprehensive pollution index (0.984 and 1.031 respectively) was still considered "relatively clean" and did not constitute substantial pollution. Of particular note is the ZK7 point, which was a "heavily polluted" core area before remediation, whose²³ 8 The U-factor index dropped to 0.193, and the comprehensive pollution index dropped to 0.121, reaching the "clean" level, indicating a highly significant remediation effect. During the dry season, the remediation effect was even more comprehensive, with all sampling points showing positive results. 8 U and ²² 6 The Ra single-factor pollution indices were all far below the WHO drinking water standard limits, falling within the ranges of 0.010–0.285 and 0.007–0.082 respectively, and the comprehensive pollution index reached the "clean" level for all. The pollution levels at multiple sampling points (such as ZK5 and ZK8–ZK10) were improved compared to before remediation. These results indicate that after PRB reactive barrier remediation, the radionuclide concentrations at all monitoring points consistently met the WHO drinking water quality standards during both wet and dry seasons, achieving a fundamental shift from "heavily polluted" to "clean / relatively clean," thus meeting the site remediation objectives.

[0063] ③ Groundwater Health Risk Assessment: This embodiment incorporates a health risk assessment in addition to conventional water quality evaluation to more comprehensively ensure drinking water safety. The assessment adopts the radiation dose model recommended by the US Environmental Protection Agency (USEPA) for radiation doses induced through drinking water. The radiation dose calculation expression is as follows: (2) In the formula, D rw The annual effective dose of the radionuclide, expressed in Sv / y; IR w The annual average water intake per person is 250, 350, and 730 L / y for infants, children, and adults, respectively; IDF is the dose conversion factor in Sv / Bq; C w This represents the activity concentration of a radionuclide, expressed in Bq / L.

[0064] Based on the health risk assessment model recommended by the U.S. Environmental Protection Agency (USEPA), the radiation dose of radionuclides ingested by different age groups in the work area through drinking water during the wet and dry seasons was calculated. The calculation results are shown in Table 14.

[0065] Table 14 Annual Effective Dose of Radionuclides in Drinking Water Surrounding the Uranium Mine Remediation Area (10) -4 ×Sv / y) Post-remediation risk assessment results show that the application of PRB technology effectively controlled health risks. Table 14 calculations indicate that, after PRB treatment, the intake of groundwater in the work area by people of all ages through drinking water during both wet and dry seasons was reduced by 2³.8 U and²² 6 The annual effective doses caused by Ra were significantly lower than the World Health Organization (WHO) recommended limit of 0.1 mSv / y. Specifically, in the most sensitive infant group, the mean annual effective doses from internal radiation during the wet and dry seasons were only 0.011 mSv / y and 0.061 mSv / y, respectively.

[0066] The assessment confirmed that after the use of PRB technology for remediation, the groundwater in the work area not only reached the "clean" level in terms of radionuclide concentration, but also reduced the resulting health risks to humans to an acceptable level, and would not cause radiation hazards to local residents, thus achieving the dual goals of environmental remediation and health and safety protection.

[0067] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any changes or substitutions conceived without inventive effort should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope defined in the claims.

[0068] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A method for in-situ remediation of radioactive nuclides in groundwater from abandoned uranium mines using radioactive rbore (PRB), characterized in that, include: S1. Conduct hydrogeological drilling downstream of the contaminated area of ​​the target uranium mine to construct a monitoring well network; The permeability coefficient K of the aquifer was obtained through on-site pumping tests, and the stable water level of each monitoring well was measured simultaneously to determine the hydraulic gradient I and the direction of groundwater flow. Groundwater samples were collected systematically, the hydrochemical parameters of each sample were measured, and a three-dimensional spatial distribution map of the pollution plume was drawn. S2. Based on the hydrochemical parameters obtained in step S1, perform geochemical simulation to identify the key migration chemical forms of target radionuclides in groundwater during different hydrological periods and their proportions. S3. Based on the parameters determined in step S1, calculate the actual average flow velocity of groundwater. v Based on the key migration patterns diagnosed in step S2, highly efficient reaction media are targeted for screening, and the hydraulic retention time t required for the reaction media to achieve the preset removal standard for the target pollutant is determined through dynamic column experiments; according to the formula Calculate the core thickness L of the PRB wall, and comprehensively determine the layout, total length and depth of the PRB wall; S4. Based on the PRB wall design in step S3, excavate a trench at the selected location and construct a seepage-proof curtain; fill the trench with the selected reaction medium according to the design sequence and proportion to form the PRB wall; wherein, the polluted groundwater flows naturally through the PRB wall under the drive of the natural hydraulic gradient. S5. After the system is in operation, the concentration of the target radionuclides in the downstream effluent of the PRB wall should be monitored regularly to assess whether the radionuclides have reached the remediation target value based on health risks.

2. The method according to claim 1, characterized in that, The water chemistry parameters mentioned in step S1 include pH value, redox potential Eh, major ion concentration, and target radionuclide concentration; the major ions include, but are not limited to, K+. + Na + Ca² + Mg² + Cl - SO4² - HCO3 - The target radionuclide is 2³ 8 U and²² 6 Ra.

3. The method according to claim 1, characterized in that, The different hydrological periods mentioned in step S2 include the high-water period, the normal-water period, and the low-water period; the key migration chemical form includes uranyl carbonate complex; wherein, , This is the dominant form.

4. The method according to claim 1, characterized in that, The formula for calculating the actual average flow velocity v of groundwater in step S3 is as follows: , where n is the effective porosity of the aquifer.

5. The method according to claim 1, characterized in that, The highly efficient reaction medium for targeted screening in step S3 includes at least anion exchange resins that have a specific adsorption effect on anionic uranium.

6. The method according to claim 1, characterized in that, The reaction medium also includes zero-valent iron composite material and activated carbon; the reaction medium is filled in the PRB wall in the following order along the direction of groundwater flow: anion exchange resin layer, zero-valent iron composite material layer, and activated carbon layer.

7. The method according to claim 1, characterized in that, In step S4, the seepage-proof curtain is composed of at least one layer of compacted clay lining and at least one layer of polymer geomembrane.

8. The method according to claim 1, characterized in that, The target value for remediation based on health risk mentioned in step S5 is: radionuclide²³ 8 The concentration of U is below 30 μg / L,²² 6 The concentration of Ra is less than 1 Bq / L, and the annual effective dose due to drinking water is less than 0.1 mSv.

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