A method for monitoring natural attenuation of hexavalent chromium contaminated groundwater

Abstract

The present application relates to the technical field of monitoring natural attenuation, and discloses a kind of monitoring natural attenuation technical method for hexavalent chromium contaminated groundwater, the method comprises: judging whether the hexavalent chromium groundwater pollution plot is suitable for using monitoring natural attenuation technology;If suitable, update the plot conceptual model and carry out simulation analysis, comprehensive various feasibility evaluation factors, judge whether monitoring natural attenuation technology is feasible;If feasible, determine the application scenario, design the monitoring scheme of monitoring natural attenuation engineering and formulate the phased goal of engineering operation;Finally, from the process evaluation and final evaluation two aspects, the effect of monitoring natural attenuation implementation is evaluated.The present application has the characteristics of scientific and efficient, strong practicality, can provide reference and technical support for the risk control of chromium contaminated site groundwater in our country.

Description

Technical Field

[0001] This invention relates to the field of natural decay monitoring technology, and in particular to a method for monitoring the natural decay of hexavalent chromium-contaminated groundwater. Background Technology

[0002] Groundwater is a crucial component of water resources, playing a vital strategic role in ensuring the livelihoods of urban and rural residents, supporting economic and social development, and maintaining ecological balance. The safety of groundwater drinking water sources is directly related to public health, and groundwater pollution prevention and control are essential for promoting ecological civilization and safeguarding national security. Chromium is involved in industries such as alloy manufacturing, electroplating, tanning, glass, dyes, pigments, and oil and gas. The improper storage of waste residue and liquid accumulated over many years has made chromium one of the most common heavy metal pollutants in my country's soil and groundwater. Chromium pollution often originates from the disorderly accumulation of chromium slag on the surface and the continuous influx of polluted soil into groundwater. Due to the high mobility of hexavalent chromium in groundwater, chromium-contaminated sites often form pollution plumes with long vertical travel. To strengthen the risk management of groundwater in chromium-contaminated sites, in-depth research and application of green and low-energy technologies such as monitoring natural attenuation, microbial remediation, and institutional control are urgently needed.

[0003] Monitored natural attenuation (MNA) is a remediation method that utilizes planned monitoring strategies to reduce the concentration, total amount, toxicity, and mobility of pollutants in soil and groundwater to acceptable risk levels within a reasonable timeframe, based on the natural physical, chemical, and biological attenuation processes that occur at contaminated sites. Compared to other "active" remediation techniques (physical, chemical, or biological remediation), monitored natural attenuation is widely used, especially for groundwater pollution remediation, due to its advantages such as minimal disruption to the subsurface environment and relatively lower remediation costs.

[0004] Currently, technical documents on monitoring natural decay issued by some developed countries such as the United States, the United Kingdom, and the Netherlands mainly focus on the overall work of monitoring natural decay. For example, some technical documents specifically target organic pollutants such as gasoline and organochlorides. These guidelines all provide evaluation procedures and methods suitable for their respective national conditions and regulatory models for contaminated sites, but they lack specific guidance for chromium-contaminated groundwater sites. Chromium, as an active heavy metal, can transform between trivalent and hexavalent states. Furthermore, hexavalent chromium complex anions have stronger water solubility and mobility than other heavy metal ions, and its natural decay mechanism differs significantly from that of organic and other inorganic substances. Existing technological achievements focus more on universal concepts and technical procedures for monitoring natural decay, without addressing the technical requirements for monitoring natural decay specific to the migration and transformation mechanisms of hexavalent chromium. This indicates a significant deficiency in guiding the monitoring of natural decay in hexavalent chromium-contaminated groundwater sites.

[0005] Currently, the management of groundwater contaminated sites in my country mainly relies on a series of guidelines such as the "Technical Guidelines for Groundwater Remediation and Risk Management of Contaminated Sites (HJ 25.6-2019)". However, specific technologies for monitoring the natural decay of hexavalent chromium are still lacking in these guidelines in my country. Existing technologies from abroad and other developed regions cannot be effectively integrated with my country's requirements for contaminated site management, thus failing to provide guidance and reference for industry practitioners in this field. Therefore, developing a monitoring system for the natural decay of chromium-contaminated groundwater, tailored to my country's contaminated site regulatory system and technical requirements, is a crucial technical means to address the lack of a comprehensive monitoring system for the natural decay of hexavalent chromium-contaminated groundwater in my country. Summary of the Invention

[0006] The purpose of this invention is to provide a monitoring and natural decay technology for hexavalent chromium-contaminated groundwater. Based on the premise that the monitoring and natural decay technology is suitable for chromium-contaminated groundwater, a comprehensive assessment of its technical feasibility is conducted to further guide engineering design and construction, ensuring the safe operation of the monitoring and natural decay project. This method is characterized by its scientific efficiency and strong practicality, and can provide reference and technical support for the risk management of groundwater in chromium-contaminated sites in my country.

[0007] To achieve the above objectives, the present invention provides the following solution:

[0008] A method for monitoring the natural decay of hexavalent chromium contaminated groundwater, comprising the following steps:

[0009] S1, Applicability Analysis: Based on the basic conditions for site selection, hydrogeological conditions, and site pollution survey data, determine whether the hexavalent chromium groundwater contaminated site is suitable for the monitoring of natural attenuation technology.

[0010] S2, Feasibility Analysis: If the hexavalent chromium groundwater contaminated site is suitable for monitoring natural attenuation technology, then a supplementary site investigation will be conducted based on the risk management objectives to update the site's conceptual model; simulation analysis will be conducted based on the updated site conceptual model, and various feasibility assessment factors will be considered to determine whether monitoring natural attenuation technology is feasible;

[0011] S3, Engineering Design and Implementation: If monitoring natural attenuation technology is feasible, then, in combination with the hydrogeological conditions of the site and the spatiotemporal evolution trend of groundwater pollutants, determine the application scenario, design a monitoring scheme for monitoring natural attenuation, and formulate phased objectives for the operation of the project.

[0012] S4, Effectiveness Evaluation: The effectiveness of monitoring natural decay is evaluated from two aspects: process evaluation and summative evaluation.

[0013] Furthermore, in S1, determining whether a site contaminated with hexavalent chromium groundwater is suitable for monitoring natural attenuation technology based on the basic site selection criteria specifically includes:

[0014] Determining whether a site contaminated with hexavalent chromium in groundwater falls under the category of unsuitable for monitoring natural attenuation techniques includes:

[0015] The control and remediation of the target pollution plume is urgent;

[0016] Located in groundwater-sensitive areas such as groundwater drinking water source protection zones;

[0017] It does not meet the relevant requirements of land use planning or environmental impact assessment;

[0018] The health risks to humans in the areas where groundwater contamination plumes are distributed are unacceptable.

[0019] The pollution source or high-concentration pollution center has not been eliminated or risk management has not been planned.

[0020] Furthermore, in S1, determining whether a site contaminated with hexavalent chromium groundwater is suitable for monitoring natural attenuation technology based on hydrogeological conditions specifically includes:

[0021] Based on four hydrogeological conditions—soil and rock pore type, aquifer medium type, stratigraphic structure, and topography—the suitability of using natural attenuation monitoring technology on hexavalent chromium groundwater contaminated sites is divided into three categories: high, medium, and low. Among them, the case with high suitability is determined to be suitable for using natural attenuation monitoring technology on hexavalent chromium groundwater contaminated sites.

[0022] Furthermore, in S1, determining whether a site contaminated with hexavalent chromium in groundwater is suitable for monitoring natural attenuation technology based on site pollution survey data specifically includes:

[0023] Acquire site pollution survey data, including groundwater pollution plume morphology and variation characteristics, as well as monitoring data of chromium-contaminated soil and aquatic media;

[0024] Based on the assessment of hydrogeological conditions and analysis of site pollution survey data, if the following two conditions are met, then the monitoring of natural attenuation technology is suitable for sites contaminated with hexavalent chromium groundwater:

[0025] (1) Based on the spatial variation of the ratio of conservative ions and pollutant concentrations in a single pollution plume, it is determined that there is a natural decay effect;

[0026] (2) Based on the changes in the morphology of multiple pollution plumes over time, it is determined that there are pollution plumes that show a stable or shrinking trend;

[0027] (3) Based on the monitoring of chromium pollution in soil and water-containing media, it is determined that there is hexavalent or trivalent chromium content exceeding the background value.

[0028] Furthermore, in S2, a supplementary investigation of the land parcel is conducted based on the risk management objective to update the land parcel conceptual model, specifically including:

[0029] Supplementary investigations were conducted on hydrogeological conditions, characteristic pollutants, biogeochemical conditions, and biogeochemical reaction parameters to obtain supplementary investigation data for the land parcel.

[0030] Identify natural attenuation effects based on the characteristics of pollution plume changes, geochemical index changes, and the types and capabilities of indigenous microorganisms.

[0031] Based on supplementary survey data of the land parcels and the effects of natural attenuation, the conceptual model of the land parcels was updated.

[0032] Furthermore, in step S2, a simulation analysis is performed based on the updated land parcel conceptual model, and multiple feasibility assessment factors are considered to determine whether the monitoring of natural attenuation technology is feasible. Specifically, this includes:

[0033] Based on the updated land parcel conceptual model, simulation analysis is achieved by setting assessment objectives, selecting assessment models, calibrating model parameters, identifying and verifying models, and conducting model prediction, sensitivity analysis, and uncertainty analysis.

[0034] The feasibility of monitoring natural decay technology is determined by considering factors related to technical feasibility, economic feasibility, and regulatory feasibility.

[0035] Furthermore, in S3, based on the site's hydrogeological conditions and the spatiotemporal evolution trends of groundwater pollutants, the application scenario is determined, a monitoring scheme for the natural attenuation monitoring project is designed, and phased objectives for the project's operation are formulated, specifically including:

[0036] S3.1, Determine the application scenario: Under the determined risk management or remediation model, based on the results of the technical feasibility assessment, select a single monitoring natural decay technology or a combination of other technologies to carry out groundwater pollution risk management and remediation; among them, other technologies include hydraulic control, in-situ reaction zone, PRB, barrier wall, contaminated soil remediation, extraction treatment, and in-situ injection technology.

[0037] S3.2, Design a monitoring scheme for monitoring natural decay projects: Update the site concept model through supplementary investigations and identification of natural decay effects during the project phase; determine the monitoring scope; design the monitoring well network, monitoring items, and monitoring frequency;

[0038] S3.3, formulate phased objectives for project operation, including: demonstrating that the pollution plume remains stationary or shrinks; no changes in key hydrogeological, geochemical, or microbial community conditions are found; setting compliance points for different stages according to expectations, and whether the compliance points are met; and whether the overall project can achieve the established control and remediation objectives in the context of multiple technologies used in combination.

[0039] Furthermore, in S3, engineering design and implementation also includes developing contingency plans; the development of contingency plans specifically includes:

[0040] Criteria for determining when to activate emergency response plans;

[0041] An emergency plan is designed based on several aspects, including the basis for implementing the emergency plan, the spatial scope and timeline of the emergency response, the technical approach of the emergency plan, the specific measures to be taken, and the evaluation of the effectiveness of the emergency plan.

[0042] Furthermore, in S4, the effectiveness of monitoring natural decay is evaluated from two aspects: process evaluation and summative evaluation, specifically including:

[0043] Process assessment was conducted using graphical and mathematical statistical methods.

[0044] A final assessment was conducted based on the Technical Guidelines for Groundwater Remediation and Risk Management of Contaminated Sites (HJ 25.6).

[0045] According to specific embodiments provided by the present invention, the following technical effects are disclosed: The method for monitoring the natural decay of hexavalent chromium-contaminated groundwater provided by the present invention includes steps such as technical applicability analysis, technical feasibility analysis, engineering design and implementation of monitoring natural decay, and evaluation of the effect of monitoring natural decay. The applicability analysis is used to determine whether the hexavalent chromium-contaminated groundwater site is suitable for using the monitoring natural decay technology. It is a prerequisite for conducting the feasibility analysis, which is an extension and deepening of the applicability analysis, providing the necessary parameter basis for engineering design and construction. The engineering design and construction are based on the parameters obtained from the feasibility analysis and in accordance with the relevant requirements of HJ 25.6, to carry out the engineering design and implementation of monitoring natural decay. The effect evaluation includes process evaluation and final evaluation. The process evaluation mainly aims to guide the operation of the project and monitoring during the process, while the final evaluation corresponds to the regulatory requirements of HJ 25.6, evaluating whether the monitoring natural decay project has achieved the project's expected results. This invention fully considers my country's regulatory needs and technical requirements regarding groundwater contaminated sites. It outlines the guiding principles, workflow, content, and technical requirements for monitoring the natural attenuation of chromium-contaminated groundwater. It is applicable to groundwater pollution prevention and remediation using single natural attenuation monitoring technologies or in combination with other technologies. The invention can be used as a reference for monitoring the natural attenuation of groundwater containing other pollutants. Attached Figure Description

[0046] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0047] Figure 1 This is a flowchart of the natural decay technology method for monitoring hexavalent chromium-contaminated groundwater according to the present invention. Detailed Implementation

[0048] The technical solutions of 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 the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0049] The regulatory requirements and technical requirements in my country regarding groundwater contaminated sites referenced in this invention include:

[0050] HJ 25.1 Technical Guidelines for Soil Pollution Status Investigation of Construction Land;

[0051] HJ 25.2 Technical Guidelines for Risk Management and Remediation Monitoring of Soil Pollution in Construction Land;

[0052] HJ 25.3 Technical Guidelines for Soil Pollution Risk Assessment of Construction Land;

[0053] HJ 25.4 Technical Guidelines for Soil Remediation of Construction Land;

[0054] HJ 25.5 Technical Guidelines for Risk Management and Soil Remediation Effectiveness Assessment of Contaminated Sites;

[0055] HJ 25.6 Technical Guidelines for Groundwater Remediation and Risk Management of Contaminated Sites;

[0056] HJ / T 166 Technical Specification for Soil Environmental Monitoring;

[0057] HJ 164 Technical Specification for Groundwater Environmental Monitoring;

[0058] HJ 610 Technical Guidelines for Environmental Impact Assessment: Groundwater Environment;

[0059] DZ / T 0282 Specification for Hydrogeological Investigation (1:50000);

[0060] GB 14848-2017 Groundwater Quality Standard;

[0061] HJ 493 Technical Specifications for the Preservation and Management of Water Quality Sampling Samples;

[0062] HJ 494 Water Quality Sampling Technical Guidelines;

[0063] Guidelines for Groundwater Pollution Simulation, Prediction and Assessment (Huanban Tushun

[2019] No. 770);

[0064] Hydrogeology Handbook

[0065] This invention specifies the guiding principles for monitoring the natural decay of chromium-contaminated groundwater:

[0066] (1) Scientific principle

[0067] Taking into account constraints such as time cost, economic cost, and work objectives, a scientific monitoring plan is determined to maximize benefits. Considering the hydrogeological conditions of contaminated sites, the uncertainty in the spatial distribution of pollutants, changes in the degree and extent of pollution, and the evolution of the biogeochemical environment in which pollutants reside, the plan is dynamically optimized based on the data acquired during technical execution.

[0068] (2) Normative principle

[0069] The work content and process of monitoring the natural decay technology of chromium-contaminated groundwater should be standardized in a procedural and systematic manner to ensure the standardized implementation of the monitoring of natural decay.

[0070] (3) Feasibility principle

[0071] Based on the hydrogeological conditions of the contaminated site, the degree and extent of pollution, the properties and historical characteristics of pollutants, the geochemical environment of pollutant occurrence, the structural characteristics of microbial communities, the condition of receptors and the surrounding environment, the natural decay capacity is analyzed and determined, and the feasibility of monitoring natural decay technology is determined.

[0072] This invention is applicable to groundwater pollution prevention and remediation using single monitoring of natural attenuation technology, as well as in combination with other technologies. It can be applied to groundwater monitoring of natural attenuation for other pollutants.

[0073] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0074] like Figure 1 As shown, the method for monitoring the natural decay of hexavalent chromium-contaminated groundwater provided by this invention includes the following steps:

[0075] S1, Applicability Analysis: Based on the basic conditions for site selection, hydrogeological conditions, and site pollution survey data, determine whether the hexavalent chromium groundwater contaminated site is suitable for the monitoring of natural attenuation technology.

[0076] S2, Feasibility Analysis: If the hexavalent chromium groundwater contaminated site is suitable for monitoring natural attenuation technology, then a supplementary site investigation will be conducted based on the risk management objectives to update the site's conceptual model; simulation analysis will be conducted based on the updated site conceptual model, and various feasibility assessment factors will be considered to determine whether monitoring natural attenuation technology is feasible;

[0077] S3, Engineering Design and Implementation: If monitoring natural attenuation technology is feasible, then, in combination with the hydrogeological conditions of the site and the spatiotemporal evolution trend of groundwater pollutants, determine the application scenario, design a monitoring scheme for monitoring natural attenuation, and formulate phased objectives for the operation of the project.

[0078] S4, Effectiveness Evaluation: The effectiveness of monitoring natural decay is evaluated from two aspects: process evaluation and summative evaluation.

[0079] Specifically, in S1, the applicability analysis includes:

[0080] S1.1, Based on the basic conditions for site selection, determine whether a site contaminated with hexavalent chromium in groundwater is suitable for monitoring natural attenuation technology, specifically including:

[0081] Determine whether the hexavalent chromium groundwater contaminated site falls under the category of unsuitable for monitoring natural attenuation technology. If not, then the hexavalent chromium groundwater contaminated site is suitable for monitoring natural attenuation technology.

[0082] Among the situations where monitoring natural attenuation technology is not suitable are:

[0083] The control and remediation of the target pollution plume is urgent;

[0084] Located in groundwater-sensitive areas such as groundwater drinking water source protection zones;

[0085] It does not meet the relevant requirements of land use planning or environmental impact assessment;

[0086] The health risks to humans in the areas where groundwater contamination plumes are distributed are unacceptable.

[0087] The pollution source or high-concentration pollution center has not been eliminated or risk management has not been planned.

[0088] S1.2, Determine whether a site contaminated with hexavalent chromium in groundwater is suitable for monitoring natural attenuation technology based on hydrogeological conditions, specifically including:

[0089] Based on the environmental surveys and risk assessments completed in accordance with HJ 25.1, HJ 25.2, and HJ 25.3, the suitability of the site's hydrogeological conditions is determined according to Table 1. For sites with high suitability, further analysis of the site's pollution survey data can be conducted to determine whether a feasibility analysis should be carried out; for sites with medium or low suitability, other technologies should be considered for groundwater pollution control and remediation as appropriate.

[0090] Table 1. Suitability Evaluation of Hydrogeological Conditions of the Site

[0091]

[0092] S1.3, Based on the site pollution survey data, determine whether the hexavalent chromium groundwater contaminated site is suitable for using the natural attenuation monitoring technology, specifically including:

[0093] S1.3.1, Obtain site pollution survey data including groundwater pollution plume morphology and variation characteristics, as well as monitoring data of chromium-contaminated soil and aquatic media;

[0094] 1.3.1.1 Morphology and Variation Characteristics of Groundwater Pollution Plume

[0095] Single-phase pollution plume: If only the pollution plume information of a certain period of time of a site is available, the migration process of chromium pollutants should be preliminarily assessed by combining the site's pollution history with the current distance of the pollution plume leading edge. The spatial variation of the ratio of conservative ions and pollutant concentrations along the groundwater flow direction can be combined to determine whether the pollutants exhibit natural decay behavior.

[0096] Multi-phase pollution plumes: If pollution plume information for a site at multiple times is available, the overall morphology of the multi-phase pollution plumes can be analyzed over time (vertical migration and diffusion distance, lateral migration and diffusion distance, etc.) and the concentration change trend at specific locations to comprehensively determine whether the pollution plumes are stabilizing or shrinking.

[0097] 1.3.1.2 Monitoring data of chromium-contaminated soil and water-containing media

[0098] The content of adsorbed hexavalent chromium and trivalent chromium in soil and groundwater aquifers at key locations was analyzed. By comparing the background values ​​of hexavalent chromium and trivalent chromium in the region, a preliminary analysis was conducted to determine whether hexavalent chromium was fixed on the aquifer through adsorption or reduction.

[0099] S1.3.2, Based on the assessment of hydrogeological conditions and analysis of site pollution survey data, if the following two conditions are met, further feasibility analysis can be conducted; otherwise, other technologies should be considered for groundwater pollution control and remediation:

[0100] (1) Based on the spatial variation of the ratio of conservative ions and pollutant concentrations in a single pollution plume, it is determined that there is a natural decay effect;

[0101] (2) Based on the changes in the morphology of multiple pollution plumes over time, it is determined that there are pollution plumes that show a stable or shrinking trend;

[0102] (3) Based on the monitoring of chromium pollution in soil and water-containing media, it is determined that there is hexavalent or trivalent chromium content exceeding the background value.

[0103] Specifically, in S2, the feasibility analysis includes:

[0104] S2.1, Based on risk management objectives, conduct supplementary land surveys and update the land conceptual model, specifically including:

[0105] 2.1.1 Supplementary Investigation

[0106] 2.1.1.1 Supplementary investigation of hydrogeological conditions

[0107] (1) Detailed characterization of aquifers: Based on the detailed results of the previous investigation of the site, hydrogeological boreholes were laid out along the migration path of chromium pollutants from source to runoff to sink, to further characterize the three-dimensional aquifer structure and distribution. Representative points were selected to collect media samples from different strata, and particle analysis was carried out to further determine the lithology of the strata.

[0108] (2) Obtaining heterogeneous hydrogeological parameters: Based on field pumping tests, infiltration tests, tracer tests and laboratory tests, hydrogeological parameters such as permeability coefficient, specific yield, storage rate, dispersion, porosity and rainfall infiltration coefficient of aquifers at different locations are obtained.

[0109] For details on how to obtain the above parameters, please refer to the "Hydrogeology Handbook" and the "Guidelines for Groundwater Pollution Simulation, Prediction and Assessment".

[0110] 2.1.1.2 Supplementary investigation of characteristic pollutants

[0111] Based on the monitoring wells established through detailed site surveys and risk assessments, supplementary hydrogeological boreholes should be drilled as needed to collect soil and groundwater samples and determine total chromium, hexavalent chromium, and other water quality indicators that require measurement. Chromium speciation analysis should be conducted on soil and aquifer samples from typical profiles, with no fewer than three control points per typical profile.

[0112] 2.1.1.3 Supplementary investigation of biogeochemical conditions

[0113] Supplementary investigations of biogeochemical conditions should include both the aquifer and groundwater geochemical characteristics. Key sites should be selected, and the following parameters should be measured: pH, temperature and conductivity, redox potential, electron acceptors (O2, Fe(III), NO3). - and SO4 2- ), reduction products (CH4, Fe(II)), concentration and alkalinity of dissolved inorganic carbon.

[0114] Microbial indicators can be determined by methods such as plate plating, metagenomics, and microbial DNA sequencing to identify the types and functional genes of microorganisms.

[0115] 2.1.1.4 Characterization of Biogeochemical Reaction Parameters

[0116] The parameters related to the natural decay of chromium in soil and groundwater mainly include adsorption equilibrium constant, adsorption rate constant and maximum adsorption capacity, reduction rate and maximum reduction capacity, microbial community composition and functional gene abundance analysis, etc., and experimental methods for obtaining relevant parameters through field experiments.

[0117] 2.1.2 Identification of Natural Attenuation Effect

[0118] Identifying natural degradation effects based on the characteristics of pollution plume changes, geochemical index changes, and the types and capabilities of indigenous microorganisms, specifically including:

[0119] The role of natural decay of chromium in groundwater can be assessed by monitoring. There are three main sources of evidence that can indicate the actual occurrence of natural decay. These three sources can provide direct or favorable evidence for the natural decay of hexavalent chromium, either individually or in combination, as shown in Table 2.

[0120] Table 2 Evidence from three aspects of monitoring natural decay

[0121]

[0122] 2.1.2.1 Characteristics of the Pollution Plume: Site pollutant characteristics demonstrate that the pollution plume is static or contracting, has not reached sensitive boundaries, and will not affect existing water supply. Once this is confirmed through groundwater characteristics, evidence can be collected to demonstrate the allocation of chromium to aquifer solids within the pollution plume, specifically including:

[0123] (1) Use historical pollutant data to determine whether pollutant concentrations show a gradient decreasing trend along the groundwater flow path;

[0124] (2) Conduct long-term concentration monitoring at specific locations and perform statistical analysis. Analyze the state of the pollution plume (shrinking, stable or expanding state) based on the trend of pollutant concentration changes.

[0125] (3) Use mass conservation analysis to determine the mass of the remaining pollutants and the mass of the degradation products after degradation;

[0126] (4) Calculate the mass flux of pollutants through a specific cross section to evaluate the state of the pollution plume (shrinking, stable or expanding state);

[0127] (5) The migration distance of the pollution plume and the time required for the pollution to reach a steady state are simulated and predicted by the solute transport analytical model, so as to evaluate the ability of natural decay.

[0128] (6) The temporal and spatial variation of pollutant concentration is characterized by a solute transport numerical model, thereby identifying the natural decay capacity and analyzing the natural decay mechanism.

[0129] Pollutants often spread in a plume-like pattern along the flow of groundwater. In the site conceptual model, it is necessary to determine the pollution source area and the outline of the pollution plume. When conducting monitoring and analysis of the characteristics of pollution plume changes, it should be noted that the decay trend of pollutants in each individual well may not be the same. Therefore, it is necessary to measure whether the pollutants show a gradient decline trend at the level of the entire pollution plume.

[0130] 2.1.2.2 Characteristics of Geochemical Indicator Changes: Geochemical data of aquifers can serve as indicators of pollutant degradation and are crucial for determining the natural decay capacity of pollutants in aquifers. The following geochemical indicators can indicate the occurrence of natural decay:

[0131] (1) Electron acceptors in polluted plumes (mainly O2, Fe(III), NO3) - and SO4 2- The concentration of ) decreased compared to the local geochemical background concentration, while the concentration of reduction products (such as CH4, Fe(II)) increased;

[0132] (2) Changes in the concentration and alkalinity of dissolved inorganic carbon: Microbial metabolism produces carbon dioxide, and an increase in carbon dioxide concentration leads to changes in groundwater alkalinity. By analyzing the groundwater alkalinity change curve, the microbial degradation process in the pollution plume can be determined.

[0133] (3) pH, temperature and conductivity: pH of groundwater affects the presence and activity of microbial populations, temperature directly affects the metabolic activities of microorganisms in groundwater, and conductivity is proportional to the amount of ions in groundwater. All of these can be used as indicators of the conditions for the biodegradation of pollutants.

[0134] (4) Oxidation-reduction potential: Some microbial degradation processes can only occur within a specific range of oxidation-reduction potential conditions. Generally, a low oxidation-reduction potential value is conducive to the occurrence of microbial degradation.

[0135] 2.1.2.3 Indigenous Microbial Species and Capabilities: Microbial community analysis can indicate the natural decay process and the ability of microorganisms to degrade target pollutants in contaminated sites. Microbial detection and analysis can assess whether the site's microorganisms have the ability to degrade target pollutants, providing direct evidence to support the determination of the site's natural decay capacity and process. Specifically, (1) in-situ microbial community detection is conducted at the site; (2) the microbial communities in the collected groundwater or soil samples are cultured in the laboratory, and their degradation rate of pollutants is quantitatively measured.

[0136] 2.1.2.4 Comprehensive Assessment of Natural Attenuation Capability: Based on the assessment and monitoring plan results, identify and determine whether monitoring natural attenuation is an acceptable remediation strategy. Monitoring natural attenuation is considered a feasible strategy when one of the following conditions is met, and the more conditions met, the higher the confidence level:

[0137] (1) Natural decay processes are identified, such as one chain of evidence showing the decay of pollutant quality and another chain of evidence showing its decay mechanism.

[0138] (2) The state of the pollution plume diffusion is identified (i.e., stable, decreasing or expanding), such as a single parameter in the pollutant, metabolite, electron donor or electron acceptor showing a decreasing trend (medium confidence), or two or more parameters showing a related decreasing trend (high confidence).

[0139] (3) As some indicators of the chain of evidence, the various monitoring trends show consistency;

[0140] (4) The observed conditions and decay characteristics were reproduced and simulated using the migration regression model;

[0141] (6) The geochemical environment is consistent with the degradation mechanism; for example, redox conditions reflect the degradation process.

[0142] (7) There are no inconsistencies between the observed conditions and the conceptual model;

[0143] (5) Microbial research provides supporting evidence for biodegradation;

[0144] (8) Sensitive parameters have been fully confirmed, such as field tests that have confirmed the range of permeability coefficients;

[0145] (9) The uncertainty regarding pollution sources, pollution plumes and pollution pathways has been significantly reduced.

[0146] The assessment of the natural decay process also needs to consider the following factors:

[0147] (1) Changes in land use or site conditions that may affect natural degradation. For example, introducing oxygen into the system and changing the aquifer environment from anaerobic to aerobic;

[0148] (2) Aquifer assimilation capacity. The assessment needs to demonstrate that the aquifer's assimilation capacity is sufficient to protect future identified sensitive targets;

[0149] (3) Desorption of pollutants. For pollutants whose natural decay processes are non-destructive, such as adsorption or precipitation, the assessment needs to demonstrate that these pollutants will not be reactivated, or that the reactivation rate is insufficient to affect the identified sensitive targets. For example, hexavalent chromium has relative mobility and toxicity. Under certain conditions, hexavalent chromium can be reduced to the less toxic trivalent chromium, which is not easily soluble and is likely to precipitate from the solution. Subsequent changes in the chemical environment may cause trivalent chromium to be oxidized back to hexavalent chromium, reactivated, and released into groundwater.

[0150] Based on supplementary survey data of the land parcels and the effects of natural attenuation, the conceptual model of the land parcels was updated.

[0151] S2.2 involves simulation analysis based on the updated land parcel conceptual model, and comprehensive feasibility assessment factors to determine the feasibility of monitoring natural attenuation technology, specifically including:

[0152] Based on the updated land parcel conceptual model, simulation analysis is achieved by setting assessment objectives, selecting assessment models, calibrating model parameters, identifying and verifying models, and conducting model prediction, sensitivity analysis, and uncertainty analysis.

[0153] The feasibility of monitoring natural decay technology is determined by considering factors related to technical feasibility, economic feasibility, and regulatory feasibility.

[0154] Section 2.2.1, the simulation analysis, is described in detail below:

[0155] (1) Determine the assessment objectives: The main objectives of the simulation analysis should be to ensure that the health and environmental risks of the sensitive receptors are acceptable, and that the target pollution plume continues to shrink and the decay rate can meet the control and remediation objectives at different stages.

[0156] (2) Selection and construction of assessment models: Depending on the complexity of the site's hydrogeological conditions and the morphological characteristics of the groundwater pollution plume, a simple analytical model or a complex numerical model should be selected to assess the long-term effectiveness of natural attenuation. If the model is found to be insufficient to reflect the attenuation process or there is insufficient monitoring data to verify the simulation results, further supplementary investigations are required. The selection of analytical models can be found in Appendix B.2 of the "Guidelines for Groundwater Pollution Simulation, Prediction and Assessment".

[0157] (3) Calibration of Model Parameters: For key parameters such as permeability coefficient and decay rate, field-measured data should be used whenever possible. The calibration process for parameter values ​​should be fully explained, including data sources, analysis methods, parameter value ranges, and uncertainties. If parameters measured off-site, such as specific yield and release (storage) coefficient, are used, it should be demonstrated that: 1. the off-site measured data is adjacent to or similar to the actual site environment; 2. the model prediction is not sensitive to this parameter; 3. the empirical range of parameter values ​​has been determined through literature review, and conservative values ​​have been used in the assessment. For example, literature records that the porosity of sand is usually between 10% and 40%. Since a lower porosity value will lead to an increase in the calculated pollutant mobility rate, a value of 10% is used to represent a conservative assessment in this case. The values ​​of off-site parameters can be referenced in Appendix C of the "Hydrogeological Handbook" and "Guidelines for Groundwater Pollution Simulation, Prediction, and Assessment".

[0158] (4) Model Identification and Validation: The model should be validated based on multiple periods of field measurement data, requiring at least one period of water level data during the normal water period of a hydrological year or the effective average of multiple periods of water level data for a complete hydrological year. Model output variables can include indicators such as head, flow rate, concentration, pollutant transport time, and pollutant removal rate; calibration should ensure that the simulated groundwater flow field or pollutant plume morphology, range, direction, and other main characteristics are basically consistent with the actual situation; calibrated hydrogeological parameters should conform to actual hydrogeological conditions; and migration model parameters should conform to solute transport characteristics and hexavalent chromium migration patterns. Unless the model's simulated values ​​agree acceptablely with the measured values ​​of hexavalent chromium concentration in the field, the model cannot be reliably used to assess the long-term effectiveness of natural attenuation.

[0159] (5) Model predictions: Model predictions should include the following: 1. Changes in pollutant concentration over time; 2. Volume of the affected aquifer (especially where plumes may migrate); 3. Pollutant migration rate and time required to reach sensitive receptors; 4. Timescale for achieving remediation targets; 5. Location of compliance points.

[0160] (6) Sensitivity analysis: The main indicators for analyzing the model's response include hydraulic head, flow velocity, and pollutant concentration. Sensitivity analysis is also used to identify uncertainties in groundwater simulation and prediction.

[0161] (7) Uncertainty Analysis: The impact of uncertainties in the conceptual model and parameter calibration on the model results should be fully considered. If the evaluation indicates insufficient certainty, further data and information should be obtained. Commonly used methods for evaluating uncertainty include sensitivity analysis, Monte Carlo methods, and first-order error analysis. Through the analysis of parameter uncertainty, the simulation results can be expressed as a range of possible results, thus reflecting the uncertainty of the simulation parameters.

[0162] 2.2.1 Comprehensive assessment of technical feasibility

[0163] A comprehensive evaluation is then conducted based on the results of the technical, economic, and regulatory feasibility analysis. The feasibility of monitoring natural decay technology is judged based on the weight of each factor. The feasibility of each factor is summarized in Table 3.

[0164] Table 3 Summary of Feasibility Assessment Elements

[0165]

[0166]

[0167] Based on the preliminary feasibility assessment results of monitoring natural decay, a comprehensive judgment will be made as to whether monitoring natural decay is a feasible option, and thus, a final decision will be made as to whether monitoring natural decay is a feasible option:

[0168] (1) The evaluation process showed that there were basically no restrictions on the use of natural attenuation technology;

[0169] (2) Based on the assessment, monitoring natural attenuation may achieve the control and restoration goals;

[0170] (3) Compared with other repair methods, it has greater cost-effectiveness.

[0171] Specifically, in S3, the engineering design and implementation, combined with the site's hydrogeological conditions and the spatiotemporal evolution trends of groundwater pollutants, determines the application scenario, designs a monitoring scheme for the natural attenuation monitoring project, and formulates phased objectives and emergency plans for the project's operation, specifically including:

[0172] S3.1 Application Scenario Analysis: Under the determined risk management or remediation model, based on the technical feasibility assessment, groundwater pollution risk management and remediation can be carried out by selecting a single monitoring natural decay technology or by combining it with other technologies, as detailed in Table 4.

[0173] Table 4. Scenarios for the availability of natural decay monitoring technologies.

[0174]

[0175]

[0176] S3.2, Engineering design for monitoring natural attenuation:

[0177] S3.2.1. Update the site concept model during the engineering phase: Conduct supplementary investigations and identify natural attenuation effects during the engineering phase, and update the site concept model to support the engineering design.

[0178] S3.2.2 Determination of monitoring scope: For sites that use natural attenuation monitoring technology alone, the monitoring scope should cover the upstream uncontaminated area, the entire pollutant plume, and the downstream range of the pollutant plume to the sensitive receptor. For sites that use natural attenuation monitoring technology in combination with other combined technologies, the monitoring scope should cover the upstream uncontaminated area, the pollutant plume outside the treatment range of other combined technologies, and the downstream range of the pollutant plume to the sensitive receptor.

[0179] S3.2.3 Monitoring well network design, including:

[0180] (1) Monitoring type

[0181] Monitoring sites can be divided into three categories: monitoring conditions outside the boundary of the pollutant plume (environmental monitoring), monitoring the natural decay process (process monitoring), and monitoring the spread of the pollutant plume and verifying its impact on receptors (migration monitoring).

[0182] Environmental monitoring should be conducted outside the pollutant plume boundary to determine control conditions and to provide insights into the potential for plume migration, assuming that redox reactions and the aquifer's ability to immobilize pollutants are the primary controlling factors. The scope and duration of environmental monitoring will be influenced by the sensitivity of aquifer chemistry to changes in recharge water quality and processes that may alter its composition.

[0183] Process monitoring is used to verify whether the degradation is occurring as predicted. If process monitoring indicates that the degradation is not occurring as expected, a change in remediation methods may be necessary. Process monitoring is specific to the contaminant.

[0184] Monitoring the spread of the contamination plume (migration monitoring) and any impacts on receptors is another important aspect of the monitoring program. This monitoring objective can be achieved by conducting multi-level monitoring below the contamination plume, near lateral gradient boundaries, and near any other boundaries specified in the contingency plan, and monitoring potential sensitive target locations (e.g., drinking water or other ecologically sensitive targets) to directly verify whether the contamination has an impact. Monitoring well locations between the contamination plume and boundaries or potential receptors should be close enough to the plume to allow contingency plans to be implemented before the contaminant migrates across warning points or affects sensitive targets. Ultimately, determining the location of monitoring wells for migration monitoring depends on a field assessment of contaminant migration and transformation.

[0185] (2) Point density design

[0186] The design of monitoring point density should follow these principles: the density (including location and number) of monitoring wells should be determined comprehensively based on the hydrogeological conditions and groundwater pollution characteristics of the site; in the initial design of the monitoring well network, a density of 40m×40m should be achieved at the boundary of the target pollution plume range for monitoring natural attenuation technology; during the monitoring process, if the pollution plume range expands or shrinks significantly, monitoring wells should be appropriately added at the boundary; existing monitoring wells (civilian wells, production wells, or springs) can be selected, and the selection should be based on meeting the monitoring needs of the project and referring to the "Technical Specification for Groundwater Environmental Monitoring" (HJ 164-2020); vertical monitoring should also consider the variation of hexavalent chromium concentration with depth, and for aquifers with large thickness or strong heterogeneity, it is recommended to construct wells or conduct sampling in layers.

[0187] (3) Design and construction of monitoring wells

[0188] (3.1) Design of monitoring wells

[0189] Screen tube design requirements: Monitoring wells should be located in the target aquifer where the pollution plume is located, and should meet the monitoring requirements for groundwater level, water temperature, water quality, and other indicators. The top of the screen tube should be at least 1.0m above the water surface during the high-water season, and the bottom of the screen tube should be at least 1.0m below the water surface during the low-water season. The specific length of the screen tube and the location of the screen opening can be optimized and adjusted according to the monitoring objectives.

[0190] Orifice and well diameter design requirements: The inner diameter of the monitoring well casing should be no less than 68mm to meet the requirements of water level measurement, well flushing, and sampling, and can be increased according to actual needs. The final orifice diameter should be determined based on the well type, the specifications of the well casing and screen pipe, and the gravel packing thickness. Relevant standards for orifice diameter can be found in DZ / T 0148. The inner diameter of the monitoring well casing should be no less than 50mm, based on the diameter required for well flushing and sampling. Other monitoring well construction requirements should be implemented in accordance with HJ 164.

[0191] Well casing material design requirements: The selection of well casing material must consider factors such as well depth, well diameter, material strength, material chemical properties, groundwater corrosivity, microbial activity, and cost. It should be made of a sturdy, corrosion-resistant material that has no impact on groundwater quality. Generally, PVC-U, steel, or stainless steel pipes can be used as well casing materials. The compatibility of the well casing material with contaminants must be considered to avoid affecting water quality monitoring results. Well casing should use threaded connections; no adhesives should be used.

[0192] (3.2) Construction of monitoring wells

[0193] Construction shall be carried out in accordance with the technical specifications such as HJ 164, DZ / T 0270, and DZ / T 0148.

[0194] S3.2.4 Monitoring Items and Monitoring Frequency

[0195] Groundwater: Pollutant indicators, including hexavalent chromium, total chromium, manganese, and other site-specific indicators; hydrogeochemical indicators, including pH, Eh, DO, conductivity, temperature, TOC, etc.; chemical indicators, such as the content of major anions and cations (potassium, sodium, calcium, magnesium, bicarbonate, carbonate, chloride, sulfate), and the content of minor ions (Fe). 2+ S 2- NO 3- NO 2- wait).

[0196] Soil and aquifer media: pollutant indicators, such as hexavalent chromium and total chromium, and other site-specific indicators; geochemical indicators, such as dry matter, moisture, pH, bulk density, redox potential, cation exchange capacity, organic matter content, electrical conductivity, sulfate, sulfides, and the occurrence form of chromium pollutants in the solid phase.

[0197] Monitoring frequency: For groundwater monitoring, the frequency will be once per quarter for the first two years of the project implementation phase. After two years, the monitoring frequency will be determined based on the trend of the pollution plume and the chemical conditions of the groundwater. If the chemical conditions of the groundwater are stable and the morphology of the pollution plume is stable or shrinking, the frequency can be adjusted to once every six months or once a year. For soil and aquifer media testing, the testing will be conducted once at the beginning of the project phase, and the subsequent monitoring frequency will be determined based on the trend of the pollution plume and the chemical conditions of the groundwater.

[0198] S3.3, Phased Objectives: The monitoring plan should include phased objectives to continuously optimize the monitoring scheme during the monitoring process. These phased objectives should include: demonstrating that the pollution plume remains stationary or shrinks; identifying no changes in key conditions such as hydrogeology, geochemistry, and microbial communities; setting compliance points for different phases based on expectations, and verifying whether these compliance points are met; and, in scenarios involving the combined use of multiple technologies, assessing whether the overall project can achieve its predetermined control and remediation objectives.

[0199] S3.4, Emergency plan development, specifically including:

[0200] 3.4.1 The criteria for determining when to activate the emergency plan are as follows:

[0201] (1) The concentration of pollutants in groundwater has increased significantly, or new pollutants have appeared in groundwater;

[0202] (2) A significant increase in pollutant concentration near the pollution source indicates the possible release of new, unknown pollution sources;

[0203] (3) Pollutants were detected in monitoring wells outside the known boundary of the pollution plume;

[0204] (4) The concentration of pollutants in the groundwater at the compliance points or downstream sensitive targets set up in the site remediation and risk management project exceeds the limit;

[0205] (5) The rate of decay of pollutant concentration in groundwater has slowed down, and it has been assessed that the treatment target cannot be achieved within the expected time frame;

[0206] (6) Changes in hydrogeological conditions or geochemical parameters may indirectly affect the chemical or biological degradation process of pollutants.

[0207] (7) Changes in the use of land or groundwater alter the human health and environmental risks of groundwater;

[0208] (8) Other pollution incidents occurred within the site.

[0209] 3.4.2 Emergency Plan Design:

[0210] Emergency response plans can consist of one or a combination of technologies. Possible measures include extraction treatment, in-situ injection, and enhanced monitoring of natural attenuation. The chosen remediation model, restoration and risk management objectives, and technological maturity should be comprehensively considered. For specific technology selection, refer to HJ 25.6. Emergency response plan content includes:

[0211] (1) Basis for implementing the emergency response plan;

[0212] (2) Spatial scope and timeframe of emergency response;

[0213] (3) Technical approach to emergency response plan;

[0214] (4) Specific measures to be taken;

[0215] (5) Evaluation of the effectiveness of emergency response plan.

[0216] Specifically, in S4, the effectiveness of monitoring natural decay is evaluated from two aspects: process evaluation and summative evaluation, including:

[0217] 4.1 Process Assessment (Engineering Operation and Monitoring Analysis)

[0218] Monitoring and analysis should be conducted according to established monitoring indicators and frequencies. Conceptual and numerical models should be updated promptly to assess changes in natural degradation conditions within the site and evaluate the effectiveness of natural degradation to determine whether the expected goals can be achieved. The assessment can be specifically conducted from the following aspects:

[0219] (1) It can be proven that natural decay is occurring as expected;

[0220] (2) It can detect any changes in environmental conditions that reduce the effectiveness of natural degradation, including changes in hydrogeology, geochemistry, microbial communities or others;

[0221] (3) It can detect products that reduce the toxicity or mobility of pollutants;

[0222] (4) It can be confirmed that the pollution plume is continuously shrinking;

[0223] (5) It can be demonstrated that the health and environmental risks to sensitive receptors are acceptable;

[0224] (6) It can detect environments where new pollutants are released, and may affect the effectiveness of natural decay;

[0225] (7) It can be proven that the control and remediation objectives can be achieved.

[0226] 4.1.1 Principles of Process Evaluation

[0227] The basis for process assessment is the detection data obtained during the monitoring process. Therefore, it is necessary to analyze the monitoring data to determine whether the natural attenuation process has achieved the expected goals. The objects of effectiveness assessment can be mainly divided into two aspects: changes in pollutants and changes in environmental factors. Changes in pollutants mainly include: the natural attenuation process of pollutants meeting expectations, the continuous reduction of the pollution plume range, and downstream sensitive targets being unaffected by polluted groundwater, thus achieving the remediation goals. Changes in environmental factors mainly include: no significant changes in hydrogeological, geochemical, and microbiological information that may affect the long-term effectiveness of natural attenuation, and no migration or release of the natural attenuation products of hexavalent chromium (adsorbed Cr(VI) and reduced Cr(III)). In summary, the key to assessing the effectiveness of natural attenuation monitoring lies in using on-site monitoring results to illustrate the spatiotemporal variation patterns of pollutants in groundwater, thereby assessing the implementation effect of natural attenuation monitoring. Effectiveness assessment should be conducted continuously throughout the implementation process of natural attenuation monitoring, at least annually, to help determine whether the natural attenuation process meets the expected goals and whether it is necessary to activate emergency plans for emergency control of groundwater pollution.

[0228] 4.1.2 Process Evaluation Methods

[0229] Graphical methods can be used to present the changes in the pollution plume by using pollutant concentration contour maps or graphs showing the trend of pollutant concentration in monitoring wells over time or distance.

[0230] (1) Pollutant concentration contour map

[0231] Based on the monitoring results of pollutant concentrations in groundwater within the site, isopleth maps of pollutant concentrations in groundwater can be drawn to understand the pollution levels in different areas within the site, including heavily polluted areas, unpolluted areas, and the extent of the pollution plume. Furthermore, monitoring results from different periods can be plotted as concentration isopleth maps to understand the spatiotemporal variation patterns of pollutant plumes in groundwater.

[0232] Data from the same quarter of different years should be selected for analysis whenever possible to minimize the impact of seasonal high and low water periods. For aquifers with significant thickness or multiple aquifers simultaneously contaminated, separate contour maps of groundwater pollutant concentrations at different depths or strata should be drawn.

[0233] (2) Trend graph of pollutant concentration in monitoring well over time or distance

[0234] By observing the trend of hexavalent chromium concentration over time in the same monitoring well, or the change of hexavalent chromium concentration with distance in monitoring wells at different times, the existence of natural decay processes can be identified.

[0235] Mathematical statistical methods can be used to assess pollution trends when the variation pattern of hexavalent chromium concentration is not obvious or when there is a large amount of monitoring data.

[0236] 4.2 Summative Assessment

[0237] If, during the monitoring of natural attenuation, the monitoring data of the target pollution plume are all below the remediation target value or the risk control target has been achieved, and the assessment indicates that the pollution still has a trend of further attenuation, the effectiveness assessment can be carried out with reference to HJ 25.6, depending on the remediation model implemented on the site.

[0238] Example:

[0239] For sites contaminated with hexavalent chromium in groundwater, this invention addresses the issue that due to the strong migration properties of hexavalent chromium, it can form long, vertically distributed plumes of contamination in the groundwater (hereinafter referred to as "contamination plumes").

[0240] For pollution plumes that extend beyond the pollution source boundary (or plant boundary), after meeting the conditions of applicability analysis, the technical methods and monitoring systems recommended in this patent can be used to monitor the natural decay of hexavalent chromium. Based on the natural decay effect identified in the feasibility study, geochemical parameter characterization experiments are carried out to calculate the natural decay rate constant. Based on the updated conceptual model and related data (strata, groundwater flow simulation, hexavalent chromium distribution and dynamic change trends, etc.), a numerical model is constructed to predict the temporal and spatial distribution of the target pollution plume without any artificial measures or certain enhanced monitoring of natural decay measures.

[0241] Following the given principles and methods, carry out engineering design and construction, select a control and repair model, formulate phased goals, develop emergency measures, and assess whether the phased goals have been achieved based on the engineering implementation and monitoring.

[0242] If the phased goals cannot be achieved, emergency measures should be initiated to achieve the phased goals. At the same time, based on the latest acquired monitoring data, the parameters of the existing prediction model should be optimized to better reflect the actual situation. Based on the revised and iterated prediction model, the setting of phased goals and engineering solutions should be further optimized.

[0243] Ultimately, this will enable the pollution plume to remain stable or continuously shrink according to the established goals, thus meeting the project's established objectives.

[0244] In summary, this invention proposes a natural attenuation technology process for monitoring hexavalent chromium pollution in groundwater. Compared to related technologies published in developed countries and regions, its structure fully considers my country's regulatory needs and technical requirements for groundwater-contaminated sites. Each technical step is structurally rigorous with clear logical relationships, forming a closed-loop structure from technology selection to project cessation. The evaluation methods, monitoring indicator requirements, monitoring network construction requirements, and data analysis methods provided in this invention throughout the technical process conform to scientific principles and are highly scientific. Specific algorithms are provided, ensuring strong operability. Furthermore, it aligns with my country's groundwater pollution prevention and control management approach, making it highly practical.

[0245] The present invention also provides an electronic device comprising one or more processors; a memory; and one or more application programs, wherein the one or more application programs are stored in the memory and configured to be executed by the one or more processors, the one or more programs being configured to perform a method for monitoring the natural decay of hexavalent chromium contaminated groundwater as described above.

[0246] Of course, those skilled in the art will understand that all or part of the processes in the above embodiments can be implemented by a computer program instructing related hardware (such as a processor, controller, etc.). The program can be stored in a computer-readable storage medium, and when executed, it can include the processes described in the above method embodiments. The storage medium can be a memory, magnetic disk, optical disk, etc.

[0247] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for monitoring the natural decay of hexavalent chromium-contaminated groundwater, characterized in that, Includes the following steps: S1, Applicability Analysis: Based on the basic site selection conditions, hydrogeological conditions, and site pollution survey data, determine whether the hexavalent chromium groundwater contaminated site is suitable for using the natural attenuation monitoring technology; among which... Based on the basic conditions for site selection, determine whether a site contaminated with hexavalent chromium in groundwater is suitable for natural attenuation monitoring. Specifically, this includes determining whether the site is unsuitable for natural attenuation monitoring, including: The control and remediation of the target pollution plume is urgent; Located in a groundwater-sensitive area within a groundwater drinking water source protection zone; It does not meet the relevant requirements of land use planning or environmental impact assessment; The health risks to humans in the areas where groundwater contamination plumes are distributed are unacceptable. The pollution source or high-concentration pollution center has not been eliminated or risk management has not been planned. Determining whether a site contaminated with hexavalent chromium in groundwater is suitable for monitoring natural attenuation techniques based on site contamination survey data includes: Acquire site pollution survey data, including groundwater pollution plume morphology and variation characteristics, as well as monitoring data of chromium-contaminated soil and aquatic media; Based on the assessment of hydrogeological conditions and analysis of site pollution survey data, if the following two conditions are met, then the monitoring of natural attenuation technology is suitable for sites contaminated with hexavalent chromium groundwater: (1) Based on the spatial variation of the ratio of conservative ions and pollutant concentrations in a single pollution plume, it is determined that there is a natural decay effect; (2) Based on the changes in the morphology of multiple pollution plumes over time, it is determined that there is a trend of pollution plumes becoming stable or shrinking; (3) Based on the monitoring of chromium pollution in soil and water-bearing media, it is determined that there are hexavalent or trivalent chromium contents exceeding the background value; S2, Feasibility Analysis: If the hexavalent chromium groundwater contaminated site is suitable for monitoring natural attenuation technology, then a supplementary site investigation will be conducted based on the risk management objectives to update the site's conceptual model; simulation analysis will be performed based on the updated site conceptual model, and various feasibility assessment factors will be considered to determine whether the monitoring natural attenuation technology is feasible; specifically, the supplementary site investigation and updating of the site conceptual model based on the risk management objectives includes: Supplementary investigations were conducted on hydrogeological conditions, characteristic pollutants, biogeochemical conditions, and biogeochemical reaction parameters to obtain supplementary investigation data for the land parcel. Identify natural attenuation effects based on the characteristics of pollution plume changes, geochemical index changes, and the types and capabilities of indigenous microorganisms. Based on supplementary survey data of the land parcels and the effects of natural attenuation, the conceptual model of the land parcels was updated. S3, Engineering Design and Implementation: If monitoring natural attenuation technology is feasible, then, in combination with the hydrogeological conditions of the site and the spatiotemporal evolution trend of groundwater pollutants, determine the application scenario, design a monitoring scheme for monitoring natural attenuation, and formulate phased objectives for the operation of the project. S4, Effectiveness Evaluation: The effectiveness of monitoring natural decay is evaluated from two aspects: process evaluation and summative evaluation.

2. The monitoring and natural decay technology method for hexavalent chromium contaminated groundwater according to claim 1, characterized in that, In step S1, determining whether a site contaminated with hexavalent chromium groundwater is suitable for monitoring natural attenuation technology based on hydrogeological conditions specifically includes: Based on four hydrogeological conditions—soil and rock pore type, aquifer medium type, stratigraphic structure, and topography—the suitability of using natural attenuation monitoring technology on hexavalent chromium groundwater contaminated sites is divided into three categories: high, medium, and low. Among them, the case with high suitability is determined to be suitable for using natural attenuation monitoring technology on hexavalent chromium groundwater contaminated sites.

3. The monitoring and natural decay technology method for hexavalent chromium contaminated groundwater according to claim 1, characterized in that, In step S2, simulation analysis is performed based on the updated land parcel conceptual model, and multiple feasibility assessment factors are considered to determine whether the monitoring of natural attenuation technology is feasible. Specifically, this includes: Based on the updated land parcel conceptual model, simulation analysis is achieved by setting assessment objectives, selecting assessment models, calibrating model parameters, identifying and verifying models, and conducting model prediction, sensitivity analysis, and uncertainty analysis. The feasibility of monitoring natural decay technology is determined by considering factors related to technical feasibility, economic feasibility, and regulatory feasibility.

4. The monitoring and natural decay technology method for hexavalent chromium contaminated groundwater according to claim 1, characterized in that, In S3, based on the site's hydrogeological conditions and the spatiotemporal evolution trends of groundwater pollutants, the application scenario is determined, a monitoring scheme for the natural attenuation monitoring project is designed, and phased objectives for the project's operation are formulated, specifically including: S3.1, Determine the application scenario: Under the determined risk management or remediation model, based on the results of the technical feasibility assessment, select a single monitoring natural decay technology or a combination of other technologies to carry out groundwater pollution risk management and remediation; among them, other technologies include hydraulic control, in-situ reaction zone, PRB, barrier wall, contaminated soil remediation, extraction treatment, and in-situ injection technology. S3.2, Design a monitoring scheme for monitoring natural decay projects: Update the site concept model through supplementary investigations and identification of natural decay effects during the project phase; determine the monitoring scope; design the monitoring well network, monitoring items, and monitoring frequency; S3.3, formulate phased objectives for project operation, including: demonstrating that the pollution plume remains stationary or shrinks; no changes in key hydrogeological, geochemical, or microbial community conditions are found; setting compliance points for different stages according to expectations, and whether the compliance points are met; and whether the overall project can achieve the established control and remediation objectives in the context of multiple technologies used in combination.

5. The monitoring and natural decay technology method for hexavalent chromium contaminated groundwater according to claim 4, characterized in that, In S3, engineering design and implementation also includes developing contingency plans; specifically, developing contingency plans includes: Determine when to activate the emergency response plan; An emergency plan is designed based on several aspects, including the basis for implementing the emergency plan, the spatial scope and timeline of the emergency response, the technical approach of the emergency plan, the specific measures to be taken, and the evaluation of the effectiveness of the emergency plan.

6. The monitoring and natural decay technology method for hexavalent chromium contaminated groundwater according to claim 1, characterized in that, In S4, the effectiveness of monitoring natural decay is evaluated from two aspects: process evaluation and summative evaluation. Specifically, this includes: Process assessment was conducted using graphical and mathematical statistical methods. A final assessment was conducted based on the Technical Guidelines for Groundwater Remediation and Risk Management of Contaminated Sites (HJ 25.6).

Images