Combined bioremediation method for shallow groundwater btx pollution

Abstract

The application discloses a combined bioremediation method for BTEX pollution in shallow groundwater, and belongs to the technical field of groundwater remediation. In view of problems such as long repair period of separate monitoring natural attenuation, and lack of diagnostic basis for enhanced bioremediation strategy, the application provides a systematic method: step 1: performing pollution site characterization and monitoring system construction; step 2: evaluating natural attenuation capacity and main control mechanism through long-term monitoring; step 3: based on the evaluation result, selecting and injecting an electron acceptor solution containing a sulfate salt, a nitrate salt or a combination thereof, compounding nutrient salt and a pH buffer, and implementing enhanced bioremediation; step 4: dynamically monitoring the concentrations of BTEX, the electron acceptor and the degradation product, and optimizing the injection strategy accordingly; and step 5: until the pollutant concentration is stably lower than the repair target. The application is suitable for shallow groundwater BTEX pollution sites caused by fuel leakage of gas stations and the like.

Description

Technical Field

[0001] This invention relates to the field of in-situ remediation technology for groundwater pollution, and more particularly to a combined bioremediation method for shallow groundwater BTEX pollution. Background Technology

[0002] Groundwater resources account for approximately 98% of the Earth's usable freshwater resources and are the most important source of water for human production and daily life. However, in the past two decades, with the acceleration of industrialization, petrochemical production, leaks in underground oil storage tanks, damage to oil pipelines, and seepage from abandoned refinery sites have led to large amounts of fuel entering the soil and groundwater. Fuel leaks mainly exist in the form of light non-aqueous phase liquids (LNAPLs), of which monocyclic aromatic hydrocarbons (BTEXs) account for only a small percentage by mass of gasoline. However, due to its high water solubility, the proportion of BTEX in the water-soluble fraction can be as high as [missing information] when gasoline comes into contact with water. BTEX is characterized by its strong biotoxicity and high migration ability. Benzene has been identified as a carcinogen by the World Health Organization, and the U.S. Environmental Protection Agency stipulates that the maximum permitted level of benzene in drinking water is [missing information]. my country's "Standards for Drinking Water Quality" (GB 5749-2006, new standard GB 5749-2022) sets the detection limit for benzene at [value missing]. BTEX has been included in the priority control pollutant lists of the US EPA and my country.

[0003] Currently, in-situ remediation technologies for BTEX pollution in groundwater mainly include monitoring natural attenuation and enhanced bioremediation. Monitoring natural attenuation utilizes the existing physical dilution, adsorption, volatilization, and microbial degradation processes to reduce pollutant concentrations, offering advantages such as low cost, no need for additional energy input, and no secondary pollution. Studies have shown that under natural conditions, BTEX can undergo various attenuation processes, including aerobic degradation, denitrification, iron reduction, sulfate reduction, and methanogenesis. For example, Chen et al. (2004) obtained the natural attenuation rate constant of benzene through sand trough experiments. Toluene is However, monitoring natural degradation has significant drawbacks: for sites with high concentrations of pollution or unfavorable hydrogeological conditions, the remediation period is extremely long. For example, Mulligan et al. (2004) reported an initial concentration... It takes approximately 250 years for benzene to naturally decay back to a safe level.

[0004] Enhanced bioremediation accelerates pollutant degradation by injecting electron acceptors (such as oxygen, nitrates, sulfates, and ferric iron), nutrients (nitrogen and phosphorus), or electron donors (such as hydrogen-releasing substrates) into the groundwater to stimulate the activity of indigenous degrading microorganisms. For example, Sublette et al. (2006) significantly improved the biodegradation rate of BTEX by injecting sulfates into gasoline-contaminated aquifers; Cunningham et al. (2001) successfully promoted the degradation of xylene and benzene at a site in California by jointly injecting nitrates and sulfates. However, existing enhanced bioremediation technologies have the following shortcomings: First, the selection of electron acceptors often lacks a scientific diagnosis of the site's natural degradation control mechanisms, and indiscriminate injection may lead to low electron acceptor utilization efficiency or even inhibition of indigenous microbial communities; second, the use of single electron acceptors makes it difficult to address the differences in the degradation difficulty of various BTEX components (especially benzene and ethylbenzene); third, the lack of a complete and systematic joint remediation process from site diagnosis and scheme design to dynamic regulation results in low standardization of technology application and high remediation costs.

[0005] Therefore, developing a method that can organically combine the diagnostic advantages of monitoring natural decay with the efficiency advantages of enhancing bioremediation to achieve precise, efficient, and economical remediation of BTEX pollution is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To achieve the above objectives, this invention provides a combined bioremediation method for BTEX contamination in shallow groundwater, comprising the following steps:

[0007] Step 1: Construction of Contaminated Site Characterization and Monitoring System

[0008] A detailed hydrogeological survey of the target shallow aquifer was conducted to determine the aquifer medium type (e.g., sand, gravel, silt, etc.), thickness H (unit: m), porosity n, and permeability coefficient K (unit: m). ), Groundwater flow direction and velocity v (unit: Key parameters such as [missing information] were used. Through drilling and sample analysis, the range of the pollution source and the three-dimensional distribution characteristics of the pollution plume were delineated. Based on this, a groundwater monitoring well network was deployed along the groundwater flow direction, including at least [missing information] located upstream of the pollution source. The monitoring wells should include background wells, source strength wells at or immediately downstream of the pollution source, wells within the degradation zone of the pollution plume, and wells at the edge of the pollution plume. The screen tubes of the monitoring wells should cover the entire vertical distribution range of the pollution.

[0009] Step 2: Assessment of Natural Attenuation Capacity

[0010] The following data were obtained through long-term monitoring over at least one hydrological year (usually more than 12 months):

[0011] Concentrations of each component in BTEX (unit: or );

[0012] Natural electron acceptor concentration: dissolved oxygen ( ), nitrates ( ), sulfate ( ), trivalent iron ( ) or total iron;

[0013] Concentration of degradation products: ferrous iron ( ), sulfides ( or ), methane ( );

[0014] Auxiliary parameters: pH, redox potential ( ,unit: ), conductivity, temperature;

[0015] Microbial community data: Functional genes (such as benzene dioxygenase gene and sulfate-reducing bacteria gene) were determined using qPCR. Genes, denitrifying bacteria The copy number of a gene.

[0016] Calculate the natural decay rate constant k (unit: ) of each component of BTEX using the mass flux method or the first-order decay model. The formula for the first-order attenuation model is:

[0017]

[0018] in Let be the pollutant concentration at time t. The initial concentration was used. By analyzing the spatiotemporal evolution of redox-sensitive parameters, the main controlling mechanism of natural site degradation was identified. The specific judgment criteria are as follows:

[0019] like and The temperature drops significantly along the streamline, with aerobic degradation being the main controlling mechanism.

[0020] like The concentration decreased significantly and or Accumulation, with denitrification as the primary controlling mechanism;

[0021] like Concentration decrease and or The rise is primarily controlled by sulfate reduction.

[0022] like A significant increase and a decrease in total iron were observed, with iron reduction as the main controlling mechanism.

[0023] like The concentration increased significantly, and the main controlling mechanism was methanogenesis.

[0024] The long-term remediation potential of the site is assessed based on the master control mechanism. If the natural decay rate constant k is less than a preset threshold (e.g., benzene), then... If the time to reach the repair target is predicted to be more than 5 years, then it is determined that enhanced bioremediation needs to be initiated.

[0025] Step 3: Enhance the development and implementation of bioremediation solutions

[0026] Based on the evaluation results of step 2, an enhanced bioremediation program is formulated and implemented, the core of which is the scientific selection and proportioning of electron acceptors:

[0027] (1) If the main mechanism of natural decay is denitrification, it indicates that the denitrifying bacteria in the site are active but the electron acceptor is weak. If insufficient, priority should be given to injecting nitrates. The injection solution is sodium nitrate ( ) or potassium nitrate ( An aqueous solution of ) with a concentration controlled at (by (Calculation). The injection rate is calculated based on groundwater flow and plume volume to ensure the integrity of the plume core area after injection. Concentration reached .

[0028] (2) If the main control mechanism of natural decay is sulfate reduction, then the injection of sulfate should be preferentially enhanced. The injected solution is sodium sulfate ( An aqueous solution of ) with a concentration controlled at (by (Calculation), after injection, the core area of ​​the contamination plume Concentration increased to .

[0029] (3) If the pollutant plume contains stubborn components such as benzene or ethylbenzene, and the natural decay assessment shows that they degrade slowly under existing conditions, a compound solution of nitrate and chelated ferric iron should be used preferentially. Ferric iron can be chelated using ferric citrate (… ) or sodium ferric ethylenediaminetetraacetate ( As a chelated form, the concentration is controlled at [specific value]. nitrate .

[0030] (4) For mixed mechanisms or complex pollution scenarios, a mixed solution of sulfate and nitrate is used, with a volume mixing ratio of 1:1 to 5:1 (sulfate solution: nitrate solution), and the total injection volume is determined based on the pore volume of the pollution plume.

[0031] In addition to electron acceptors, phosphorus-based nutrients (such as phosphorus sources) should be selectively added based on the site's background nutrient status. or final concentration by (Calculation) and nitrogen source (if only sulfate is added and ammonia nitrogen in the water is insufficient, supplementation can be made). final concentration by (Calculation). Sodium bicarbonate ( As a pH buffer, it maintains the pH of groundwater at a certain level. The appropriate range.

[0032] The injection method can be continuous injection or pulsed injection. The injection well should be located downstream of the pollution source. Or the core region of the plume. The principle of injection velocity control is to avoid hydraulic fracturing of the aquifer (i.e., the injection pressure does not exceed the overlying pressure) and to prevent uncontrolled diffusion of the plume; the typical injection velocity range is... Per meter of aquifer thickness. The injection cycle is designed based on the pollutant degradation half-life, typically... sky.

[0033] Step 4: Dynamic monitoring and process optimization

[0034] During the injection process and the recovery period after cessation of injection, the following indicators should be continuously monitored at a frequency of once every two weeks to once a month:

[0035] Concentrations of each component in BTEX;

[0036] Electron acceptor ( , ) and its reduction products ( , , The concentration of );

[0037] pH Dissolved oxygen;

[0038] If iron is injected, monitor total iron and ferrous iron.

[0039] Dynamic optimization is performed based on monitoring data, and the specific decision-making logic is as follows:

[0040] When a significant decrease in electron acceptor concentration is detected (e.g., a decrease compared to the injected concentration) Furthermore, an increase in the concentration of its reduction products indicates high microbial degradation activity, and this should be maintained or gradually increased (increased rate). The injection rate is adjusted to continuously supply reactants.

[0041] When electron acceptor accumulation is detected (e.g.) concentration (and there is no downward trend) or pollutant degradation stagnation (BTEX concentration changes monitored twice consecutively). If the pH level is too low, it indicates the presence of other limiting factors, such as nutrient deficiency, unsuitable pH, microbial metabolic inhibition, or toxicity accumulation. In this case, injection should be reduced or suspended, and a supplementary assessment should be conducted: measure the concentrations of ammonia nitrogen and phosphate in the water, and supplement as necessary; if the pH deviates from the target range... If the buffer dosage is not adjusted, and if toxic metabolites (such as high concentrations of ferrous iron or sulfides) are detected, the addition of circulation wells or intermittent injection may be considered.

[0042] If a particular electron acceptor is ineffective, another electron acceptor can be switched or combined. For example, if benzene degradation does not accelerate significantly after 60 days of using sulfate alone, nitrate or ferric iron can be added as a supplement.

[0043] Step 5: Determine if the repair objective has been achieved

[0044] Continue the above steps until the concentrations of all BTEX components in all monitoring wells are consistently below the preset remediation target (refer to GB 5749-2022: Benzene). Toluene xylene Furthermore, monitoring was conducted quarterly over a complete hydrological year (12 months), and there was no rebound trend (the concentration increase did not exceed the target value). If the condition is met, the repair is considered complete, the injection can be stopped, and the process can proceed to the long-term monitoring phase.

[0045] Preferably, to reduce the risks of large-scale field application, a small-scale field pilot test is conducted before or in the early stages of step 3. The pilot test range is typically within the radius of influence of the injection well. ,continued Days. Encrypted monitoring during pilot testing (weekly). (This process is repeated several times) to verify the effectiveness of the selected electron acceptor, obtain the degradation rate constant, optimize the injection concentration and rate, and then apply the optimized parameters to the whole field remediation.

[0046] The beneficial effects of this invention are:

[0047] 1. This invention does not blindly implement biostimulation, but rather scientifically diagnoses the degradation potential and bottlenecks of a site through a pre-emptive, systematic assessment of its natural degradation capacity (step 2). Based on the assessment results (whether the main controlling mechanism is denitrification, sulfate reduction, or iron reduction), electron acceptors are precisely selected and proportioned, achieving targeted treatment. Compared with existing technologies that randomly or empirically select electron acceptors, this invention avoids cost waste and inefficiency caused by inappropriate or excessive use of electron acceptor types. For example, if a site is naturally in a sulfate-reducing state but a large amount of nitrate is injected, it may inhibit the original sulfate-reducing bacteria. However, this invention, through diagnosis, prioritizes and strengthens existing dominant bacteria, significantly improving the targeting and success rate of remediation projects.

[0048] 2. This invention organically integrates long-term, slow monitoring of natural degradation with rapid, efficient enhanced bioremediation, forming a closed-loop system of "assessment first, then enhancement, and finally regulation" (steps 1 to 4). Through dynamic monitoring and process optimization, the optimal microbial degradation environment can be maintained at all times, effectively overcoming the shortcomings of single natural degradation remediation cycles that are too long (decades to hundreds of years), unstable effects of single bioremediation, or targeting only specific components. In particular, the mixed electron acceptor strategy (such as nitrate + chelated iron) proposed for recalcitrant components such as benzene can effectively initiate and accelerate their anaerobic degradation process. Example 1 shows that after using the method of this invention, the degradation rate of benzene increased from... Upgraded to The repair time has been shortened from the estimated >10 years to <1 year;

[0049] 3. This invention relies entirely on stimulating native microorganisms at the site, without introducing exogenous non-native strains, posing no ecological safety risks and aligning with green remediation principles. The remediation process is conducted in situ, requiring no large-scale excavation or extraction, minimizing site disturbance and avoiding secondary surface pollution and treatment costs associated with extraction methods. Furthermore, accurate pre-treatment diagnosis and dynamic optimization during the process prevent ineffective chemical application, significantly reducing material and operating costs. Economic calculations show that compared to traditional extraction methods, the overall cost of this invention can be reduced. Compared to blindly enhancing repair methods, the dosage of the medicine can be reduced. This invention provides a green and sustainable groundwater remediation paradigm that achieves a good balance between environmental and economic benefits. Attached Figure Description

[0050] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only for this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0051] Figure 1 This is a flowchart of the steps of the method of the present invention. Detailed Implementation

[0052] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. It should also be noted that, to make the embodiments more comprehensive, the following embodiments are the best and preferred embodiments, and those skilled in the art can use other alternative methods to implement some well-known technologies; moreover, the accompanying drawings are only for more specific description of the embodiments and are not intended to specifically limit the present invention.

[0053] Example 1

[0054] An experiment on sulfate-nitrate combined enhancement of bioremediation based on natural degradation diagnosis;

[0055] according to Figure 1 As shown, this embodiment uses a large-scale laboratory physical sand tank model to simulate a traditional gasoline leak scenario at a gas station, verifying the core process of the present invention.

[0056] 1. Characterization of contaminated sites and construction of monitoring systems (Step 1);

[0057] Construct a geometric dimension of A laboratory sand trough (length × width × height). The trough is filled with homogeneous quartz sand with a median particle size of [missing information]. Porosity n=0.38, permeability coefficient A water inlet tank is installed at the left end of the sand trough to maintain a constant water head supply and a groundwater flow rate. At a distance from the inlet tank An oil source chamber was set up to simulate a continuous leak from an underground oil storage tank, and conventional gasoline was injected into it. Along the longitudinal direction of the sand trough (simulating the direction of groundwater flow) in , , , , Five rows of monitoring sampling wells are set up, with six different depths in each row, simulating a "monitoring well network".

[0058] 2. Assessment of natural attenuation capacity (Step 2);

[0059] After simulating a leak, the system operated stably for 30 days, followed by a 120-day period of natural decay monitoring (Phase I). Monitoring indicators included BTEX concentration, , , , , Key monitoring results:

[0060] The initial total concentration of BTEX was approximately benzene Toluene ethylbenzene xylene .

[0061] After 120 days of natural decay, the benzene concentration decreased to... (attenuation Toluene decreased (attenuation ), ethylbenzene decreased (attenuation xylene decreased (attenuation ).

[0062] The decay rate constant of benzene was obtained by fitting the data using a first-order decay model. Toluene .

[0063] Analysis of redox-sensitive parameters: exist between, Below the detection limit From background value Downstream ,at the same time from Rise to , Concentration reached downstream of the pollution plume .

[0064] Assessment Conclusion: The main controlling mechanisms of natural BTEX degradation in this simulated site are sulfate reduction and methanogenesis, with active sulfate-reducing bacteria. The natural degradation rate, especially the degradation rate of benzene, is too low to achieve the remediation target (benzene). The estimated timeframe exceeds 10 years. Therefore, it is necessary to initiate enhanced bioremediation, with sulfate as the preferred electron acceptor.

[0065] 3. Enhance the development and implementation of bioremediation plans (Step 3);

[0066] Based on the evaluation results, a biostimulation solution was prepared: the main component is... The concentration is (by (plan), supplemented by of (Provide nitrogen source) and of (Provide phosphorus source), and use Adjust the pH to 7.2. Downstream of the oil source chamber. Set an injection point at the location to... The solution was continuously injected at a constant flow rate for 60 days (Phase II).

[0067] 4. Dynamic monitoring and process optimization (step 4);

[0068] After the injection begins, encryption monitoring will be conducted weekly. Monitoring data shows:

[0069] On the 15th day after injection, the core area of ​​the contamination plume Concentration from Significantly increased to ,at the same time from Descending to , Concentration from Down to This indicates that sulfate-reducing bacteria were effectively stimulated, competitively inhibiting methanogenesis.

[0070] On day 30, the toluene concentration decreased from... Down to (Removal rate) ), ethylbenzene from Down to (Removal rate) xylene from Down to (Removal rate) ).

[0071] Benzene concentration decreased slowly during the first 30 days of Phase II, only from... Down to The degradation rate k did not increase significantly.

[0072] Process optimization decision: Based on the monitoring results of benzene degradation stagnation, it was determined that a single sulfate-reducing environment was insufficient for benzene degradation. According to the strategy of this invention (step 4 optimization logic), the injection scheme was adjusted on day 40, changing the injection solution to a "sulfate + nitrate" mixture. ( (Calculation) and ( (Calculation) Mix, continue with Injection (Phase III).

[0073] 5. The repair objective has been achieved (step 5);

[0074] After adjusting the injection scheme, monitoring revealed the following:

[0075] Day 50 Concentration drops rapidly in the core region of the plume and is accompanied by The appearance of this indicates that denitrification has been activated.

[0076] On day 60, the benzene concentration dropped to On day 90 (after the end of Phase III and continued operation for 30 days), the benzene concentration decreased to By day 120, the concentrations of all BTEX components had decreased. The following steps achieve the simulated repair goal.

[0077] This embodiment fully verifies the core technical path of "assessment-intervention-optimization" of this invention. Through early monitoring of natural degradation, sulfate reduction was accurately diagnosed as the controlling mechanism, and benzene was identified as a challenging pollutant. Based on this, sulfate-enhanced remediation was first implemented, effectively removing toluene, ethylbenzene, and xylene. Subsequently, based on the degradation response of benzene, a mixed electron acceptor strategy was dynamically optimized, successfully overcoming the benzene degradation challenge. Ultimately, the overall apparent degradation rate of benzene increased from natural degradation... Upgraded to The repair time has been shortened from over 10 years to 120 days. This process fully demonstrates the scientific nature, precision, and efficiency of the method of this invention.

[0078] Example 2

[0079] Nitrate-enhanced bioremediation under ethanol gasoline leak conditions and its on-site pilot verification;

[0080] In this embodiment, the sand trough model with the same structure is used to inject [a substance]. Ethanol gasoline (EG) with volumetric ethanol was used to simulate an oxygenated fuel leak scenario and a field pilot test was introduced.

[0081] 1. Characterization and monitoring system construction for contaminated sites;

[0082] The sand tank parameters are the same as in Example 1. Ethanol gasoline is then dispensed. The initial concentration of ethanol was approximately The initial total concentration of BTEX was approximately .

[0083] 2. Assessment of natural attenuation capacity;

[0084] 90-day natural degradation monitoring showed that ethanol degrades extremely rapidly, with removal occurring within 15 days. However, BTEX degradation was inhibited. Benzene concentration only increased from... Down to Toluene from Down to Redox parameters are displayed. and It was quickly depleted. No significant decrease Mass production (reaching) Assessment Conclusion: Ethanol preferential degradation depletes electron acceptors, leading to BTEX being under methanogenic conditions and experiencing slow degradation. The primary controlling mechanism is methanogenesis, but it is inefficient. Remediation Recommendation: Supplement with readily available electron acceptors to reverse the methanogenic conditions.

[0085] 3. Small-scale field pilot test (optimization step);

[0086] Before large-scale injection, select in the sand tank × × The local area (equivalent to the radius of influence of the injection well) Pilot testing was conducted. A single-port injection well was set up to... injection Solution ( by (Calculation), lasting 30 days, with intensive monitoring twice a week. Pilot-scale results showed that within 10 days after nitrate injection, Consumed rapidly Concentration decrease The benzene concentration began to decrease. At the end of the 30-day pilot test, the benzene concentration had decreased. Based on this, the total injection concentration was determined to be... Injection rate .

[0087] 4. Enhance the implementation and dynamic optimization of bioremediation;

[0088] The entire site was injected with the aforementioned nitrate solution for 90 consecutive days. Monitoring showed that all BTEX components decreased rapidly during the first 60 days, with benzene decreasing from... Down to Toluene dropped to Observed after 60 days Accumulation occurs ( Furthermore, the degradation rate slowed down. Based on the optimization logic, it was determined that the phosphorus source might be insufficient. Therefore, phosphorus was added to the injection solution. to Final concentration Within two weeks after supplementation, It was rapidly consumed again, and the benzene concentration continued to drop. By day 120, all BTEX components had met the standards.

[0089] This embodiment demonstrates that the field pilot-scale step in the method of the present invention can effectively reduce the risk of uncertainty in repair parameters, while dynamic nutrient supplementation further ensures the repair effect.

[0090] Comparative Example 1

[0091] Individual natural decay experiment (compared with Example 1);

[0092] In the same sand tank model as in Example 1, only natural decay monitoring after BTEX leakage was performed, without any electron acceptor injection. Monitoring continued for 360 days. Results showed that after 360 days, the toluene concentration decreased by approximately [missing information]. (from Down to xylene decreases by approximately (from Down to However, the concentration of benzene is still as high as [a certain value]. (initial ), only reduce Based on the degradation rate of benzene, it is estimated that... The target requires at least 10 years. This indicates that relying solely on monitoring natural degradation results in a repair cycle that is unacceptable in engineering practice.

[0093] Comparative Example 2

[0094] Unproven blind enhancement of bioremediation (compared to Example 1);

[0095] In the same model as in Example 1, without prior assessment of natural decay capacity (skipping step 2), a high concentration of nitrate solution was directly injected into the plume. ,by (Form). Results showed that due to the dominance of endogenous sulfate-reducing bacteria at the site, the injected nitrate was not effectively utilized, and monitoring indicated... Almost no consumption within 30 days, at the same time The concentration continued to decrease, indicating that the addition of nitrate inhibited the activity of the previously active sulfate-reducing bacteria. Ultimately, not only was the degradation of benzene not improved (it remained at a certain level after 60 days), but the concentration was also reduced. The degradation rates of toluene and xylene were also lower than in Example 1 when sulfate was used alone (the 60-day removal rate of toluene was only...). This indicates that blindly enhancing remediation without proper site diagnosis not only wastes reagents but may also disrupt the existing degradation microecology, leading to remediation failure.

[0096] Comparative Example 3

[0097] Single electron acceptor does not switch (compared to benzene repair in Example 1);

[0098] Under the same conditions as in Example 1, sulfate was injected for 60 days initially, as in Example 1, but benzene degradation was observed to be halted ( After the concentration stopped decreasing, no process optimization was performed (no switching to the mixed electron acceptor), and sulfate injection continued alone for 180 days. Results: Toluene, ethylbenzene, and xylene continued to decrease slowly, but benzene concentration remained consistently high. Between then and at the end of 180 days, benzene was still at This indicates that if the electron acceptor strategy is not adjusted in a timely manner based on dynamic monitoring results, benzene, a recalcitrant component, will remain for a long time, making it impossible to achieve the remediation goal. The dynamic optimization step (step 4) of this invention is crucial for overcoming the "remediation bottleneck".

[0099] Industrial applicability;

[0100] This invention provides a combined bioremediation method for shallow groundwater BTEX contamination. Based on well-defined hydrogeological and microbial geochemical principles, it achieves efficient, precise, and low-cost remediation of BTEX contamination through systematic monitoring, assessment, intervention, and process optimization. This method is highly operable, requires minimal equipment, and utilizes common, inexpensive, and environmentally friendly chemicals, including electron acceptors (sulfates and nitrates) and nutrients. The injection process can be integrated with conventional groundwater monitoring well networks. This method can be widely applied to various shallow groundwater BTEX contaminated sites caused by leaks in underground oil storage tanks at gas stations, damaged oil pipelines, and seepage at refinery sites, and is particularly suitable for sites with complex hydrogeological conditions, diverse pollutant compositions, and tight remediation timelines. Compared to traditional extraction and chemical oxidation methods, this invention significantly reduces costs and produces no secondary pollution, possessing high industrial practical value and broad application prospects.

[0101] This invention encompasses any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of this invention. To provide the public with a thorough understanding of this invention, specific details are described in detail in the following preferred embodiments; however, those skilled in the art will fully understand the invention even without these details. Furthermore, to avoid unnecessary misunderstanding of the essence of this invention, well-known methods, processes, procedures, components, and circuits are not described in detail.

[0102] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A combined bioremediation method for BTEX contamination in shallow groundwater, characterized in that, Includes the following steps: Step 1: Characterization of contaminated sites and construction of monitoring systems. Conduct hydrogeological surveys of the target shallow aquifers to determine the range of pollution sources and the distribution characteristics of pollution plumes, and deploy a groundwater monitoring well network. Step 2: Assessment of natural degradation capacity. Through long-term monitoring, data on the concentration of each BTEX component, the concentration of natural electron acceptors, and the degradation microbial community are obtained. The natural degradation rate constant is calculated to assess the main control mechanism of natural degradation and the remediation potential of the site. Step 3: Enhance the formulation and implementation of bioremediation schemes. When the natural decay rate in Step 2 is lower than the preset threshold or the remediation standard is not reached within the target time frame, based on the natural decay control mechanism, select and inject a biostimulation solution containing one or more electron acceptors into the groundwater. The electron acceptors are selected from sulfates, nitrates or combinations thereof. Step 4: Dynamic monitoring and process optimization. During and after injection, continuously monitor the concentration changes of BTEX, electron acceptor and their reduction products, and dynamically adjust the injection well layout, injection rate and electron acceptor formulation based on the monitoring data. Step 5: Determining whether the repair target has been achieved. When the concentrations of all BTEX components in all monitoring wells are consistently lower than the preset repair target and there is no rebound trend within a hydrological year, the repair is deemed complete.

2. The combined bioremediation method for BTEX contamination in shallow groundwater according to claim 1, characterized in that, The hydrogeological survey in step 1 includes determining the aquifer medium type, thickness, porosity, permeability coefficient, groundwater flow direction and velocity parameters; the monitoring well network includes background wells, pollution source wells, intra-feather monitoring wells and feather edge monitoring wells laid out along the groundwater flow direction.

3. The combined bioremediation method for BTEX contamination in shallow groundwater according to claim 1, characterized in that, The natural decay rate constant in step 2 is calculated based on the mass flux method or a first-order decay model, wherein the first-order decay model is expressed as follows: , The concentration at time t, The initial concentration is given by , and k is the natural decay rate constant. The main control mechanism identifies redox-sensitive parameters, including the concentration changes and spatial zoning characteristics of dissolved oxygen, nitrate, ferrous iron, sulfate, and methane.

4. The combined bioremediation method for BTEX contamination in shallow groundwater according to claim 1, characterized in that, The principle for selecting electron acceptors in step 3 is as follows: if the main control mechanism of natural decay is denitrification, then nitrate injection should be strengthened first; if the main control mechanism is sulfate reduction, then sulfate injection should be strengthened first; when benzene or ethylbenzene persistent components are present in the pollutant plume, a compound solution of nitrate and chelated ferric iron should be used first.

5. The combined bioremediation method for BTEX contamination in shallow groundwater according to claim 1 or 4, characterized in that, The electron acceptor solution injected in step 3 has a concentration of Sodium sulfate solution, or solution with a concentration of Sodium nitrate solution, or a mixture of the two in a volume ratio of 1:1 to 5:

1.

6. The combined bioremediation method for BTEX contamination in shallow groundwater according to claim 1, characterized in that, The biostimulating solution in step 3 also contains auxiliary components, which are selected from nutrient salts using dipotassium hydrogen phosphate or potassium dihydrogen phosphate as the phosphorus source, and components for maintaining pH at a certain level. Sodium bicarbonate buffer in the range.

7. The combined bioremediation method for BTEX contamination in shallow groundwater according to claim 1, characterized in that, In step 3, the injection method employs either continuous injection or pulsed injection. The injection well is located downstream of the pollution source or in the core area of ​​the pollution plume. The injection flow rate is controlled to avoid hydraulic fracturing of the aquifer or uncontrolled diffusion of the pollution plume; the injection flow rate range is specified as follows: Thickness of the aquifer per meter.

8. The combined bioremediation method for BTEX contamination in shallow groundwater according to claim 1, characterized in that, The dynamic monitoring frequency in step 4 is once every two weeks to once a month. The process optimization includes: maintaining or gradually increasing the injection rate when a significant decrease in electron acceptor concentration and an increase in degradation product concentration are detected; reducing or pausing the injection when electron acceptor accumulation or degradation stagnation are detected, and assessing the limiting factors of microbial activity.

9. The combined bioremediation method for BTEX contamination in shallow groundwater according to claim 1, characterized in that, The combined bioremediation method further includes conducting a small-scale field pilot test before or during step 3 to verify the effectiveness of the selected electron acceptor and optimize injection parameters; the scale of the field pilot test is the radius of influence of the injection well. The duration is sky.

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