Field Demonstration of In Situ Slow-Release Oxygen Chemicals Coupled with Microbial Agents for Injection to Remediate BTEX Contamination

Abstract

The global concern for risk control of organic contaminated sites is becoming more and more prominent. Traditional ex situ remediation techniques are costly and damage the site, seriously destroying the soil structure and ecological functions. Therefore, in situ means of combining material injection and microbial remediation have become a potential pathway for the green, economical, and efficient remediation of contaminated sites. In this work, a 200 m² test block was selected for the coupled injection of slow-release oxygen materials and microbial agents, and long-term monitoring of groundwater was carried out. The results showed that the slow-release materials could release oxygen for a period of 90 days, which provided an oxidizing environment for microorganisms to rapidly degrade BTEX. For the pre-adapted indigenous degradation bacterial agent test group, the degradation degree of BTEX was up to 98% after 40 days of injection. The results of the application on the field scale proved the feasibility of reinforcing biostimulation for remediation of underground organic contamination through the coupled injection of slow-release oxygen materials and microbial agents. The results provided theoretical and technical support for the in situ remediation of petroleum hydrocarbon-contaminated sites.

1. Introduction

BTEX (benzene, toluene, ethylbenzene, and xylene) is a common raw and auxiliary material in the process of petroleum extraction and refining. The structure of the benzene ring means that it can exist stably in the environment, and it is carcinogenic, teratogenic, mutagenic, and highly biotoxic [1]. It causes ecological damage to the ecological environment when it enters into the ground due to accidents, leakage, and other reasons during the production process [2,3]. For a long time, as a difficult point in the treatment of petroleum hydrocarbon pollutants, it has received great attention [4,5,6,7]. BTEX contamination remediation commonly uses technologies including gas-phase extraction, enhanced desorption, and chemical oxidation. However, the above remediation technologies have different degrees of technical limitations, such as high safety risks, large treatment costs, and secondary pollution [8]. Microbial degradation technology has received widespread attention because of its advantages of high economic efficiency, simple operation, greenness, and absence of secondary pollution [5,9,10].

There are many microorganisms that can degrade BTEX in nature. Up to now, hundreds of microorganisms that can degrade BTEX have been found around the world, with most of them distributed in ecosystems contaminated by BTEX, and microbial degradation is an effective way to remove BTEX pollutants [11]. Most of the research has focused on the basic principle, the method of biodegradation, and the growth conditions of the BTEX-degrading bacterial group [12,13]. The degradation pathway and microbial community structure have also received significant attention [14,15]. A variety of microorganisms (e.g., Pseudomonas spp., Pseudomonas syringae, Erythrobacter spp., Sphingomonas spp., and Streptomyces spp.) can degrade BTEX aerobically, with oxygen as the electron acceptor, and anaerobically under anoxic conditions, with NO₃⁻, Fe³⁺, or SO₄²⁻ as the electron acceptor [16,17,18,19]. Wu et al. showed that the microbial community could be remodeled by changing the electron acceptor and the microbial function to improve the degradation of BTEX [10]. Currently, studies on the addition of electron acceptors to enhance the degradation of BTEX are mostly at the stage of indoor experiments or small-scale trials [12,20]. Yang Chufang collected groundwater from a real contaminated site and utilized indoor batch experiments to improve the degradation of BTEX by providing sufficient DO (dissolved oxygen) and obtained suitable parameters for microbial remediation of the contaminated site [21]. Ali et al. added vegetable oils to the contaminated soil and used indoor simulation experiments to conclude that microbial function was significantly enhanced, and therefore vegetable oils can be used as biostimulants to enhance the remediation of BTEX-contaminated soils [22]. Groundwater and soil contamination is a multiscale process and the development of efficient remediation technologies requires research across multiple spatial and temporal scales [23]. Microscale experiments under well-controlled and reproducible setups provide fundamental insights into the mechanisms and optimal conditions of biodegradation [24]. However, the transition from the laboratory to the pilot scale and, ultimately, to the field scale remains challenging as greater uncertainties exist under real-world conditions [25]. The paper by Ellis et al. (2000) presents a landmark study on the successful demonstration of field-scale bioremediation of soil contaminated by trichloroethene (TCE) through the coupled injection of chemical and microbial agents [26]. Therefore, field demonstrations are of far-reaching significance to verify whether the available means of enhanced biostimulation can work for contamination degradation in a real site [27].

BTEX-degrading bacteria can rapidly degrade BTEX under aerobic conditions [28]. The main limitation of in situ microbial remediation technology that failed to be engineered for application and promotion is as follows: the environmental oxygen content of the aquifer location is low [17,29]. After the injection of aerobic bacteria, it cannot be adapted to the micro-oxygenated environment for the bio-utilization of BTEX. It is more likely to die due to the lack of oxygen and the existence of competing flora under the ground, which makes it difficult for the degradation effects to meet expectations [30]. The underground matrix is anisotropic and the pollution is non-homogeneous, which limits microbial activity [31]. It is necessary to define the status of the pollution and utilize the equipment to inject the bacterial agent into the underground pollution-enriched area precisely to make the bacteria play a role [32,33]. Through long-term research and practice, it has been proven that microbial degradation technology has achieved remarkable results in the treatment of petroleum hydrocarbon pollution. In this study, the feasibility and efficiency of microbial degradation technology were further improved by screening and optimizing efficient degradation strains and fine-tuning environmental conditions. This provides strong support for the application of microbial degradation technology to the remediation of actual contaminated sites. Therefore, it is important to further investigate the coupled injection of degrading bacteria and other materials to produce a lasting degradation effect and the precise injection of bacteria and materials to validate the feasibility of in situ enhanced biostimulation for the remediation of BTEX contamination at petrochemical sites. We screened and cultivated highly efficient BTEX indigenous degrading bacterial strains suitable for low-temperature environments based on the contaminated sites, and prepared green and low-cost slow-release oxygen agents to ensure suitable environments for microbial degradation. The degradation effects of contamination under different coupling conditions through indoor batch experiments and sand-box experiments determined the key conditions of the agents and the ratio of the agents, laying the groundwork for the on-site application of the present project. The main objectives of this work are investigating the spatial and temporal injection conditions of materials/bacteria at the site scale by monitoring the groundwater quality during the injection process and evaluating the pollution degradation effects of commercial and pre-adapted bacterial agents at the site scale by determining the concentration of pollutants in the monitoring wells after injection and remediation. In summary, this study shows remarkable novelty in the microbial degradation of petroleum pollutants (especially BTEX) in groundwater. By exploring the optimization strategy and application conditions of microbial degradation technology, a new idea and method are provided for the ecological restoration of groundwater pollution.

2. Experimental Methodology

2.1. Experimental Materials

The slow-release oxygen material is pre-adapted and based on the reaction between Ca(OH)₂ (calcium hydroxide, Sigma-Aldrich, St. Louis, MO, USA, 450146) and hydrogen peroxide (Sigma-Aldrich, HX0636). The purity of the micron-sized material is more than 70% and the oxygen release period is 90–220 days. The slow-release oxygen material was used to provide a suitable aerobic environment for the microbial degradation of pollutants.

Indigenous degrading bacteria were screened from contaminated areas at the test site. The acclimation factor gradient method was used to screen BTEX-degrading bacteria. A mixed bacterial solution that can withstand a high concentration of BTEX and a low-temperature environment was obtained. After further domestication and purification, the highly efficient benzene-degrading strain F was obtained. It belongs to the Bacillus sp. F, as determined by molecular biology. The commercial benzene-degrading strain is a ring-opening bacterial agent (VlandSaide, KH, VlandSaide Co., Ltd., Qingdao, China) produced by VlandSaide Enterprises, which is a functional microorganism that can decompose all kinds of BTEX under aerobic conditions to break the benzene ring into small molecules of organic acids, thus reducing the content of BTEX.

2.2. Hydrogeologic Conditions of the Test Site

The test site is located in the Ordos Basin, in the hinterland of Mao Wusu Desert. The terrain is relatively flat and the surface is mostly covered by the wind-deposited sands. The layer is mainly composed of fine sand, the thickness of which is about 40 m. The lithology of this set of strata in the horizontal and vertical directions does not have a big difference. The groundwater table is relatively shallow, about 4.0 m, and the main source of its recharge is atmospheric precipitation and direct infiltration. The main discharge pathway is atmospheric evaporation and lateral runoff. The hydraulic gradient of groundwater is relatively small, about 0.3%, and the annual change of the water level at the site is relatively small, ranging from 0.6 to 1.0 m, as shown in Figure 1. The permeability coefficient of the aquifer is 18.9 m/d according to the pumping test.

2.3. Injection Impact Radius Test

In order to detect the instantaneous influence radius of the injection process under different pressures and the range of influence of the diffusion of the injected material, two test groups were set up. One injection point was set for each test group, and monitoring wells were set up along the direction of the groundwater flow at a distance of 0.5, 1, 2, and 3 m from the injection point. The arrangements of the injection point and the monitoring point are shown in Figure 2. Two test groups were injected with 110 kg of slow-release oxygen material (configured as a suspension with water at a mass ratio of 1:3 before injection) in sequence with two different injection pressures, a high pressure of 5 MPa and a low pressure of 0.5 MPa, and stirred continuously during the process to ensure the homogeneity of the injected material. After the injection was completed, water quality monitoring was carried out in different monitoring wells at the same time, and soil cores were collected at different distances from the injection point to visually determine the instantaneous influence radius of the material in the horizontal direction under different pressures. In order to investigate the long-term diffusion influence range of the slow-release oxygen material, long-term monitoring of DO value, redox potential value, and other parameters in each monitoring well was carried out for a period of 90 days.

2.4. Monitoring Well Placement and Test Program

In the selected test site, four test groups were set up. The four experimental groups were a blank control group, slow-release oxygen material, coupled slow-release oxygen material/commercial bacteria, and coupled slow-release oxygen material/screening soil bacteria. Combined with the direction of groundwater flow in the test site, each group set up four groundwater monitoring points and one background monitoring point, as shown in Figure 3. According to the design of the test program, a total of 19 groundwater monitoring wells were constructed, and the depth of the wells was 12 m to ensure that groundwater samples could be taken from all layers.

2.5. Initial Contamination Conditions at the Site

Next to the test site is the wastewater treatment plant of an enterprise, and the main pollutants in the wastewater are BTEX, halogenated hydrocarbons, petroleum hydrocarbons, etc. In the previous stage, the underground pollution of the wastewater treatment plant was detected using an MIP (membrane interface probe), and the planar distribution range of the wastewater treatment plant is shown in Figure 4. In order to explore the feasibility of coupled injection for the remediation of the BTEX contamination, the edge area of the contaminated plume was selected for this test (shown by the blue box in Figure 4). The current status of BTEX contamination in groundwater is shown in Figure 5. All samples were taken from 50 cm below the groundwater surface. BETX was detected by the purge-and-trap gas chromatography/mass spectrometry method (HJ 605-2011). The monitoring results showed that the distribution of BTEX concentration in the groundwater was basically the same between 7 days before injection and 1 day before injection, indicating that the concentration field in the test area was basically stable at this time and suggesting that the natural attenuation effect was not obvious. The concentration of BTEX was high at the location close to the contaminated plume of the plant, with the concentration reaching 3058 μg/L at the point of J11. The concentration in the blank control group, which was far away from the contaminated plume, was low. The concentration at the lowest point, J13, was 74 μg/L. In terms of composition, m/p-xylene was the main component of BTEX, accounting for about 50%, and benzene accounted for about 25%. The composition was basically the same at all points, proving that they had a consistent source of pollution. In addition, the saturation degree of BTEX in this area is relatively low, and there is no NAPL phase pooling.

2.6. Coupling Injection Test

According to the indoor microbial remediation batch experimental data, the bacterial load Log (CFU/g) was ≥9.7. The best mass ratio of pollutant and degrading bacterial input under a low temperature (17 °C) was 1:6. Combined with the initial concentration of groundwater contamination in the field test plots and the hydrogeological conditions, it was determined that the amount of bacterial products injected into each group was 115 kg, and the bacterial products were 80 mesh powder formulations. Before injection, the bacterial product was mixed with activation matrix alginate (0.5%) and water in a ratio of 1:9 to form a suspension for activation and then injected 1 h after mixing. The oxygen demand of the bacteria was used to calculate the amount of slow-release oxygen material injected into each group of tests for 330 kg. The slow-release materials and water were mixed in a ratio of 1:3 and configured into a suspension, with constant stirring during the injection process to ensure the homogeneity of the injected material.

Geoprobe 7822DT (Geoprobe Co., Ltd., Salina, KS, USA) was used for injection during the test, and the injection system included storage tanks, injection pumps, mixing equipment, flow meters, pressure gauges, and so on. According to the parameters determined by the injection impact radius test, a reasonable injection point and radius were set up, as shown in Figure 6. In order to ensure the degradation effect of coupled pollution, each group of microorganisms/agents was set up with three injection points, slow-release oxygen material injection points, and microorganisms, using upstream and downstream double-point injection. The upstream point for the slow-release oxygen material injection was 2.5 m downstream of the microorganism injection position. Vertical injection was performed at a depth of 7, 6, 5, 4, and 4.5 m in five layers.

A long-term monitoring program was designed for water quality in the groundwater monitoring wells, which were carried out on the 1st, 3rd, 5th, 10th, 20th, 40th, 60th, and 90th days after the injection. The indicators of the tests were BTEX and microbiological indicators, including the structure of microbial communities, the total number of microorganisms, the activity of lipase, dehydrogenase activity, monooxygenase activity, and so on. The degradation effect of pollutants in different test groups was compared with the groundwater samples before and after the test through the collection and analysis of the samples, so as to validate the effectiveness of the present test. The samples were sampled and detected by the Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, as previously described [34,35].

3. Results and Discussion

3.1. Injection Instantaneous Influence Radius

Slow-release materials will lead to increases in pH and dissolved oxygen in direct contact with groundwater. Both of the phenomena were detected instantaneously in the monitoring wells 0.5 m away from the injection point under both high-pressure and low-pressure injection modes. The dissolved oxygen concentrations of t1 and T5 were 22.5 mg/L and 14.2 mg/L, and the pH levels were 12.3 and 12.5, respectively, proving that the instantaneous influence radius of high-pressure and low-pressure injection can reach 0.5 m. The T2 wells were also found to have a rising dissolved oxygen concentration of 19.2 mg/L and a pH of 10.5, proving that the instantaneous influence radius of high-pressure injection can reach 1 m. The T6 well did not register obvious changes, proving that the instantaneous influence radius of low-pressure injection is greater than 0.5 m and less than 1 m. The soil sampling work was carried out at different distances of 0.75 m and 1.2 m from high-pressure and low-pressure injection points. The low-pressure group did not show obvious traces of material injection, while the high-pressure group showed traces of material injection at 0.75 m. Non-detection at 1.2 m determined that the instantaneous influence radius of high-pressure and low-pressure injection was 1 m and 0.5 m, respectively.

3.2. Changes in Groundwater Dissolved Oxygen Concentration after Injection of Slow-Release Oxygen Materials

The dissolved oxygen concentrations of groundwater monitoring wells in the test area were continuously monitored. Under the effect of groundwater flow, the dissolved oxygen has always been maintained at a high level, averaging around 20 mg/L, which creates an oxidizing environment suitable for the microbial aerobic degradation of BTEX in the groundwater aquifer. The influence range of the slow-release material expanded continuously with the groundwater flow. The T3 well, which is 1 m away from the T2 well, showed DO elevation 7 days after the injection. The diffusion rate of dissolved oxygen is about 0.15 m/d. The results of pH monitoring found that, except for the phenomenon of pH elevation in the monitoring wells within the transient radius of influence, the pH elevation phenomenon was not found in the downstream monitoring wells. It was presumed that the pH elevation caused by material injection was buffered by soil and did not spread downstream. The monitoring results after 90 days in wells T2, T3, T4, T5, T6, and T7 were 17.67 mg/L, 15.8 mg/L, 13.71 mg/L, 19.58 mg/L, 15.66 mg/L, and 11.71 mg/L, respectively. It shows that the slow-release oxygen material can release oxygen stably for a long period.

3.3. Characterization of Spatial and Temporal Changes in Microbial Abundance after Coupled Injection

In comparing the results of the samples with the effects of different bacterial agents, we observed significant differences, as shown in Figure 7. In the pre-adapted bacterial agent group (group 2), after the addition of Bacillus spp., its relative abundance climbed rapidly, not only being stable at more than 10% but also increasing to values more than 50% under certain conditions, showing strong adaptability and growth vigor. At the same time, the abundance of Bacillus spp. in the commercial bacterial agent group (group 1) also increased rapidly after its addition, but the average value was stable at about 10%, which indicated that the added bacterial agent could be effectively dispersed in the aquifer and survive for a long time. However, group 3 did not perform well in stimulating indigenous degrading bacteria. Despite the addition of slow-release oxygen materials, the abundance of Bacillus spp. in this test group always remained below 1%, indicating that the addition of slow-release oxygen materials alone could not effectively stimulate the growth of the indigenous Bacillus spp. In contrast, the abundance of Bacillus spp. in the control group also remained at a low level, which further verified that the injection of Bacillus spp. was significant in promoting the growth of Bacillus spp. Combined with the efficiency of pollutant degradation in different experimental groups, it was found that the injected Bacillus spp. could persistently and efficiently participate in the degradation of pollutants in the process of pollution degradation in conjunction with the slow-release material.

3.4. Characterization of Spatial and Temporal Changes in Pollution after Coupled Injection

BTEX concentration in each well was examined after coupling injection, and the results are shown in Figure 8. The pollutant concentration in the test plots on the first day after injection increased slightly, and this phenomenon also appeared in other cases [36], mainly due to the pressure pulse and turbulence of the injected suspension. On the 3rd–10th days after injection, the test area’s pollution plume concentration showed a significant reduction. The pollution concentration in the circled areas of J4, J9, J10, and J11 was reduced by an average of 50%. The pollution distribution was the same as before the injection, and the pollution near the plant was more serious. In the late stage of the experiment, the concentrations in the monitoring wells generally showed a large increase, and the results of pollution interpolation are shown in Figure 8. Affected by the migration of lateral pollution to the experimental area, BTEX concentration in the test area has increased. The highest increase in the concentration appeared in the blank control group of J13, which increased from 63.5 μg/L to 3180 μg/L. The average concentration in the blank control group was the lowest level in the test area before the injection, while the concentration continued to rise and became the highest concentration of BTEX in the test area. As shown in Figure 8h, the concentration of the test group closer to the pollution source was lower than that of the blank control group, which demonstrated that the injection of slow-release materials and microorganisms played a significant role in the process of pollutant degradation.

3.5. Actual BTEX Degradation Efficiencies after Pollution Effects

In order to deduct the influence of flanking pollution, the actual degradation efficiency of BTEX was calculated using two methods: ratio analysis and computer numerical simulation. The pollutant concentration in the control groups of J14, J15, and J16 was not affected by the injection of slow-release materials and bacteria. It was affected by the spread of the flanks of the contaminated plume. Since the test area is very small compared to the whole area of the pollution plume, the test area can be approximated as homogeneous. To calculate the degradation efficiency of the injection of the slow-release materials and fungicides, the growth rate of pollutant concentrations in the control group can be used to speculate on the impact of the dispersion pollution plume at other sites.

The growth rate of BTEX concentrations in the blank control group is shown in

Table 1. Growth rate of BTEX concentration pre- and post-injection in the control group.

Well	7 d before Injection BTEX (μg/L)	90 d after Injection BTEX (μg/L)	Growth Rate (%)
J14	514.72	2876.67	458.88
J15	446.80	3098.78	593.55
J16	474.46	3642.29	667.67
Average value	478.66	3205.91	573.37

. According to this growth rate, the theoretical pollutant concentration at other points after 90 days is calculated, as shown as

Table 2. BTEX degradation efficiencies calculated by ratio analysis.

Group	Well	7 d before Injection BTEX (μg/L)	90 d after Injection BTEX Ratio Analysis Value (μg/L)	90 d after Injection BTEX Detection Value (μg/L)	Calculated BTEX Degradation Efficiencies (%)
Group of slow-release materials	J04	445.15	2995.85	35.34	98.82
	J05	602.64	4055.77	3307.19	18.46
	J06	624.96	4205.97	3792.03	9.84
	Average value				42.37
Group of slow-release materials and pre-adapted bacterial agents	J07	1033.84	6957.75	2228.64	67.97
	J09	850.20	5721.85	12.48	99.78
	J10	690.10	4644.36	205.13	95.58
	Average value				87.78
Group of slow-release materials coupled with commercial fungicides	J08	2019.71	13,592.65	3080.80	77.33
	J11	2740.37	18,442.66	1102.06	94.02
	J12	1231.42	8287.46	3452.41	58.34
	Average value				76.57

. The real degradation efficiency after the injection of slow-release materials and bacterial agents can be calculated by combining the actual test concentrations. The degradation degree of BTEX in the group of slow-release materials is 9.84–99.78%, with an average value of 42.37%. The degradation efficiency of BTEX in the coupling test group of slow-release materials and pre-adapted bacterial agents ranged from 67.97% to 98.82%, with a mean value of 87.78%. The degradation efficiency of benzene in the test group of slow-release materials coupled with commercial fungicides ranged from 58.34% to 94.02%, with an average value of 76.57%.

Groundwater flow and solute transport were modeled using the GSM 10.2(groundwater modeling system) to simulate the transport of BTEX without injection of degrading bacteria and slow-release materials and test site concentrations. The actual degradation efficiency of degrading bacteria and slow-release materials was calculated according to the measured and simulated concentrations, which are shown in

Table 3. BTEX degradation efficiencies calculated by computer numerical simulation.

Group	Well	90 d after Injection BTEX-Detection Value (μg/L)	90 d after Injection BTEX-Simulation Value (μg/L)	Calculated BTEX Degradation Efficiencies (%)
Group of slow-release materials	J04	35.34	5376.23	99.34
	J05	3307.19	5794.89	42.93
	J06	3792.03	6025.91	37.07
	Average value			59.78
Group of slow-release materials and pre-adapted bacterial agents	J07	2228.64	6849.18	67.46
	J09	12.48	8780.12	99.86
	J10	205.13	7952.25	97.42
	Average value			88.25
Group of slow-release materials coupled with commercial fungicides	J08	3080.8	8428.11	63.45
	J11	1102.06	15,936.54	93.08
	J12	3452.41	15,959.31	78.37
	Average value			78.29

. Compared with the three test groups, the highest BTEX degradation efficiency occurred in the group of slow-release materials and pre-adapted fungicides, with an average degradation efficiency of 88.25%. The results were similar to those of the ratio analysis method, proving the feasibility of these two methods.

In summary, the comprehensive calculation of the actual degradation efficiency at different time points in this test was carried out. The change in the BTEX degradation efficiency, calculated by numerical simulation, is shown in Figure 9. The highest degradation efficiency of 98% was reached at 40 d. Both groups in the coupled injection test group showed good pollution degradation effects, in which the indigenous bacterial agent was more effective.

Previous studies have focused on petroleum hydrocarbon pollution in groundwater and BTEX degradation. However, they focus on different aspects, such as the degradation ability of a single strain, comparisons of different degradation techniques, and microbial community changes during degradation. This study focuses on the microbial degradation of petroleum-based pollutants, especially BTEX, in groundwater. The optimization strategy and application conditions of microbial degradation technology were studied by screening and optimizing efficient degradation strains and fine-tuning environmental conditions. The degradation efficiency of a simple microbial injection is usually between 70% and 90%, and we further improve the degradation efficiency to more than 90% by injecting pre-adapted microbial agents under finely regulated environmental conditions. It is verified that the technology can be used in the treatment of medium- and low-concentration site pollution.

4. Conclusions

(1) Through the on-site injection test, it was verified that the radius of influence of the injection transient under the two pressures of high pressure (5 MPa) and low pressure (0.5 MPa) were 1 m and 0.5 m, respectively. The upstream and downstream dual-point coupled injection of slow-release oxygen materials and microbial agents was determined based on the results, with spacing between injection points being not less than 1.5 m.

(2) The monitoring results of dissolved oxygen concentrations after the injection of slow-release materials showed that the groundwater changed to an oxidizing environment. The dissolved oxygen content in the groundwater is continuously maintained above 15 mg/L, which ensures that microorganisms require the aerobic degradation of pollution. Elevated pH levels due to material injection are buffered by the soil medium and do not diffuse downstream.

(3) Both groups of coupled injection tests achieved positive pollutant degradation effects, in which the pre-adapted bacterial agent has a faster start rate of pollution degradation and higher pollution degradation efficiency due to the screening of local soil samples from the purification plant. The degradation effect has a long-lasting effect, reaching the highest BTEX degradation efficiency of 98% at 40 d. There were no significant changes in dissolved oxygen concentration except for redox conditions and dissolved oxygen, demonstrating that the coupled injection did not result in secondary contamination.

(4) The scope of application of this technology is areas with low- and medium-concentration pollution. When designing risk control programs for specific enterprises, it is necessary to cooperate with the use of pollution source cutting technology, and then continue to carry out in situ long-term remediation projects using the green and low-risk pollution control technology in this study. It will ensure that companies can safely and cost-effectively manage the risk of stock contamination.