## 4 Results and Discussion
### Microbial Remediation with Chemotaxis Optimization  
In the context of groundwater pollution by petroleum contaminants resulting from a chemical factory's leakage, the use of chemotactic bacteria for bioremediation proves more effective compared to traditional physical methods, such as hydraulic flushing. This efficacy stems from the inherent characteristics of petroleum contaminants, including their tendency to adhere to soil and their low volatility, making them less susceptible to removal by hydraulic flushing. Instead, they often become trapped in areas of lower groundwater permeability, making it challenging to remove them solely through water flow. Moreover, this can lead to the further spread of pollutants, exacerbating the contamination. In contrast, the application of chemotactic bacteria (e.g., Pseudomonas putida) in bioremediation can overcome these limitations. First, chemotactic bacteria have the capability to directly degrade and absorb petroleum contaminants. Second, due to their chemotaxis, they can actively move towards areas with higher pollutant concentrations, attracted to low-permeability zones where they degrade  
Figure 8: Final volatile pollutant content at different adsorption constants. Figure 7: Relationship between total EPH and adsorption constant. pollutants that are difficult to remove through hydraulic flushing. Lastly, as pollutants are locally degraded, bacterial populations increase, continuously enhancing the bioremediation capacity of the contaminated site. Thus, compared to traditional physical methods, chemotactic bacteria can achieve more efficient and controllable in situ pollutant degradation, mitigating spread, and are a more suitable bioremediation approach in such scenarios. To attract chemotactic bacteria, the attractant is set as VOC with a concentration of 1 mol/m\({}^{3}\), causing chemotactic bacteria to migrate with the injected water. After one year of extraction simulation, the distribution of chemotactic and non-chemotactic bacteria is illustrated in Figure 9. The results from the figure show that non-chemotactic bacteria predominantly accumulate near the injection well and are mainly present in high-permeability areas. Conversely, chemotactic bacteria exhibit a wider distribution due to their attraction to pollutants. A cross-sectional plane is taken near the injection well, as depicted in Figure 10. Bacterial concentrations and streamlines are plotted on this plane, as shown in Figure 11. The total concentration of chemotactic bacteria on this plane is 356.8 mol, while non-chemotactic bacteria have a concentration of 278.58 mol. Non-chemotactic bacteria primarily disperse along the direction of streamlines, while the distribution of chemotactic bacteria is less correlated with the flow field. Observations indicate that chemotactic bacteria do not exhibit strong chemotaxis in the direction of VOC concentration gradients. This is due to the initially high bacterial  
Figure 11: Concentration distribution of non-chemotactic and chemotactic bacteria on the cross-section plane with streamlines. Figure 9: Distribution of non-chemotactic and chemotactic bacteria after one year of water injection. growth concentration set, which leads to decreased pollutant concentrations after water extraction. For lower pollutant concentrations, chemotactic bacteria become less active. In practical remediation work, the removal of low-concentration pollutants is equally important. Therefore, reducing the optimal growth concentration for chemotactic bacteria is necessary. The resulting cross-sectional image is shown in Figure 12a. Reducing the optimal growth concentration for chemotactic bacteria does enhance chemotaxis to some extent. However, the effect is not significant due to the strong heterogeneity of the soil, resulting in minimal bacterial movement into low-permeability areas. Additionally, chemotactic bacteria with reduced optimal growth concentrations exhibit less activity when pollutant concentrations are initially high, which limits their spread.