## 3 Results and Discussion
### Evaluation of Different Methods for Groundwater Treatment  
Based on the simulated current distribution of 1,4-dioxane in groundwater at the study site, two possible treatment methods, that is, passive and active treatments, were proposed and evaluated.  
Passive treatment refers to the natural attenuation of 1,4-dioxane in groundwater. The simulation results of the distribution of 1,4-dioxane in the study site under natural attenuation over the next 40 years (2020-2060) are shown in Figure 4. The hydraulic environment of this site is relatively stable because no artificial activities have been conducted. Figure 4 suggests that as time passes, 1,4-dioxane moves with the groundwater flows, its concentration gradually decreasing. After 10 years of natural attenuation, most areas of the dumping site have 1,4-dioxane concentrations \(\leq\)10\({}^{-3}\) mg/L, with very few areas in the range of 10\({}^{-3}\)-10\({}^{-1}\) mg/L. The concentration of 1,4-dioxane is relatively high near the impermeable wall, even after 40 years of natural attenuation, which may be attributed to the stagnation of groundwater flow near this area.  
With the active treatment method, groundwater pumping, followed by treatment technologies, is applied. It is recommended to pump water through the monitoring wells,  
Figure 4: 1,4-Dioxane concentrations in groundwater by passive treatment over time. Contour lines indicate groundwater levels and vectors indicate flow directions. The color of the legend indicates the concentration of 1,4-dioxane.  
which are more widely installed than the current pumping wells--monitoring wells 7, 8, and 9 are close to the impermeable wall, which has an accumulation od 1,4-dioxane. A constant pumping rate of 250 m\({}^{3}\)/day is suggested.  
The distribution of 1,4-dioxane under active treatment is shown in Figure 5. The groundwater levels and groundwater directions near the monitoring wells and the impermeable wall were significantly influenced by groundwater pumping actions. These changes make the groundwater more dynamic and the groundwater environment becomes more complex, which also influences the movement of 1,4-dioxane. The northern areas with relatively high 1,4-dioxane concentrations (10\({}^{-2}\)-10\({}^{-1}\) mg/L) were quickly remediated within five years, with the 1,4-dioxane concentration decreasing to \(<\)10\({}^{-3}\) mg/L. After 30 years of active treatment, the 1,4-dioxane concentration in the groundwater in most areas of the site was \(\leq\)10\({}^{-3}\) mg/L, and did not accumulate near the impermeable wall.  
Comparing the simulation results of 1,4-dioxane distribution in the groundwater shown in Figures 4 and 5, a wider 1,4-dioxane distribution may occur in the case of the active treatment (e.g., after 30 years of treatment). It has been proposed that the pumping actions during active treatment may result in a more dynamic groundwater flow than during passive treatment, which promotes 1,4-dioxane transport to areas where it would otherwise be difficult to reach. This phenomenon was unanticipated and would influence the groundwater treatment efficiency; therefore, when designing pumping well locations, both the natural hydraulic environment and artificially induced groundwater flows should be taken into consideration. Finally, we suggest further research to study better prediction models and remediation strategies for groundwater pollution based on the findings of the present study.  
Figure 5: 1,4-Dioxane concentrations in groundwater by active treatment over time. Contour lines indicate groundwater levels and vectors indicate flow directions. The color of the legend indicates the concentration of 1,4-dioxane.