Introduction: Coastal aquifers are the main source of freshwater supply in most coastal regions. Their proximity to seas and oceans exposes them to the risk of salinization. Hence, providing an effective solution to reduce saltwater intrusion can play a significant role in coastal water resource management. In this study, an effort was made to evaluate the effects of recharge wells and abstraction wells as hydraulic barriers, and cutoff walls as a physical barrier, both separately and in combination, on the mitigation of seawater intrusion using different experiments.
Methodology: The experimental model used in this study was a flow tank with a thickness of 1 centimeters. The flow tank was divided into a central chamber with internal dimensions of 140×58×5 centimeters and two side chambers with internal dimensions of 10×58×5 centimeters. The central chamber served as the porous medium, while the left and right side chambers were used to control the head of saline and fresh water entering the porous medium during the experiments. Glass beads were employed to create the porous medium. The fresh water used in the experiments was supplied from the tap water, with a density of approximately 998 kg/m3. The saline water used in the experiments was prepared by adding 35 grams of purified salt and 2 milliliters of red food coloring to each liter of tap water. An aluminum sheet with a thickness of 7 millimeters, an approximate width of 5 centimeters, and a height of 54 centimeters was used to construct the cutoff wall. Two separate polyvinyl chloride (PVC) pipes were used as extraction and injection wells. The extraction well was placed 22 centimeters from the saltwater boundary, with its mouth penetrating to a depth of 52 centimeters from the top edge of the experimental model. The injection well was placed 40 centimeters from the saltwater boundary, with its mouth penetrating to a depth of 56 centimeters from the top edge of the model. To prepare the required freshwater for the injection well, one cubic centimeter of blue food coloring was added to each liter of tap water.
Results and Discussion: In this study, injectioning action was performed at flow rates of 0.65, 1.30, and 2.0 liters per minute. The fed water flow caused the return of the salt wedge. The reason is that when the injection well is activated, the local velocity in the aquifer increases because the recharge flow accumulates with the normal velocity inside the aquifer, resulting in a greater velocity at the outlet of the well towards the saltwater boundary. This increased flow washes and transports salt grains of the salt wedge along the common boundary towards the sea, leading to their removal from the porous medium. Thus, greater local velocity, the greater the washing and salt transport, resulting in a smaller salt wedge or, in other words, a greater return. For investigating the combined effect of cutoff walls and injection wells, the intrusion experiment was first conducted in the presence of a cutoff wall. Therefore, the distance of the salt wedge to the saltwater boundary was 38.5 centimeters. Then, injectioning action started at a rate of 0.65 liters per minute. The injected water began to move within the porous medium, and after approximately 50 minutes from the start of the injectioning process, the salt wedge reached a stable state. In the stable conditions, the length of the salt wedge was 27 centimeters, indicating a return of 17 centimeters. Therefore, the effectiveness of the simultaneous use of an cutoff wall and a injection well in preventing saltwater intrusion is approximately 38.6%. To investigate the combined effect of injection and extraction wells in reducing saltwater intrusion, both wells were activated simultaneously at a rate of 0.65 liters per minute each. After about 35 minutes, the salt wedge reached a stable state with a length of 29.6 centimeters, indicating a return of approximately 16.7 centimeters. Thus, the simultaneous use of extraction and injection wells at a rate of 0.65 liters per minute each is approximately 32.7% more effective than using the extraction well alone.
Conclusion: The simultaneous use of extraction and injection wells showed a 5.22% better performance in preventing saltwater intrusion compared to using the extraction well alone, albeit with approximately 2% lower effectiveness in retreating the salt wedge compared to using the injection well alone. By comparing the combined effect of all three barriers with each individual method, it was evident that the combined method of cutoff wall, extraction well, and injection well had a better performance in preventing saltwater intrusion by 17% and 19.3% compared to the cutoff wall and extraction well alone, respectively. However, their effectiveness was 4.59% weaker compared to the individual effect of the injection well.