Numerical assessment of compressed air injection for mitigating seawater intrusion in a coastal unconfined aquifer

Subsurface fluids injection is a viable alternative for controlling seawater intrusion in coastal regions, and freshwater is usually served for valid fluids to generate pressure ridge towards the ocean. But other fluids injection, such as compressed air, which presents clean and readily available, is a more attractive solution for repulsing the intruded seawater where freshwater is insufficient. In this study, the performance of compressed air injection (i.e., air barriers) for mitigating seawater intrusion in a coastal unconfined aquifer was quantificationally assessed using a coupled water-air two-phase flow and saltwater transport model. As compressed air is introduced into the aquifer at the toe of saltwater wedge, the seaward hydraulic gradient adjacent to the coast is produced driven by the generated airflow, thereby causing salt-freshwater interface to retreat to the ocean. The magnitude of air injection rate, related to the operational cost of air barriers, does not increases significantly given the progressively decreasing change rate. Compared to the workability of air barriers in the confined aquifer under similar conditions, the reduction in intruded seawater at 365.0 d in the unconfined aquifer reaches only about 0.45 times that in the confined aquifer, which indicates the significance of the boundary condition of aquifer top. The sensitivity analysis of the overlying unsaturated zone reveals that with its permeability decreasing or its thickness increasing, the performance of air barriers is enhanced owing to the blocking effect of the overlying layer on the escape of generated airflow and tends progressively to be stable. Particularly, the efficacy of air barriers in the confined aquifer, including the reduction in intruded seawater and the air injection rate, also can be achieved in an unconfined aquifer with an overlaying sufficient-semipermeable layer.

Automated summary

The study showed that using compressed air injection as a method to control seawater intrusion is effective. By injecting compressed air into the toe of the saltwater wedge in the aquifer, a hydraulic gradient driving seawater back towards the ocean is produced. The injection rate of air is related to operational costs, but the increase is not significant as the change rate decreases.