A simulation-optimization integrated framework for subsurface air barrier strategies to mitigate seawater intrusion in a 3D coastal aquifer

Subsurface air barrier, owing to readily accessible and sufficiently clean of injected air, is a more attractive alternative for mitigating seawater intrusion (SWI) in arid and semi-arid coastal regions, compared to hydraulic barrier. Nevertheless, it remains an enormous challenge to recognize the appropriate layout scheme of air barrier system in a 3D coastal aquifer, including the spatial structure of air-injection zone and the adopted injection pressure, simultaneously considering the environmental and economic performances. This study established a simulation-optimization (S-O) integrated framework for determining the optimal air barrier scheme with minimizing total cost while effectively controlling the extent of SWI. Within the proposed S-O framework, the liquid-gas two-phase flow and solute transport (LGST) numerical model was linked with the micro-population genetic algorithm (mGA). Layout parameters of air barriers, salinity at constraint points and air injection rates were continuously exchanged between the LGST model and mGA at each optimization step. A hypothetical 3D confined aquifer was then employed to evaluate the stability of this S-O framework. The total cost of air barriers increasingly declines to a stable minimum value with the evolutionary processes. With the optimal scheme, there forms a complete and continuous air barrier system between adjacent air-boreholes, giving rise to the intrusive seawater decreasing by 69.7 % and the 0.5 isochlor retreating by 43.4 % at 365.0 day. Furthermore, the 3D aquifer system gradually reaches a quasi-steady state with time and the air injection rate cannot indefinitely increase in response to air injection. The proposed S-O framework can therefore provide a decision support system for designing subsurface air barrier with the least cost on the promise of controlling SWI in 3D coastal aquifers.