Modeling the fresh–saline water interface dynamics in coastal aquifers under managed aquifer recharge (MAR)

Seawater intrusion (SWI) is one of the most serious threats to groundwater resources worldwide, particularly in densely populated coastal areas where groundwater is heavily exploited. Managed Aquifer Recharge (MAR) has emerged as a viable technique to mitigate SWI and enhance freshwater storage. This study employs the density-dependent numerical code SEAWAT to simulate and analyze saline water dynamics in a coastal unconfined aquifer under MAR conditions. The numerical model was calibrated and validated against controlled sand-tank experiments, followed by sensitivity and grid-resolution analyses to ensure numerical reliability. The results demonstrate that MAR effectiveness strongly depends on aquifer hydraulic conductivity, saline water density, and injection rate. In highly permeable formations, injected water disperses rapidly, forming a smaller water table mound and reducing the seaward retreat of the fresh-saline water interface. Conversely, lower hydraulic conductivity enhances water table mound buildup and improves SWI control. Increasing seawater concentration by 30% (from 50 g/L to 65 g/L) required a 100% increase in the injected freshwater volume to achieve a similar seaward retreat of the fresh-saline water interface. A twofold increase in injection volume produced a more than 20% reduction in the intrusion extent. This study provides quantitative insights into how MAR design parameters and aquifer characteristics interact to control saline intrusion, offering guidance for optimizing MAR implementation in coastal aquifers of arid environments.