Impact of land subsidence and drainage infrastructure on estuarine aquifer salinization

Flood management through drainage infrastructures effectively removes inundated water yet creates a critical hydrogeological trade-off. By artificially altering hydraulic gradients, these systems can accelerate saltwater intrusion into freshwater aquifers—particularly in regions where land subsidence from groundwater extraction has increased flooding vulnerability. In this study, we used a coupled surface–subsurface flow model to assess how land subsidence and drainage infrastructure affect shallow groundwater salinization in subsided estuarine zones. Comparison of simulated groundwater salinities with field measurements indicates that the model captures relatively well the primary spatial patterns in salinity distributions. However, agreement with 1D resistivity inversion results varied across locations, with some sites showing close correspondence while others exhibited marked discrepancies—likely reflecting spatial heterogeneity and complexity beyond the model's representation. The results showed that although pumping stations effectively reduce surface inundation, they alter hydraulic gradients by removing surface water, thereby promoting the inland transport of high-salinity water. These findings demonstrated that conventional flood-management strategies can exacerbate aquifer salinization in subsided coastal areas. While land subsidence is often a localized phenomenon, the mechanisms identified here may have broader relevance to coastal regions under sea-level rise globally, as both subsidence and sea-level rise reduce relative land elevation and intensify the hydraulic gradient driving saltwater intrusion. This suggests the need for coupled surface–groundwater assessment frameworks that account for alterations to both flooding patterns and subsurface salinity transport in vulnerable coastal regions.