Impacts of Spatial Heterogeneity on Residual Saltwater Removal Upstream of Subsurface Dams in Coastal Aquifers

Although subsurface dams can effectively control seawater intrusion, the trapped saltwater behind the dam significantly impedes groundwater resource utilisation. Previous studies have largely neglected the impact of spatial heterogeneity on residual saltwater desalination processes. To address this research gap, this study provides the first field-scale simulation of how spatial heterogeneity and connectivity features influence the mechanism and timescale of residual saltwater removal. The results show that under certain conditions, Tr (timescale of residual saltwater removal) in heterogeneous aquifers varies by approximately −69% to 145% relative to Tr in homogeneous aquifers. Although spatial heterogeneity and connectivity features significantly change the time-varying course of residual saltwater removal rate, they do not alter the mechanism of residual saltwater removal. Additionally, in homogeneous scenarios, Tr increases with the increase of dam height or decrease of dam distance from the sea boundary (dam location). In heterogeneous aquifers, Tr maintains a significant positive correlation with dam height under isotropic conditions; however, this relationship does not necessarily hold under anisotropic conditions. Regardless of heterogeneity types, there is no significant relationship between Tr and dam location. Sensitivity analyses demonstrate a non-monotonic relationship between Tr and three key heterogeneity parameters: (i) the variance of logarithmic hydraulic conductivity (${\sigma }_Y^2$), (ii) vertical correlation length (${\delta }_Y^{\upsilon }$), and (iii) anisotropy ratio (anis). Further analysis shows that, in general, Tr is more sensitive to ${\sigma }_Y^2$, ${\delta }_Y^{\upsilon }$, and anis in aquifers with multivariate Gaussian structures compared to those containing either connected high-K structures or connected low-K structures. These results highlight the importance of spatial heterogeneity, including connectivity features, in assessing timescale of residual saltwater removal, which is valuable for protecting of coastal groundwater resources.