Seawater intrusion in response to sea-level rise in a coastal aquifer with a general-head inland boundary

Seawater intrusion in response to sea-level rise has been studied extensively in recent years by assuming largely a constant-head or constant-flux inland boundary condition. However, these two types of boundary conditions are not sufficient when the inland edge of the model domain is neither natural groundwater nor known hydraulic boundaries. Under these circumstances, a general-head inland boundary condition capable of characterizing the hydraulic response of model boundaries is needed. Previous studies adopting the general-head inland boundary condition to assess coastal aquifer vulnerability to sea-level rise are limited and all based on numerical modeling. In this study, we derive analytical solutions of the interface toe location in both confined and unconfined coastal systems with a general-head inland boundary condition. Comparison among the performances of the three different types of inland boundary conditions in evaluating the sea-level rise impact on aquifer salinization is carried out by assuming the same initial system condition. It is found that the displacement of the interface toe predicted using a general-head inland boundary is between those of using a constant-head (upper bound) and constant-flux (lower bound) inland boundary, depending on the values of two general-head boundary parameters (i.e., hydraulic conductance and reference head). More importantly, analytical solutions developed could serve as a tool for calibrating the two boundary parameters defined in the general-head boundary condition for site-specific assessments