A semi-analytical method for delineating steady-state seawater-freshwater interface in three-dimensional stochastic heterogeneous confined coastal aquifers

Seawater intrusion threatens the freshwater resources and the ecosystem in coastal areas. Spatial heterogeneity of hydraulic conductivity is a common feature of coastal aquifers and can significantly influence the location of the seawater-freshwater interface. Conventional approaches primarily rely on numerically solving the advection-dispersion models (e.g. SEAWAT) to determine the interface in three-dimensional heterogeneous aquifers. However, these approaches are mostly computationally intensive. This study developed a semi-analytical approach for determining the location of the sharp seawater-freshwater interface, considering the steady-state seawater intrusion in three-dimensional heterogeneous confined aquifers. The approach decouples the hydraulic conductivity as a spatial variable from the conventional potential definition and defines a new potential expression. A two-dimensional partial differential equation is employed to describe the three-dimensional heterogeneous aquifer system by calculating the transmissivity above the sharp interface. Finally, the potential distribution can be obtained by solving the governing equation using finite-difference method, allowing for determining of the morphology of the sharp interface. 2D cross-sectional and 3D confined coastal aquifers with Gaussian random hydraulic conductivity fields were generated to validate our approach. The seawater-freshwater interfaces were estimated using both the semi-analytical approach and SEAWAT. Comparison analysis showed that the developed semi-analytical approach can accurately simulate the interfaces by correcting the dispersion effect. This study provides a cost-effective means for estimating the seawater-freshwater interface location in three-dimensional heterogeneous confined aquifers. It is useful for future seawater intrusion study of three-dimensional systems and for managing subsurface freshwater resources efficiently.

Key Words: seawater intrusion, semi-analytical, three-dimensional, stochastic heterogeneity