A Semianalytical Method to Fast Delineate Seawater-Freshwater Interface in Two-Dimensional Heterogeneous Coastal Aquifers

The freshwater-seawater interface is one of the most important regions in coastal aquifer systems, delineating the subsurface into zones with distinct fluid density and biogeochemical properties. Heterogeneity in hydraulic conductivity is inherent to geological formations, resulting in distinct interface profiles. Currently available analytical solutions are limited to homogeneous and stratified aquifers, and numerical simulations of variable-density flow and transport are the only option to characterize the interface in a two- or three-dimensional heterogeneous aquifer. This study presents a novel semianalytical method to fast delineate the steady seawater-freshwater interface in a two-dimensional, confined, heterogeneous aquifer with a constant flux inland boundary and indistinct seepage face at the vertical coastal boundary. The heterogeneous domain is conceptualized as a series of thin columns of stratified aquifers with the horizontal flow in each of them. The proposed approach is based on the concept of local transmissivity parameters and shows high accuracy in the interface delineation. Leveraging the ability to delineate the interface rapidly along with the Monte Carlo approach, we numerically investigate the stochastic behavior of the interface profile. The uncertainty in seawater intrusion is heavily influenced by the degree of heterogeneity and weakly influenced by the scale of heterogeneity. To homogenize the aquifer, we found that geometric mean generally underestimates the seawater intrusion and cannot be used as an effective parameter. We also showed that the near-coast, near-top region of the aquifer is the most influential region and should be characterized at a higher resolution to reduce uncertainties in estimating seawater intrusion.