Numerical modeling and sensitivity analysis of seawater intrusion in a dual-permeability coastal karst aquifer with conduit networks

In a coastal karst aquifer, seawater intrudes significantly landward through the highly permeable subsurface conduit system, and contaminates the groundwater resources in the porous medium. In this study, a two-dimensional coupled density-dependent flow and transport SEAWAT model is developed to study seawater intrusion in the dual-permeability karst aquifer. To provide guideline for modeling seawater intrusion in such an aquifer, local and global sensitivity analysis are conducted to evaluate the parameters of boundary conditions and hydrological characteristics, including hydraulic conductivity, effective porosity, specific storage and dispersivity of the conduit and the porous medium. In the local sensitivity analysis, simulations are more sensitive to all parameters at the seawater and freshwater mixing zone than elsewhere. The most important parameter for simulations in both domains is salinity at the submarine spring, which is also the boundary condition of the conduit. The hydrological characteristics of the conduit network are not only important to the simulations in the conduit, but also significantly affect the simulations in the porous medium, due to the interactions between the two systems. Therefore, salinity and head observations in the conduits and karst features have more values for calibrating the models and understanding seawater intrusion in a coastal karst aquifer. Compared to the local sensitivity analysis, the global sensitivity results are different in several parameters (hydraulic conductivity, porosity and the boundary conditions at the submarine spring), mainly due to the non-linear relationship of the parameters with respect to the simulations. The results of global sensitivity analysis also indicate that the Darcy's equation does not accurately calculate the conduit flow rate with hydraulic conductivity in the continuum SEAWAT model. Dispersivity is no longer an important parameter in the advection-dominated transport aquifer system with conduit, compared to the sensitivity results in a homogeneous porous medium. Based on the sensitivity analysis, the extents of seawater intrusion are quantitatively evaluated with the identified important parameters, including salinity at the submarine spring with rainfall recharge, sea level rise and longer simulation time under an extended low rainfall period.