Variable-density numerical modeling of seawater intrusion in coastal aquifer with well-developed conduits

Abstract:

Karst aquifer is an important drinking water supply for nearly 25% of the world’s population. Well-developed subground conduit systems usually can be found in a well-developed karst aquifer, as a dual permeability system. Hydraulic characteristics of non-laminar flow in conduits could be significantly different from darcian flow in porous medium; therefore, hybrid model and different governing equations are necessary in numerical modeling of karst hydrogeology. On the other hand, seawater intrusion has been observed and studied for several decades, also become a worldwidely problem due to groundwater over-pumping and rising sea level. The density difference between freshwater and seawater is recognized as the major factor governing the movements of two fluids in coastal aquifer.

Several models have been developed to simulate groundwater flow in karst aquifer, but hardly describe seawater intrusion through the conduits without coupling variable density flow and solute transport. In this study, a numerical SEAWAT model has been developed to simulate variable density flow and transport in heterogeneous karst aquifer. High-density seawater is verified to intrude further inland through high permeability conduit network rather than porous medium. The numerical model also predicts the effect of different cases on seawater intrusion in coastal karst aquifer, such as rising sea level, tide stages and freshwater discharge effects. A series of local and global uncertainty analysis have been taken to evaluate the sensitivity of hydraulic conductivity, porosity, groundwater pumping, sea level, salinity and dispersivity. Heterogeneous conduit and porous medium hydraulic characteristics play an important role in groundwater flow and solute transport simulation. Meanwhile, another hybrid model VDFST-CFP model is currently under development to couple turbulent conduit flow and variable density groundwater flow in porous media, which provides a new method and better description in seawater intrusion modeling.

Keywords: Seawater intrusion, Karst conduit, Sea-level rise, Numerical modeling, Variable-density flow