Development of a discrete‐continuum VDFST‐CFP numerical model for simulating seawater intrusion to a coastal karst aquifer with a conduit system

Abstract

A hybrid discrete‐continuum numerical model, Variable‐Density Flow and Solute Transport—Conduit Flow Process (VDFST‐CFP), is developed to simulate seawater intrusion to a coastal karst aquifer with a conduit network. The Darcy‐Weisbach equation is applied to simulate the nonlaminar groundwater flow in the conduit system that is conceptualized as pipes, while the Darcy equation is used for laminar groundwater flow in the continuum porous medium. Density‐dependent groundwater flow with appropriate additional density terms in the conduit is analytically derived. The flow and transport equations are coupled, and numerically solved by the finite difference method with an implicit iteration procedure. Two synthetic benchmarks are developed to compare the VDFST‐CFP model results with other numerical models, such as the variable‐density SEAWAT, constant‐density continuum MODFLOW/MT3DMS, and constant‐density discrete‐continuum CFPv2/UMT3D models. The VDFST‐CFP model compares reasonably well with the other model results in both conduit and porous medium domains, and well describes water and salt exchange between the two systems. Under turbulent flow conditions within the conduit, the Darcy‐Weisbach equation calculates the flow rate more accurately without overestimation by the Darcy equation. Sensitivity analysis indicates that conduit diameter, friction factor, matrix hydraulic conductivity, and effective medium porosity are important parameters in the VDFST‐CFP model. The pros and cons of the VDFST‐CFP model are discussed, including the model assumptions and simplifications, limitations of the discrete‐continuum modeling method, and the convergence criteria. In general, the newly developed VDFST‐CFP model provides a new numerical modeling method for simulating seawater intrusion in a coastal karst aquifer with conduits.

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

“…In summary, the modeling and field observations of the karst features, including the subsurface conduit network, the submarine spring, and karst windows, are critical for understanding seawater intrusion in a coastal karst aquifer and important for model calibration. The discrete-continuum density-dependent flow and transport model, for example, the VDFST-CFP in Xu and Hu (2017), is important to accurately simulate seawater intrusion and assess parameter sensitivities in the coastal karst aquifer with conduit networks. Advanced numerical methods and/or high-performance computing are expected to solve the issue of Peclet number limitation and reduce the uncertainty of the dispersion solution with a finer spatial resolution in this study.…”

“…The VDFST-CFP, developed by Xu and Hu (2017), is based on a density-dependent discrete-continuum modeling approach to study seawater intrusion in a coastal karst aquifer with conduits. However, VDFST-CFP is not able to simulate the seawater intrusion processes addressed in this study due to the computational constraints and the numerical method limitations associated with the aquifer geometry and the domain scale.…”

Experiment and Numerical Simulation of Seawater Intrusion under the Influences of Tidal Fluctuation and Groundwater Exploitation in Coastal Multilayered Aquifers

“…The physical and chemical equilibrium of the coastal aquifer system is easily destroyed by the human activities, resulting in the problems of land subsidence, seawater intrusion, and environment deterioration. Among them, the seawater intrusion is a problem of worldwide concern, which is caused by the overpumping of groundwater, sea level rising, change of climate, and change of land use in the coastal area [3][4][5][6][7][8][9]. Among these factors, groundwater pumping is considered to be one of the most important chal-lenges that promote the extent and severity of seawater intrusion [6,9,10].…”

Modelling Actual and Future Seawater Intrusion in the Variconi Coastal Wetland (Italy) Due to Climate and Landscape Changes

“…SWI occurs in coastal freshwater aquifers when different densities of freshwater and saltwater allow saltwater to intrude into freshwater aquifers that are in hydraulic continuity with seawater. SWI is a physically density-dependent problem [3,4] where two different equations need to be coupled: (i) groundwater flow equation and (ii) solute (salt) transport equation. There are many numerical models that can simulate and predict SWI [5,6] like: FEFLOW [7], SEAWAT [8,9], and SUTRA [10].…”