Numerical modeling of karst water systems is a challenge due to vastly different flow regimes in the solution channels and the porous medium, and because of the complex interactions between them. Recent developments with unstructured grid models have enabled robust simulation of such systems due to full coupling of the system of equations and the interactions, within a flexible gridding framework that allows these domains to be discretized independently of each other. These capabilities have been incorporated into the MODFLOW-USG code, which has been developed and released in the public domain by the United States Geological Survey. Further developments to MODFLOW-USG have continued and have also been released in the public domain. The significant features that enhance simulations in karst environments include various optional turbulent flow formulations, solute transport capability and a density-dependent flow simulation option that can be used to model transport of nonaqueous phase liquids (NAPLs) or freshwater–seawater interactions, for example. A dual-porosity-flow and dual-porosity-transport formulation is also included to further expand model scale and flexibility. A physically based representation of flow and transport within the karst conduits (channels) and porous medium eliminates the need for various surrogate modeling approaches in karst, thus enabling solutions to some of the common problems that groundwater professionals face when dealing with groundwater resources management in coastal karst areas. A demonstrative example shows the impact of karst on the saltwater system in a coastal aquifer.

岩溶水系统的数值模拟是一个挑战，因为溶液通道和多孔介质中的流动方式截然不同，并且它们之间的相互作用很复杂。由于方程式系统和相互作用的完全耦合，在非结构化网格模型的最新发展中，由于这些系统的鲁棒仿真，使得在灵活的网格化框架内，这些域可以彼此独立地离散化。这些功能已合并到MODFLOW-USG代码中，该代码已由美国地质调查局开发并在公共领域发布。MODFLOW-USG的进一步开发仍在继续，并且已在公共领域发布。增强岩溶环境中模拟的重要功能包括各种可选的湍流公式，溶质传输能力以及与密度有关的流量模拟选项，例如，这些选项可用于模拟非水相液体（NAPL）的传输或淡水与海水的相互作用。还包括双重孔隙流动和双重孔隙运输配方，以进一步扩大模型规模和灵活性。基于物理的岩溶管道（通道）和多孔介质内流动和传输的表示方法消除了对岩溶中各种替代建模方法的需要，从而能够解决地下水专业人员在处理沿海地区地下水资源管理时面临的一些常见问题喀斯特地区。