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MODFLOW 6 is the latest in a line of six "core" versions of MODFLOW released by the U.S. Geological Survey. The MODFLOW 6 architecture supports incorporation of additional hydrologic processes, in addition to groundwater flow, and allows interaction between processes. The architecture supports multiple model instances and multiple types of models within a single simulation, a flexible approach to formulating and solving the equations that represent hydrologic processes, and recent advances in interoperability, which allow MODFLOW to be accessed and controlled by external programs. The present version of MODFLOW 6 consolidates popular capabilities available in MODFLOW variants, such as the unstructured grid support in MODFLOW‐USG, the Newton‐Raphson formulation in MODFLOW‐NWT, and the support for partitioned stress boundaries in MODFLOW‐CDSS. The flexible multi‐model capability allows users to configure MODFLOW 6 simulations to represent the local‐grid refinement (LGR) capabilities available in MODFLOW‐LGR, the multi‐species transport capabilities in MT3DMS, and the coupled variable‐density capabilities available in SEAWAT. This paper provides a new, holistic and integrated overview of simulation capabilities made possible by the MODFLOW 6 architecture, and describes how ongoing and future development can take advantage of the program architecture to integrate new capabilities in a way that is minimally invasive and automatically compatible with the existing MODFLOW 6 code. [ABSTRACT FROM AUTHOR]

The χMD matrix solver package is incorporated into USGS groundwater modeling software, such as MODFLOW-NWT, MODFLOW-USG, and MT3D. The solver is used to solve matrices assembled through numerical discretization of the groundwater flow equation, and solute transport equations. χMD has demonstrated its higher robustness, faster execution speed, and more efficient memory usage compared to the existing solvers for many types of groundwater flow problems. χMD uses preconditioned iterative Krylov-subspace methods and consists of preconditioning and acceleration modules. Because the solver package uses a variety of preconditioning features including level-based incomplete lower-upper (ILU) factorization method with a drop tolerance scheme, users must choose optimal preconditioning parameters to improve execution speed and robustness. In order to examine how the preconditioning parameters, ILU factorization level, and drop tolerance values affect the overall performance of the matrix solver, we evaluated five different groundwater model applications using MODFLOWUSG that include different numerical complexities. For those five cases, the number of discretization nodes varied from 10,000 cells to 730,300 cells. From the analysis, we found that the preconditioning parameters greatly affect execution times and memory usage of the preconditioning and acceleration procedures. In addition, a combination of the ILU level between five to seven and the drop tolerance value between 10-2 and 10-3 usually resulted in shorter overall execution time. Our study suggests that the users can elicit higher performance and robustness of the χMD matrix solver using this combination of the parameters and enhance computational efficiency of solving groundwater and solute transport problems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Density‐dependent flow and transport solutions for coastal saltwater intrusion investigations, analyses of fluid injection into deep brines, and studies of convective fingering and instabilities of denser fluids moving through less dense fluids typically formulate the groundwater flow equation in terms of pressure or equivalent freshwater head. A formulation of the flow equation in terms of hydraulic head is presented here as an alternative. The hydraulic‐head formulation can facilitate adaptation of existing constant‐density groundwater flow codes to include density‐driven flow by avoiding the need to convert between freshwater head and hydraulic head within the code and by incorporating density‐dependent terms as a compartmentalized "correction" to constant‐density calculations already performed by the code. The hydraulic‐head formulation also accommodates complexities such as unconfined groundwater flow and Newton‐Raphson solution schemes more readily than the freshwater‐head formulation. Simulation results are presented for four example problems solved using an implementation of the hydraulic‐head formulation in MODFLOW. [ABSTRACT FROM AUTHOR]

Axisymmetric groundwater models are used for simulating radially symmetric conditions. Groundwater simulators built specifically to model axisymmetric conditions are most commonly used for simulating aquifer tests. Although some numerical models capable of simulating flow and solute transport that are developed in the cartesian coordinate system framework offer flexibility to simulate axisymmetric conditions, most of the numerical groundwater models, such as the MODFLOW family of codes, are based on structured grids in which axisymmetric flows cannot be directly simulated. Researchers in the past have provided methods to manipulate aquifer properties to mimic axisymmetric conditions. This study presents a methodology that takes advantage of the unstructured grids of MODFLOW‐USG to simulate axisymmetric models within the MODFLOW framework. To develop axisymmetric models, the intercell interface area arrays of MODFLOW‐USG were calculated to accurately represent coaxial cylindrical model cells. Three examples are presented to demonstrate the application of MODFLOW‐USG for axisymmetric modeling: a pumping well with delayed yield effects, a vadose zone flow model simulating an infiltration basin, and a density‐dependent saltwater intrusion problem for a circular island. Results were verified against analytical solutions and published numerical codes. Article impact statement: This study presents a new methodology to create axisymmetric models using MODFLOW‐USG. [ABSTRACT FROM AUTHOR]

Abstract: As an alternative to grid refinement, the concept of a ghost node, which was developed for nested grid applications, has been extended towards improving sub-grid scale accuracy of flow to conduits, wells, rivers or other boundary features that interact with a finite-difference groundwater flow model. The formulation is presented for correcting the regular finite-difference groundwater flow equations for confined and unconfined cases, with or without Newton Raphson linearization of the nonlinearities, to include the Ghost Node Correction (GNC) for location displacement. The correction may be applied on the right-hand side vector for a symmetric finite-difference Picard implementation, or on the left-hand side matrix for an implicit but asymmetric implementation. The finite-difference matrix connectivity structure may be maintained for an implicit implementation by only selecting contributing nodes that are a part of the finite-difference connectivity. Proof of concept example problems are provided to demonstrate the improved accuracy that may be achieved through sub-grid scale corrections using the GNC schemes. [Copyright &y& Elsevier]

Various approaches have been proposed to manage the nonlinearities associated with the unconfined flow equation and to simulate perched groundwater conditions using the MODFLOW family of codes. The approaches comprise a variety of numerical techniques to prevent dry cells from becoming inactive and to achieve a stable solution focused on formulations of the unconfined, partially-saturated, groundwater flow equation. Keeping dry cells active avoids a discontinuous head solution which in turn improves the effectiveness of parameter estimation software that relies on continuous derivatives. Most approaches implement an upstream weighting of intercell conductance and Newton-Raphson linearization to obtain robust convergence. In this study, several published approaches were implemented in a stepwise manner into MODFLOW for comparative analysis. First, a comparative analysis of the methods is presented using synthetic examples that create convergence issues or difficulty in handling perched conditions with the more common dry-cell simulation capabilities of MODFLOW. Next, a field-scale three-dimensional simulation is presented to examine the stability and performance of the discussed approaches in larger, practical, simulation settings. [ABSTRACT FROM AUTHOR]

[Display omitted] • Finite difference code used to assess the aquifer thermal perturbation of a GSHP. • CLN package of open source Modflow-USG adapted to simulate BHE operation in aquifer. • Validation of the BHE model using CLN package coupled to unstructured grid. • Possible implementation of Borefield in one numerical model by means of Modflow-USG. Simulating heat transfer in an aquifer with one or more vertical Borehole Heat Exchangers (BHEs) of a Ground Source Heat Pump (GSHP) system by means of a finite difference code is difficult because of the square or rectangular geometry grid and computational times thus limiting the types of evaluations that can be performed. The aim of this work is to explore through MODFLOW-USG code (public domain software) a different approach towards simulating a borefield that would be more efficient computationally, in order to enable simulations of larger domains with multiple BHEs. The Connected Linear Network (CLN) package, introduced in MODFLOW-USG, generally simulates 1-D linear computational cells in a 3-D grid, such as hydraulic pipes in subsoil, but for the first time has been adapted to reproduce vertical closed loop U-pipe of a BHE. Therefore, this work evaluates the MODFLOW-USG and CLN package capability to reproduce the yearly operation of one or more BHEs in an aquifer as a simpler and faster approach compared to a very fine finite-difference discretization. Once the CLN package was adapted, a sensitivity analysis on the grid size refinement was performed. There were several findings from this work. The results of the different numerical models were in good agreement with an already validated model, in terms of exchanged energies and aquifer thermal perturbation. Same analyses were carried out for different groundwater flow velocities and it was confirmed that the exchanged energy by a BHE increases with the groundwater flow velocity in accordance with literature studies. At last, a borefield of 7 BHEs was implemented in a numerical model in a more expeditious and efficient way and without any computational effort. [ABSTRACT FROM AUTHOR]