MODFLOW Model of Mine Dewatering

By Darrel Dunn, Ph.D., Hydrogeologist 

(View Résumé 🔳)

Purpose and Scope

The purpose of this nontechnical webpage is to present a MODFLOW groundwater model that demonstrates the use of dewatering wells for underground mine dewatering that can be compared to an analytic element model (AEM) of the same system.  The application of the two modeling techniques is limited to this simple dewatering problem.  Extrapolation of findings to other modeling scenarios is not intended.  The webpage uses relatively non-technical language with some technical footnotes and references.  The AEM model of the dewatering system is described on another webpage (mine dewatering).  The two models simulate dewatering of a sub-surface mine with two tunnels to ore veins.  The two modeling techniques are tools used in mining hydrogeology.

MODFLOW and AEM Modeling

MODFLOW is a modular three-dimensional finite-difference1 groundwater modeling computer code maintained by the United States Geological Survey (USGS).  Finite-difference groundwater modeling was introduced in 1967 (Early Groundwater Model), and more sophisticated models were developed in subsequent years.  The technique became widely used after MODFLOW was developed in 1984 (McDonald and Harbaugh, 1984).  It has been evolving into a comprehensive groundwater modeling program as additional modules that expand its capabilities are added.  Analytic element modeling began evolving in the 1980s (Strack, 1989) and is not as widely used as finite-difference modeling.  A brief description of AEM is provided on the webpage titled Analytic Element Groundwater Modeling.  Whereas  AEM uses relatively simple mathematical equations to compute the effect of such elements as water wells on groundwater systems; a finite-difference model divides the system into rectangular cells, applies an equation to each cell and solves the equations simultaneously to get the water table elevation in each cell.  AEM models are generally used for relatively simple groundwater systems, whereas finite-difference models like MODFLOW can simulate very complex systems.  AEM equations apply to extensive areas with no definite boundaries, whereas the extent of a finite-difference model is limited by defined boundaries.  Conditions at the boundaries of a finite-difference model must be specified.  Boundary conditions may be (1) impermeable boundary, (2) constant head2 boundary, or (3) general head boundary3.  Other differences between AEM and finite-difference models exist.  Consequently, a comparison of results of AEM and finite-difference models of the same dewatering system is of interest.

MODFLOW Method

FloPy was used to generate the input files for the MODFLOW model.  FloPy is a set of Python programs developed by the USGS that can be used to produce input files for various USGS groundwater models, including MODFLOW.  Python is a computer programming language that was introduced in 1991 and has evolved into a powerful tool increasingly used in science and technology.  It includes matrix manipulation modules that can be used for development of cell by cell MODFLOW input and modules for plotting maps and graphs of model results.

MODFLOW Model Setup - Constant Head Boundary Model

For this MODFLOW model, the gridded area is the same as the area on the maps used to present the results of the AEM model.  The grid is shown in Figure 1 superimposed on the pre-mining water table map from the AEM mine dewatering page.