A Nested Rediscretization Method to Improve Pathline Resolution by Eliminating Weak Sinks Representing Wells

Abstract

This paper discusses a method for overcoming the problem of weak sinks representing wells that result from spatial discretization effects when using MODPATH, the particle-tracking postprocessor for the ground water flow model MODFLOW. Weak sink cells are model cells that represent a well that does not discharge at a sufficiently large rate to capture all of the flow entering the cell; therefore, flowpaths within these cells cannot be uniquely defined because it is impossible to know whether a given water particle discharges to the well or passes through the cell. Creating a submodel of the well cell by using the nested rediscretization method can eliminate this ambiguity by converting the weak sink cell into a strong sink cell. The method is designed to be run manually for each well and for steady-state conditions. Other advantages, disadvantages, technical considerations, and limitations of the method are presented. Software created for the method consists of five Fortran programs that are operated using a set of instructions. A practical application of the method is presented by using an example wellhead-protection problem that demonstrates that nested rediscretization can provide more accurate particle-tracking results than those obtained by using a coarsely discretized model alone.

On Modeling Weak Sinks in MODPATH

“…Either a strong sink cell or a weak sink cell may represent an actual physical strong sink. An example of the latter is a strong sink well whose capture zone does not fully cover the well cell (Spitz et al 2001). Likewise, either type of sink cell may also represent a physical weak sink.…”

Mathematical development and verification of a non-orthogonal finite volume model for groundwater flow applications

“…discontinuities, or very thin units) requires relatively small grid spacing. Many authors choose to reduce or eliminate the FD limitations by using techniques of mesh refinement [33,39,45]. However, fine grids can result in long execution times that prohibit the many model runs often needed to understand the system dynamics and calibrate the model.…”

A fully coupled numerical modeling for regional unsaturated–saturated water flow

“…However, FDM is not flexible to arbitrary boundary geometries and may lose accuracy when predicting hydraulic heads in the vicinity of irregular boundaries. The only way to fit MODFLOW with arbitrary boundary geometries is by using the mesh refinement, but could greatly increase the computational cost (Spitz et al, 2001;Mehl and Hill, 2004). Moreover, FEM is notorious for its mass imbalance at the local level (Di Giammarco et al, 1996).…”