Modeling Axially Symmetric and Nonsymmetric Flow to a Well with MODFLOW, and Application to Goddard2 Well Test, Boise, Idaho

Abstract

Tools for analyzing aquifer test data at the pumping or injection well are limited for aquifer conditions that are not axially symmetric about the pumping well. Accurate simulations of head change at a pumping well can be generated with MODFLOW using a discretization scheme that (1) captures steep gradients adjacent to the cell(s) containing the pumping or injection well and (2) approximates radial flow despite rectangular prism well cell(s). This scheme is based on a very small incremental cell width adjacent to the pumping or injection well cell, and then a logarithmic increase in cell width outward with the expansion factor, α, in the range of 1.2 to 1.5. The validity of this scheme has been demonstrated by comparing model results with analytical solutions and RADMOD (axisymmetric adaptation of MODFLOW) for three radially symmetric confined aquifer scenarios, and with the analytical solution for a nonaxisymmetric confined aquifer scenario. Utility of this scheme is demonstrated with simulation of time-drawdown behavior at the Goddard2 pumping well in Boise, Idaho where four observation wells did not respond during a pumping test and where 28.8 m of drawdown occurred in the pumping well during the first 2 min of the test. The hydrogeologic setting for the test is interpreted to be a partially penetrating well pumping from a sand-stringer aquifer that receives leakage from surrounding finer grained sediments, and includes a fault (no-flow boundary) truncating the aquifer

Flow to horizontal drains in isotropic unconfined aquifers

“…For determining the flow rate accurately, the cell dimension of 0.001 m is considered for the cells next to the drain periphery. To improve the model efficiency, the cell dimensions are gradually enlarged through getting far from the drain by an expansion factor of 1.44 (Barrash and Dougherty, 1997). The length of drain cells, along its axis and at its ends are 0.001 m and gradually increase toward the midpoint of the drain by extension factor of 1.44.…”

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

“…rectilinear rows and columns). Barrash and Dougherty [6] found that the FD formulation underestimated large head gradients that can occur in the vicinity of pumping (or injection) wells. Generally, a detailed representation of the flow field in the vicinity of hydrological features and through irregular hydrogeological units (e.g.…”

Correction of Discretization Errors Simulated at Supply Wells

“…Barrash and Dougherty (1997) describe a variable grid refinement strategy through which a finite difference model can be discretized in order to better estimate the drawdown at a well. Various methods can be used to minimize s d when simulating drawdown at a pumping well.…”

“…This method was made available for MODFLOW 2005 (Mehl and Hill 2005). These grid refinement strategies would facilitate the implementation of Barrash and Dougherty's (1997) while decreasing the computational burden as compared to global or variable grid refinement approaches. These grid refinement strategies would facilitate the implementation of Barrash and Dougherty's (1997) while decreasing the computational burden as compared to global or variable grid refinement approaches.…”