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Copyright 2007

Last Update:
July 31, 2007

Groundwater Contaminant Transport
Dr. Darrell Pepper , Dr. David Carrington

Overview
GWADAPT is a finite-element model which incorporates mesh (h-) adaptation to calculate ground-water flow and pollutant transport. The formulation is based on the equations for conservation of mass, Darcy's law for an anisotropic medium, and the time-dependent species transport equation. Modifications are used in the finite-element formulation to enhance computational speed and reduce storage; Petrov-Galerkin weighting of the advection terms provides numerical stability. An explicit time marching scheme is used to solve the transient equations. By utilizing unstructured adaptive meshing, species concentration and location of steep fronts are accurately resolved, even though one begins with a coarse mesh. The algorithm is written in C/C++, and runs on PCs under WINDOWS; versions are also available for workstations and a Cray super-computers.

Introduction

Groundwater contamination has become an important environmental issue which poses a serious threat to drinking water quality. Such problems involve complicated physics, chemistry, and multi phase flow phenomena. The simulation of contaminant transport through the subsurface is necessary in order to effectively design mitigation methods for cleanup and prevention of the deterioration of ground water. The transport of contaminants in groundwater systems requires reliable predictions to assess potential hazards to the public. Analytical methods have limited capability to accurately predict complex dispersion patterns where geometry and soil conditions vary over a wide range. Numerical models are more flexible in solving such complex processes and yield realistic solutions, but the cost and amount of time involved in the computation of these multi dimensional problems can be considerable.

Current Results
An adapting finite element model, GWADAPT, has been developed for calculating contaminant transport and ground water flow. Simple modifications to the basic Galerkin formulations are used to enhance speed and reduce storage; a Petrov-Galerkin weighting scheme is used for the advection terms. Mesh adaptation is achieved using interpolation based commands and averaging to refine/unrefine the mesh. An explicit, second-order accurate in time Runge-Kutta method is employed to advance the transient solutions; the solution times are fast on PCs, depending on the mesh density, and indicate the potential for significant payoff in overall solution time for three-dimensional problems. GWADAPT is fast enough in 2-D to allow transient solutions to be generated on enhanced 486 and Pentium class PCs without significant waiting, providing moderately sized meshes are used. GWADAPT has been written in C/C++ to provide ease of portability to other machines, and quickly display graphical output of the model results, as well as mesh changes. GWADAPT also runs quickly on workstation class machines (which allow easy interface to real time graphics capability). Savings in computer time and storage amount to several factors using locally adapting meshes. Preliminary tests of the model show promise for execution on vector super-computers as well as massively parallel computers. Extension of the two-dimensional version of GWADAPT to three dimensions has been undertaken. Testing of the 3-D version is still being conducted. The ability to perform 3-D adaptation with speed and prevent CPU memory overload (as a result of new nodal numbering and bandwidth increases) is rather tricky, especially when running on small machines. The 3-D version of GWADAPT should be available in 1997, and will be primarily aimed at the workstation level (although a reduced version will be available for the PC).





Sponsor: Environmental Protection Agency: Experimental Program to Stimulate Competitive Research (EPA EPSCoR)


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Nevada Center for Advanced Computational Methods