j.g. johnson, a.s. mayer & s.a. sorby - wit press · 2014. 5. 16. · value, which can be used to...

Development of an efficient pre- and

post-processing framework for groundwater

flow and transport models

J.G. Johnson, A.S. Mayer & S.A. Sorby

Department of Civil and Environmental Engineering,

Michigan Technological University, Houghton, Michigan,

Abstract

The use of groundwater flow and transport models is hindered by the lack ofadequate graphical tools for assembling input data sets and analyzing output.This work involves the development of a software interface between a power-ful, graphical pre- and post-processing package and widely-used groundwaterflow and transport models.

Background

Previous waste disposal practices have not included adequate protection forgroundwater resources, resulting in the degradation of groundwater quality. Animportant component of groundwater contamination assessment and remedia-tion efforts is the mathematical modeling of groundwater flow and contaminanttransport. Mathematical models are used to predict the potential exposure toharmful contaminants in the groundwater and to design and predict the perfor-mance of groundwater restoration measures. However, there are several factorswhich limit the widespread use of these models. Modeling efforts for a typicaldegraded groundwater site require large amounts of input data. The step ofinputting the data can consume as much time as it takes to execute the modelsimulations.

This limitation has serious implications, because several simulations often arerequired for an effective characterization of the problem at hand. More than onesimulation is needed because it is often the case that the input data is uncertainand/or incomplete. Thus, there is often a need to perform a series of simulationsto test the sensitivity of flow or transport results as a function of the possiblerange of data values. An additional limiting factor is the ability to visualize the

Transactions on Ecology and the Environment vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3541

78 Computer Techniques in Environmental Studies

output of the model, that is, the patterns of groundwater flow and contaminantsas a function of space and time. A typical model simulation results in the outputof hundreds to tens of thousands of numerical values. The objective of thiswork is to develop an interface between groundwater modeling codes and agraphical pre- and post-processor, in order to enhance groundwater modelingvisualization capabilities and increase the productivity of current groundwatermodeling efforts.

Approach

The approach taken in this project consists of developing a software packagethat combines existing groundwater flow and transport models with a graphicalpre- and post-processor. In the following discussion, the groundwater models,the pre- and post-processing software, and the methods for linking and execut-ing the groundwater models with the pre- and post-processing software aredescribed.

Groundwater Models. The most widely-used groundwater flow model is the

MODFLOW model \ Because of its general acceptance in the modeling com-munity, this model was used as the flow model in this project. The MODFLOWmodel is based on a three-dimensional (3D), finite-difference solution to thegroundwater flow equation. The model may be used to simulate flow in uncon-fined and confined aquifers and provides for flows associated with externalstresses such as wells, areal recharge, evapotranspiration, drains, and streams.The model source code is written in standard FORTRAN. Application of themodel involves the superposition of a 3D grid over the groundwater aquifer ofinterest. The aquifer is subdivided into a series of cells, with nodes centeredwithin the cells. The unknown of interest, groundwater hydraulic head, issolved for at these nodal locations. The locations of the nodes and the corre-sponding dimensions of the cells must be specified for each node and cell.

The MT3D model* was selected as the contaminant transport model for

this project This model has been shown to be a robust and accurate model* andwas written to accept MODFLOW output. The MT3D source code is written instandard FORTRAN. This model simulates 3D solute advection, dispersion,and chemical reactions. The numerical solution used to solve the governingtransport equations is based on the modified method of characteristics. Theapplication of the MT3D model involves the superposition of the same 3D gridused for MODFLOW. Velocities, reaction rates and dispersivities are specifiedfor the solute transport model, along with appropriate boundary and initial con-ditions. The model can account for solute mass leaving the system by way ofpumped wells, which is the most common technique of recovering contami-nated groundwater. The output from this model consists of concentration val-ues as a function of both space and time.


Computer Techniques in Environmental Studies 79

Visualization Software. I-DEAS* software was adopted for pre- and post-pro-cessing. The database manager found in I-DEAS enables a user to create a filetranslator for use with virtually any other finite element or finite difference pro-gram. Using the PEARL database transfer, data files for other programs can beread or written and converted to/from standard I-DEAS format. In this way, theuser can take advantage of the meshing capabilities of I-DEAS while still usingthe solver which is most suited to the problem at hand. The meshing and post-processing capabilities of I-DEAS are interactive, versatile and relatively easyto master. I-DEAS is widely available at U.S. universities. It is also popular atindustrial and governmental institutions which are involved in mechanical andaerospace engineering design, and is likely to become widely used in otherengineering design disciplines in the near future.

Software Linkages and Execution. The software development is occurring inthe following steps: (1) development of I-DEAS programs for geologic modelbuilding; model grid building; assignment of material properties, boundaryconditions and other ancillary parameters required by the groundwater models;(2) development of an I-DEAS file translator to create input files for thegroundwater models; (3) modification of the groundwater models to allow foroutput of I-DEAS-usable data sets; and (4) development of I-DEAS programsfor post-processing of groundwater flow and transport model outputs.

The pre-processor allows the user to graphically build a geologic model inthree dimensions using vertical layering geometry from well log data. Hydro-geologic features, such as different geologic materials, surface water bodies, orcontaminant source locations are graphically designated, generating a 3Dvisual model. Once the geological model has been generated, a 3D finite-differ-ence grid can be overlain on the geologic model. Model inputs such as materialproperties, boundary conditions, and sources and sinks can be designatedgraphically for individual or blocks of cells, or directly input numerically. Themodel grid can be modified easily to adjust for horizontal and vertical heteroge-neities or sources or sinks. The pre-processor was constructed using I-DEASprogramming language. The pre-processor is menu- and query-driven; the I-DEAS programming is invisible to the user.

The file input structures of the groundwater models were analyzed and afile translator program was created to be run from within I-DEAS. The filetranslator draws upon the grid, material property, and other data generated frompre-processing and writes out a data file that is suitable for direct input to thegroundwater models. This activity also is invisible to the user. The user canexecute the groundwater models using the I-DEAS-generated input files.

Next, a link between the output of the groundwater models and I-DEASwas developed. The link involves the addition of subroutines to the sourcecodes of the groundwater models such that they will generate a universal file asa part of its output. Universal files are ASCII files which contain geometric data



in a format which is acceptable to I-DEAS. In this way, results from the analy-sis can be read directly into I-DEAS and post-processing can be achieved. Themain programs of the groundwater model source codes also were modifiedslightly to allow activation of the new output subroutines. Conventional outputfiles can still be generated from the groundwater models; generation of the uni-versal file does not require additional effort by the user.

Post-processing within I-DEAS can take the form of surface contours ofconstant hydraulic heads or concentrations or shaded images which representthese entities as a continuum. Cross-sectional or cutaway views of the interiorof the model can be produced. The user also has the ability to show only thoseelements where the concentration of the contaminant is greater than a givenvalue, which can be used to represent contaminant plumes as 3D solids, which

is a valuable visualization technique for mapping contaminant plumeŝ . Any ofthese images can be rotated about the x-, y-, or z-axes. Post-processing on sev-eral data sets may be stored so that an animated display of the contaminanttransport may be observed. All of the post-processing activities rely on stan-dard I-DEAS menu items.

Example Application

This section demonstrates the application of the modeling framework to a fieldproblem involving the remediation of a contaminated aquifer. The site and datadescribed by the example problem in Section 7.7 of the MT3D manual was

used̂ . The site consists of a single unconfined aquifer consisting of medium-grained sand with a porosity of approximately 30%. Two distinct hydraulicconductivity zones are present, a shallow zone with a hydraulic conductivity of60 ft./day and a deep zone with a hydraulic conductivity of 600 ft./day. The sitereceives about 5 in/yr. of recharge. An organic chemical is found in an area thatis 2200 ft by 1300 ft., with concentrations exceeding 200 parts per billion.

The model grid used for the site consisted of four layers, 61 rows, and 40columns, with the greatest amount of cells concentrated in the center of themesh, where the contaminant plume and wells are located. Eight extractionwells were placed in this center section, extracting a total of 700 gpm from thethird model layer. The longitudinal dispersivity was 10 ft. and the horizontaland vertical transverse dispersivities were 2 ft. The retardation factor was equalto 2.

The modeling framework allowed for the model to be developed in a seriesof steps. First, a geologic model was set up through the input of layer depthsfrom well-log data. Figure 1 shows the geologic model for the example prob-lem. In this case, the layers are flat, but the modeling framework also is capableof interpolating curved surfaces through the well-log points to model slopingand irregularly shaped layers. A finite difference mesh can then be developed



Contamination Source

Well Logs

Figure 1. Geologic Model

and refined with or without the use of the geologic model. The finite differencemesh used in the example problem is shown in Figure 2.

With the finite difference grid in place, boundary and initial conditions areinput through the selection of elements and the use of pull-down menus. Sym-bols and color changes show the user where conditions have been assigned. Theexample problem involved selecting the elements for well placement, as well asdefining areal recharge over the region. Constant head cells were used along theouter boundaries; Figure 2 shows the model with constant head conditions inplace. A different program section allows for the similar input of hydraulic con-ductivity, dispersivity, and the other material and chemical properties. Elementsthat are hidden or clustered in the center of the model can be exposed by view-ing individual layers or cutting away model sections. Figure 3 shows the isola-tion of the center section of the third layer in the example grid to expose wellelements.

With all data in place, the modeling framework will then create input filesfor all of the necessary MODFLOW and MT3D program packages. The flowand transport models can then be executed. The model codes have been slightlymodified to produce I-DEAS readable files as output for post-processing. Thepost-processing capabilities include 3D viewing and color contour and continu-ous shaded images. The 3D views can be rotated on the screen to gain addi-



Variable HeadElements

Constant HeadElements

Figure 2. Model grid with boundary conditions

Wells

Figure 3. Magnification of model grid showing well elements



tional perspective on irregular features. Heads and drawdowns around wells orother features can be viewed in full, with cutting possible to view inner headcontours. Figure 4 shows a cut-away view of the heads around the wells of theexample problem. Similarly, concentration plumes can be viewed and cut to

781.0

780.5

780.0

779.5

779.0

778.5

778.0

777.5

Figure 4. Head distributions (in feet) near extraction wells

reveal plume sizes and features. Figure 5 shows a concentration plume pro-duced for the example problem. Animation is possible through the productionof multiple images to view plume movement or drawdown over time.

Conclusions

Pre- and post-processors were developed for use wrth groundwater flow andtransport models. The pre-processor gives the user the capacity to build geo-logic models of sites; construct model grids; and assign material properties,boundary conditions, and other ancillary parameters to model grids. Theseactivities are conducted using a combination of user menus and 3D, graphicalutilities. Post-processing of model output includes 3D shaded contours andsurfaces, which can be observed at any view angle and through cutaway views.Software linkages were developed to allow the pre-processor to generate inputfiles for the groundwater models and to allow the groundwater models to pro-duce output files for the post-processor. The pre- and post-processor wereapplied to an example problem, indicating the capabilities of the visualizationtools.



time = 1 day time = 500 days

time= 1,000 days

Figure 5. Contaminant plume through time with concentrations rangingfrom 200 ppb (darkest) to 0 ppb (lightest)

Acknowledgment

This work has been supported by the Michigan Research Excellence Fund.

References

1. McDonald, M. G. and A. W. Harbaugh, 'Techniques of Water-ResourcesInvestigations of the United States Geological Survey: Chapter Al, A ModularThree-Dimensional Finite-Difference Ground-Water Flow Model," U.S. Gov-ernment Printing Office, Washington, D.C., 1988.

2. Zheng, C, "MT3D, A Modular Three-Dimensional Transport Model," S. S.Papadopulos and Associates, Rockville, Maryland, 1990.

3. Zheng, C., "Extension of the/ Method of Characteristics for Simulation ofSolute Transport in Three Dimensions," Ground Water, 31(3), pp. 456-465,1993.

4. Structural Dynamics Research Corporation, "I-DEAS User's Guide," Struc-tural Dynamics Research Corporation, Milford, Ohio, 1992.

5. Nichols, R. L., Looney, B. B., and J. E. Huddleston, "3-D Digital Imaging,"Environmental Science and Technology, 26(4), pp. 642-649, 1992.


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