Map of modeled area showing locations of wells, the interim corrective measure, and extent of modeled area.
¹ The use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
An compliance order issued by the state required implementing an interim corrective measure in the summer of 1996 to contain the ground-water plume onsite. The selected containment method consisted of a subsurface treatment wall interrupted by four treatment gates; the wall funneled ground-water flow into the gates for in-place treatment of the ground water as it passed through (Starr and Cherry, 1994). A series of site-characterization tasks were begun to determine specifications for the containment. Because of time constraints associated with the project, these characterizations were underway during mobilization of construction activities. Model findings were rapidly incorporated into the final implementation of the interim corrective measure.
A two-dimensional, finite-difference model of ground-water flow in the alluvium was developed by the USGS using MODFLOW (McDonald and Harbaugh, 1988). The model was based on limited data from previous investigations as well as new data from the current site-characterization studies. Several possible funnel-and-gate configurations were simulated using the flow model. The site-specific modeling allowed consideration of different environmental factors, such as lateral variations in aquifer thickness and hydraulic conductivity, as well as physical constraints on the location of the system. The final configuration was a wall approximately 1,200 feet long including four separate gates, each 40 feet wide, which is the largest multiple-gate and funnel system implemented to date. This paper discusses how ArcInfo was used to assist the site-characterization and modeling effort and how the ease of editing model-input parameters and visualization in the GIS made modeling possible within a short timeframe.
Grid surfaces of bedrock elevation (bottom of aquifer) and water-table elevation were generated using the GRID function SPLINE from an attributed point coverage of well locations, and a saturated thickness surface was calculated from the bedrock- and water-elevation grids. Vector coverages, also used as input to the model, included polygon coverages of impervious areas, polygons defining active and inactive portions of the model area, boundary conditions, polygons defining the spatial extent of remediation features, and polygons defining areas of different hydraulic conductivity.
Because the distribution of data points (well locations) around the study area did not extend to the edge of the modeled area, the GRID function SPLINE was used instead of the TIN module.
Map of modeled area showing locations of wells, the interim corrective
measure, and extent of modeled area.
It was a requirement of this model that data extend beyond the irregular area interpolated by TIN all the way to the stream boundary to the south and to the model area boundaries to the west, north, and east. Because the spatial extent of an ArcInfo grid is a box enclosing all the data points, the SPLINE function required little or no use of artificial points to force extrapolation to the boundaries of the active model area.
Map showing the differences in boundaries calculated by TIN and SPLINE
interpolation methods.
ZONEARRAY created input files that were based on vector coverages. Polygons defined discrete zones such as hydraulic conductivity, impervious areas, and active versus inactive areas of the model. In this model, a zone was usually a polygon attributed with an integer number to represent a feature or value. The AML executed ARC commands to overlay the various polygon themes with the model mesh to determine which zone each cell in the mesh belonged to for each theme. The appropriate zone value was added as an attribute to the polygon attribute table (PAT) of the model mesh.
Map showing sample overlay of the model mesh with a polygon coverage of hydraulic
conductivity zones.
The AML then rearranges the values from the PAT and stores them in a sorted temporary INFO file. To create the ASCII file that would be given to MODFLOW, the AML passed the zone values in each cell from the temporary INFO file to a Fortran program. The Fortran program then output the values in a predefined format to a MODFLOW-readable ASCII file.
The second AML, REALGRIDARRAY, was used to create input files based on continuous surfaces such as water-table elevation, bedrock elevation, and saturated thickness. The AML executed GRID commands that rasterized the polygon coverage of the model mesh and used the resulting grid to compute zonal statistics (geometric mean or arithmetic mean) for each cell of the model mesh and added that value as an attribute to the PAT of the model mesh. As with ZONEARRAY, this AML passed values from an INFO file to a Fortran program that formatted the values for input into MODFLOW .
Example of an ASCII file created by running the AML RELAGRIDARRAY which
required a file defining output format, a surface grid, and the model mesh coverage.
These AMLs made updating the input ASCII files for MODFLOW much easier than manually editing the thousands of numbers by simply running them again using updated coverage and grid themes. For example, editing the spatially referenced hydraulic conductivity coverage while it was overlaid on other coverage information and rerunning the parameterizing AMLs was much easier than trying to determine and modify corresponding numbers in the ASCII MODFLOW input files as cells values changed in the GIS. The AMLs were re-run as needed, as changes were made to surfaces and polygon coverages for model calibration and sensitivity analysis.
Map of MODFLOW-calculated water-table elevation contours displayed in ARC
with other spatially referenced information.
The MODFLOW graphics files were CGM files that imported directly into ARC using the ARCPLOT command PLOT. Scaling the graphics was established by matching the boundary box of the MODFLOW graphic with that of the geo-referenced model mesh boundary.
MODFLOW also produces unformatted binary files that contain either model-calculated head or drawdown values. The AML, SHADEMODFLOWCELLS (Jennifer Sieverling and Ned Banta, U.S. Geological Survey, written commun., 1996), helped with visualization of these data. The AML generated a postscript file for a graphic that showed a gradational shading of the head or drawdown values. The postscript graphic also contained an annotated legend about the source data, data type, model layer number, model stress period, time step, and shading legend.
The final AML used from the model-interface suite was ROWCOLOFFSET (Jennifer Sieverling and Ned Banta, U.S. Geological Survey, written commun., 1996). This AML calculated the row and column identification of the mesh cell that each well point fell within and added it to the well coverage PAT. The x and y offset of the wells from the centroid of its corresponding mesh cell also was recorded in the well coverage PAT. These values are required by MODPATH3, a particle-tracking package that computes particle-path lines based on output from simulations created by MODFLOW (Pollock, 1989).
McDonald, M.G., and Harbaugh, A.W., 1988, A modular three-dimensional finite-difference ground-water flow model: Techniques of Water Resources Investigations of the U.S. Geological Survey, Book 6, Chapter A1.
Pollock, David W., 1989, Documentation of computer programs to compute and display pathlines using results from the U.S. Geological Survey modular three-dimensional finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 89-381, 188 p.
Starr, R.C., and Cherry, J.A., 1994, In-situ remediation of contaminated ground water-The funnel-and-gate system; Ground Water, v. 32, p. 465-476.
Jennifer B. Sieverling
Hydrologist/GIS Specialist
U.S. Geological Survey - Water Resources Division, Colorado District
Box 25046, MS 415
Denver Federal Center
Denver, CO 80225
Phone: (303) 236-4882
Fax: (303) 236-4912