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

Abstract

The illustration on the cover of this report shows the local, intermediate, and regional scale flow systems that develop in reponse to sinusoidal variations in water table elevation about an average linear regional gradient In this figure, a regional groundwater divide exists at the right boundary and the regional discharge area is in the upper left corner of the system. The analysis on which this illustration is based was originally done by Toth (1963) using analytical solutions for the groundwater flow equation. The cover illustration was generated by a particle tracking analysis based on a finite difference solution of case 2f in Toth's paper. The finite-difference solution used 100 cells in the vertical direction and 200 cells in the horizontal direction.

Hydrogeology, simulated ground-water flow, and ground-water quality at two landfills in Bristol, Vermont

“…Distinctions should not be made in the upper aquifer (model layer 1) between flow-path discharges to the stream and ground-water flow past the stream through the dolomite gap. MODPATH cannot accurately partition flow paths to the sinks in a cell (in this case the stream and the aquifer) because all sinks are uniformly distributed within the cells (Pollock, 1989). Although the total flow budget is acceptable, model layer discretization is probably not fine enough to accurately represent vertical partitioning of flow at the stream cells in layer 1.…”

“…A particle-tracking program developed by Pollock (1989), MODPATH, was used to compute pathlines of ground-water flow. MODPATH is designed as a postprocessor to MODFLOW; MODFLOW output files are used directly in computational schemes.…”

Hydrogeology and results of tracer tests at the old Tampa well field in Hillsborough County, with implications for wellhead-protection strategies in west-central Florida

“…Vecchioli and others (1989) presented an evaluation of the methodology for delineation of wellhead-protection zones in west-central Florida. McDonald and Harbaugh (1988) documented the USGS modular ground-water flow model (MODFLOW), and Pollock (1989) documented a particle-tracking program (MODPATH) that uses the output of MODFLOW to calculate ground-water pathlines and time of travel.…”

“…The objectives of the modeling were to: (1) provide additional estimates of the hydraulic coefficients of the Upper Floridan aquifer; (2) test the conceptual model of the ground-water flow system; and (3) generate the velocity-vector field required by the particle-tracking program to simulate the groundwater travel times measured with tracer tests. The USGS modular model "MODFLOW" (McDonald and Harbaugh, 1988) was selected because of its ability to simulate layered aquifers and because the particletracking program "MODPATH" (Pollock, 1989) uses the output from the modular model. The MODFLOW model can use the quasi-three-dimensional approach to simulate flow between multiaquifer systems (twodimensional (x,y) horizontal flow through aquifers and vertical (z) flow through confining units using leakance terms and ignoring storage) or the three-dimensional approach by specifying a model layer for each hydrogeologic unit.…”

“…The particle-tracking program MODPATH (Pollock, 1989) was used to simulate the ground-water travel times measured during the deep tracer test. The objective of the particle-tracking simulation was to compare ground-water travel times in an equivalent porous media model of the ground-water flow system with observed ground-water travel times.…”